--- jsr166/src/jsr166y/ForkJoinPool.java 2010/05/27 16:46:48 1.56 +++ jsr166/src/jsr166y/ForkJoinPool.java 2013/02/16 20:50:29 1.185 @@ -1,62 +1,59 @@ /* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at - * http://creativecommons.org/licenses/publicdomain + * http://creativecommons.org/publicdomain/zero/1.0/ */ package jsr166y; -import java.util.concurrent.*; - import java.util.ArrayList; import java.util.Arrays; import java.util.Collection; import java.util.Collections; import java.util.List; -import java.util.concurrent.locks.LockSupport; -import java.util.concurrent.locks.ReentrantLock; -import java.util.concurrent.atomic.AtomicInteger; -import java.util.concurrent.CountDownLatch; +import java.util.concurrent.AbstractExecutorService; +import java.util.concurrent.Callable; +import java.util.concurrent.ExecutorService; +import java.util.concurrent.Future; +import java.util.concurrent.RejectedExecutionException; +import java.util.concurrent.RunnableFuture; +import java.util.concurrent.TimeUnit; /** * An {@link ExecutorService} for running {@link ForkJoinTask}s. * A {@code ForkJoinPool} provides the entry point for submissions - * from non-{@code ForkJoinTask}s, as well as management and + * from non-{@code ForkJoinTask} clients, as well as management and * monitoring operations. * *

A {@code ForkJoinPool} differs from other kinds of {@link * ExecutorService} mainly by virtue of employing * work-stealing: all threads in the pool attempt to find and - * execute subtasks created by other active tasks (eventually blocking - * waiting for work if none exist). This enables efficient processing - * when most tasks spawn other subtasks (as do most {@code - * ForkJoinTask}s). A {@code ForkJoinPool} may also be used for mixed - * execution of some plain {@code Runnable}- or {@code Callable}- - * based activities along with {@code ForkJoinTask}s. When setting - * {@linkplain #setAsyncMode async mode}, a {@code ForkJoinPool} may - * also be appropriate for use with fine-grained tasks of any form - * that are never joined. Otherwise, other {@code ExecutorService} - * implementations are typically more appropriate choices. + * execute tasks submitted to the pool and/or created by other active + * tasks (eventually blocking waiting for work if none exist). This + * enables efficient processing when most tasks spawn other subtasks + * (as do most {@code ForkJoinTask}s), as well as when many small + * tasks are submitted to the pool from external clients. Especially + * when setting asyncMode to true in constructors, {@code + * ForkJoinPool}s may also be appropriate for use with event-style + * tasks that are never joined. + * + *

A static {@link #commonPool()} is available and appropriate for + * most applications. The common pool is used by any ForkJoinTask that + * is not explicitly submitted to a specified pool. Using the common + * pool normally reduces resource usage (its threads are slowly + * reclaimed during periods of non-use, and reinstated upon subsequent + * use). * - *

A {@code ForkJoinPool} is constructed with a given target - * parallelism level; by default, equal to the number of available - * processors. Unless configured otherwise via {@link - * #setMaintainsParallelism}, the pool attempts to maintain this - * number of active (or available) threads by dynamically adding, - * suspending, or resuming internal worker threads, even if some tasks - * are stalled waiting to join others. However, no such adjustments - * are performed in the face of blocked IO or other unmanaged - * synchronization. The nested {@link ManagedBlocker} interface - * enables extension of the kinds of synchronization accommodated. - * The target parallelism level may also be changed dynamically - * ({@link #setParallelism}). The total number of threads may be - * limited using method {@link #setMaximumPoolSize}, in which case it - * may become possible for the activities of a pool to stall due to - * the lack of available threads to process new tasks. When the pool - * is executing tasks, these and other configuration setting methods - * may only gradually affect actual pool sizes. It is normally best - * practice to invoke these methods only when the pool is known to be - * quiescent. + *

For applications that require separate or custom pools, a {@code + * ForkJoinPool} may be constructed with a given target parallelism + * level; by default, equal to the number of available processors. The + * pool attempts to maintain enough active (or available) threads by + * dynamically adding, suspending, or resuming internal worker + * threads, even if some tasks are stalled waiting to join + * others. However, no such adjustments are guaranteed in the face of + * blocked I/O or other unmanaged synchronization. The nested {@link + * ManagedBlocker} interface enables extension of the kinds of + * synchronization accommodated. * *

In addition to execution and lifecycle control methods, this * class provides status check methods (for example @@ -65,23 +62,51 @@ import java.util.concurrent.CountDownLat * {@link #toString} returns indications of pool state in a * convenient form for informal monitoring. * - *

Sample Usage. Normally a single {@code ForkJoinPool} is - * used for all parallel task execution in a program or subsystem. - * Otherwise, use would not usually outweigh the construction and - * bookkeeping overhead of creating a large set of threads. For - * example, a common pool could be used for the {@code SortTasks} - * illustrated in {@link RecursiveAction}. Because {@code - * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon - * daemon} mode, there is typically no need to explicitly {@link - * #shutdown} such a pool upon program exit. + *

As is the case with other ExecutorServices, there are three + * main task execution methods summarized in the following table. + * These are designed to be used primarily by clients not already + * engaged in fork/join computations in the current pool. The main + * forms of these methods accept instances of {@code ForkJoinTask}, + * but overloaded forms also allow mixed execution of plain {@code + * Runnable}- or {@code Callable}- based activities as well. However, + * tasks that are already executing in a pool should normally instead + * use the within-computation forms listed in the table unless using + * async event-style tasks that are not usually joined, in which case + * there is little difference among choice of methods. + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + *
Call from non-fork/join clients Call from within fork/join computations
Arrange async execution {@link #execute(ForkJoinTask)} {@link ForkJoinTask#fork}
Await and obtain result {@link #invoke(ForkJoinTask)} {@link ForkJoinTask#invoke}
Arrange exec and obtain Future {@link #submit(ForkJoinTask)} {@link ForkJoinTask#fork} (ForkJoinTasks are Futures)
* - * 

- * static final ForkJoinPool mainPool = new ForkJoinPool();
- * ...
- * public void sort(long[] array) {
- *   mainPool.invoke(new SortTask(array, 0, array.length));
- * }
- *
+ *

The common pool is by default constructed with default + * parameters, but these may be controlled by setting three {@link + * System#getProperty system properties} with prefix {@code + * java.util.concurrent.ForkJoinPool.common}: {@code parallelism} -- + * an integer greater than zero, {@code threadFactory} -- the class + * name of a {@link ForkJoinWorkerThreadFactory}, and {@code + * exceptionHandler} -- the class name of a {@link + * java.lang.Thread.UncaughtExceptionHandler + * Thread.UncaughtExceptionHandler}. Upon any error in establishing + * these settings, default parameters are used. * *

Implementation notes: This implementation restricts the * maximum number of running threads to 32767. Attempts to create @@ -89,7 +114,8 @@ import java.util.concurrent.CountDownLat * {@code IllegalArgumentException}. * *

This implementation rejects submitted tasks (that is, by throwing - * {@link RejectedExecutionException}) only when the pool is shut down. + * {@link RejectedExecutionException}) only when the pool is shut down + * or internal resources have been exhausted. * * @since 1.7 * @author Doug Lea @@ -99,233 +125,388 @@ public class ForkJoinPool extends Abstra /* * Implementation Overview * - * This class provides the central bookkeeping and control for a - * set of worker threads: Submissions from non-FJ threads enter - * into a submission queue. Workers take these tasks and typically - * split them into subtasks that may be stolen by other workers. - * The main work-stealing mechanics implemented in class - * ForkJoinWorkerThread give first priority to processing tasks - * from their own queues (LIFO or FIFO, depending on mode), then - * to randomized FIFO steals of tasks in other worker queues, and - * lastly to new submissions. These mechanics do not consider - * affinities, loads, cache localities, etc, so rarely provide the - * best possible performance on a given machine, but portably - * provide good throughput by averaging over these factors. - * (Further, even if we did try to use such information, we do not - * usually have a basis for exploiting it. For example, some sets - * of tasks profit from cache affinities, but others are harmed by - * cache pollution effects.) + * This class and its nested classes provide the main + * functionality and control for a set of worker threads: + * Submissions from non-FJ threads enter into submission queues. + * Workers take these tasks and typically split them into subtasks + * that may be stolen by other workers. Preference rules give + * first priority to processing tasks from their own queues (LIFO + * or FIFO, depending on mode), then to randomized FIFO steals of + * tasks in other queues. + * + * WorkQueues + * ========== + * + * Most operations occur within work-stealing queues (in nested + * class WorkQueue). These are special forms of Deques that + * support only three of the four possible end-operations -- push, + * pop, and poll (aka steal), under the further constraints that + * push and pop are called only from the owning thread (or, as + * extended here, under a lock), while poll may be called from + * other threads. (If you are unfamiliar with them, you probably + * want to read Herlihy and Shavit's book "The Art of + * Multiprocessor programming", chapter 16 describing these in + * more detail before proceeding.) The main work-stealing queue + * design is roughly similar to those in the papers "Dynamic + * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005 + * (http://research.sun.com/scalable/pubs/index.html) and + * "Idempotent work stealing" by Michael, Saraswat, and Vechev, + * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186). + * The main differences ultimately stem from GC requirements that + * we null out taken slots as soon as we can, to maintain as small + * a footprint as possible even in programs generating huge + * numbers of tasks. To accomplish this, we shift the CAS + * arbitrating pop vs poll (steal) from being on the indices + * ("base" and "top") to the slots themselves. So, both a + * successful pop and poll mainly entail a CAS of a slot from + * non-null to null. Because we rely on CASes of references, we + * do not need tag bits on base or top. They are simple ints as + * used in any circular array-based queue (see for example + * ArrayDeque). Updates to the indices must still be ordered in a + * way that guarantees that top == base means the queue is empty, + * but otherwise may err on the side of possibly making the queue + * appear nonempty when a push, pop, or poll have not fully + * committed. Note that this means that the poll operation, + * considered individually, is not wait-free. One thief cannot + * successfully continue until another in-progress one (or, if + * previously empty, a push) completes. However, in the + * aggregate, we ensure at least probabilistic non-blockingness. + * If an attempted steal fails, a thief always chooses a different + * random victim target to try next. So, in order for one thief to + * progress, it suffices for any in-progress poll or new push on + * any empty queue to complete. (This is why we normally use + * method pollAt and its variants that try once at the apparent + * base index, else consider alternative actions, rather than + * method poll.) + * + * This approach also enables support of a user mode in which local + * task processing is in FIFO, not LIFO order, simply by using + * poll rather than pop. This can be useful in message-passing + * frameworks in which tasks are never joined. However neither + * mode considers affinities, loads, cache localities, etc, so + * rarely provide the best possible performance on a given + * machine, but portably provide good throughput by averaging over + * these factors. (Further, even if we did try to use such + * information, we do not usually have a basis for exploiting it. + * For example, some sets of tasks profit from cache affinities, + * but others are harmed by cache pollution effects.) + * + * WorkQueues are also used in a similar way for tasks submitted + * to the pool. We cannot mix these tasks in the same queues used + * for work-stealing (this would contaminate lifo/fifo + * processing). Instead, we randomly associate submission queues + * with submitting threads, using a form of hashing. The + * ThreadLocal Submitter class contains a value initially used as + * a hash code for choosing existing queues, but may be randomly + * repositioned upon contention with other submitters. In + * essence, submitters act like workers except that they are + * restricted to executing local tasks that they submitted (or in + * the case of CountedCompleters, others with the same root task). + * However, because most shared/external queue operations are more + * expensive than internal, and because, at steady state, external + * submitters will compete for CPU with workers, ForkJoinTask.join + * and related methods disable them from repeatedly helping to + * process tasks if all workers are active. Insertion of tasks in + * shared mode requires a lock (mainly to protect in the case of + * resizing) but we use only a simple spinlock (using bits in + * field qlock), because submitters encountering a busy queue move + * on to try or create other queues -- they block only when + * creating and registering new queues. + * + * Management + * ========== * * The main throughput advantages of work-stealing stem from - * decentralized control -- workers mostly steal tasks from each - * other. We do not want to negate this by creating bottlenecks - * implementing the management responsibilities of this class. So - * we use a collection of techniques that avoid, reduce, or cope - * well with contention. These entail several instances of - * bit-packing into CASable fields to maintain only the minimally - * required atomicity. To enable such packing, we restrict maximum - * parallelism to (1<<15)-1 (enabling twice this to fit into a 16 - * bit field), which is far in excess of normal operating range. - * Even though updates to some of these bookkeeping fields do - * sometimes contend with each other, they don't normally - * cache-contend with updates to others enough to warrant memory - * padding or isolation. So they are all held as fields of - * ForkJoinPool objects. The main capabilities are as follows: - * - * 1. Creating and removing workers. Workers are recorded in the - * "workers" array. This is an array as opposed to some other data - * structure to support index-based random steals by workers. - * Updates to the array recording new workers and unrecording - * terminated ones are protected from each other by a lock - * (workerLock) but the array is otherwise concurrently readable, - * and accessed directly by workers. To simplify index-based - * operations, the array size is always a power of two, and all - * readers must tolerate null slots. Currently, all worker thread - * creation is on-demand, triggered by task submissions, - * replacement of terminated workers, and/or compensation for - * blocked workers. However, all other support code is set up to - * work with other policies. - * - * 2. Bookkeeping for dynamically adding and removing workers. We - * maintain a given level of parallelism (or, if - * maintainsParallelism is false, at least avoid starvation). When - * some workers are known to be blocked (on joins or via - * ManagedBlocker), we may create or resume others to take their - * place until they unblock (see below). Implementing this - * requires counts of the number of "running" threads (i.e., those - * that are neither blocked nor artifically suspended) as well as - * the total number. These two values are packed into one field, - * "workerCounts" because we need accurate snapshots when deciding - * to create, resume or suspend. To support these decisions, - * updates to spare counts must be prospective (not - * retrospective). For example, the running count is decremented - * before blocking by a thread about to block as a spare, but - * incremented by the thread about to unblock it. Updates upon - * resumption ofr threads blocking in awaitJoin or awaitBlocker - * cannot usually be prospective, so the running count is in - * general an upper bound of the number of productively running - * threads Updates to the workerCounts field sometimes transiently - * encounter a fair amount of contention when join dependencies - * are such that many threads block or unblock at about the same - * time. We alleviate this by sometimes bundling updates (for - * example blocking one thread on join and resuming a spare cancel - * each other out), and in most other cases performing an - * alternative action like releasing waiters or locating spares. - * - * 3. Maintaining global run state. The run state of the pool - * consists of a runLevel (SHUTDOWN, TERMINATING, etc) similar to - * those in other Executor implementations, as well as a count of - * "active" workers -- those that are, or soon will be, or - * recently were executing tasks. The runLevel and active count - * are packed together in order to correctly trigger shutdown and - * termination. Without care, active counts can be subject to very - * high contention. We substantially reduce this contention by - * relaxing update rules. A worker must claim active status - * prospectively, by activating if it sees that a submitted or - * stealable task exists (it may find after activating that the - * task no longer exists). It stays active while processing this - * task (if it exists) and any other local subtasks it produces, - * until it cannot find any other tasks. It then tries - * inactivating (see method preStep), but upon update contention - * instead scans for more tasks, later retrying inactivation if it - * doesn't find any. - * - * 4. Managing idle workers waiting for tasks. We cannot let - * workers spin indefinitely scanning for tasks when none are - * available. On the other hand, we must quickly prod them into - * action when new tasks are submitted or generated. We - * park/unpark these idle workers using an event-count scheme. - * Field eventCount is incremented upon events that may enable - * workers that previously could not find a task to now find one: - * Submission of a new task to the pool, or another worker pushing - * a task onto a previously empty queue. (We also use this - * mechanism for termination and reconfiguration actions that - * require wakeups of idle workers). Each worker maintains its - * last known event count, and blocks when a scan for work did not - * find a task AND its lastEventCount matches the current - * eventCount. Waiting idle workers are recorded in a variant of - * Treiber stack headed by field eventWaiters which, when nonzero, - * encodes the thread index and count awaited for by the worker - * thread most recently calling eventSync. This thread in turn has - * a record (field nextEventWaiter) for the next waiting worker. - * In addition to allowing simpler decisions about need for - * wakeup, the event count bits in eventWaiters serve the role of - * tags to avoid ABA errors in Treiber stacks. To reduce delays - * in task diffusion, workers not otherwise occupied may invoke - * method releaseWaiters, that removes and signals (unparks) - * workers not waiting on current count. To minimize task - * production stalls associate with signalling, any worker pushing - * a task on an empty queue invokes the weaker method signalWork, - * that only releases idle workers until it detects interference - * by other threads trying to release, and lets them take - * over. The net effect is a tree-like diffusion of signals, where - * released threads (and possibly others) help with unparks. To - * further reduce contention effects a bit, failed CASes to - * increment field eventCount are tolerated without retries. - * Conceptually they are merged into the same event, which is OK - * when their only purpose is to enable workers to scan for work. - * - * 5. Managing suspension of extra workers. When a worker is about - * to block waiting for a join (or via ManagedBlockers), we may - * create a new thread to maintain parallelism level, or at least - * avoid starvation (see below). Usually, extra threads are needed - * for only very short periods, yet join dependencies are such - * that we sometimes need them in bursts. Rather than create new - * threads each time this happens, we suspend no-longer-needed - * extra ones as "spares". For most purposes, we don't distinguish - * "extra" spare threads from normal "core" threads: On each call - * to preStep (the only point at which we can do this) a worker - * checks to see if there are now too many running workers, and if - * so, suspends itself. Methods awaitJoin and awaitBlocker look - * for suspended threads to resume before considering creating a - * new replacement. We don't need a special data structure to - * maintain spares; simply scanning the workers array looking for - * worker.isSuspended() is fine because the calling thread is - * otherwise not doing anything useful anyway; we are at least as - * happy if after locating a spare, the caller doesn't actually - * block because the join is ready before we try to adjust and - * compensate. Note that this is intrinsically racy. One thread - * may become a spare at about the same time as another is - * needlessly being created. We counteract this and related slop - * in part by requiring resumed spares to immediately recheck (in - * preStep) to see whether they they should re-suspend. The only - * effective difference between "extra" and "core" threads is that - * we allow the "extra" ones to time out and die if they are not - * resumed within a keep-alive interval of a few seconds. This is - * implemented mainly within ForkJoinWorkerThread, but requires - * some coordination (isTrimmed() -- meaning killed while - * suspended) to correctly maintain pool counts. - * - * 6. Deciding when to create new workers. The main dynamic - * control in this class is deciding when to create extra threads, - * in methods awaitJoin and awaitBlocker. We always - * need to create one when the number of running threads becomes - * zero. But because blocked joins are typically dependent, we - * don't necessarily need or want one-to-one replacement. Using a - * one-to-one compensation rule often leads to enough useless - * overhead creating, suspending, resuming, and/or killing threads - * to signficantly degrade throughput. We use a rule reflecting - * the idea that, the more spare threads you already have, the - * more evidence you need to create another one. The "evidence" - * here takes two forms: (1) Using a creation threshold expressed - * in terms of the current deficit -- target minus running - * threads. To reduce flickering and drift around target values, - * the relation is quadratic: adding a spare if (dc*dc)>=(sc*pc) - * (where dc is deficit, sc is number of spare threads and pc is - * target parallelism.) (2) Using a form of adaptive - * spionning. requiring a number of threshold checks proportional - * to the number of spare threads. This effectively reduces churn - * at the price of systematically undershooting target parallelism - * when many threads are blocked. However, biasing toward - * undeshooting partially compensates for the above mechanics to - * suspend extra threads, that normally lead to overshoot because - * we can only suspend workers in-between top-level actions. It - * also better copes with the fact that some of the methods in - * this class tend to never become compiled (but are interpreted), - * so some components of the entire set of controls might execute - * many times faster than others. And similarly for cases where - * the apparent lack of work is just due to GC stalls and other - * transient system activity. - * - * 7. Maintaining other configuration parameters and monitoring - * statistics. Updates to fields controlling parallelism level, - * max size, etc can only meaningfully take effect for individual - * threads upon their next top-level actions; i.e., between - * stealing/running tasks/submission, which are separated by calls - * to preStep. Memory ordering for these (assumed infrequent) - * reconfiguration calls is ensured by using reads and writes to - * volatile field workerCounts (that must be read in preStep anyway) - * as "fences" -- user-level reads are preceded by reads of - * workCounts, and writes are followed by no-op CAS to - * workerCounts. The values reported by other management and - * monitoring methods are either computed on demand, or are kept - * in fields that are only updated when threads are otherwise - * idle. - * - * Beware that there is a lot of representation-level coupling - * among classes ForkJoinPool, ForkJoinWorkerThread, and - * ForkJoinTask. For example, direct access to "workers" array by - * workers, and direct access to ForkJoinTask.status by both - * ForkJoinPool and ForkJoinWorkerThread. There is little point + * decentralized control -- workers mostly take tasks from + * themselves or each other. We cannot negate this in the + * implementation of other management responsibilities. The main + * tactic for avoiding bottlenecks is packing nearly all + * essentially atomic control state into two volatile variables + * that are by far most often read (not written) as status and + * consistency checks. + * + * Field "ctl" contains 64 bits holding all the information needed + * to atomically decide to add, inactivate, enqueue (on an event + * queue), dequeue, and/or re-activate workers. To enable this + * packing, we restrict maximum parallelism to (1<<15)-1 (which is + * far in excess of normal operating range) to allow ids, counts, + * and their negations (used for thresholding) to fit into 16bit + * fields. + * + * Field "plock" is a form of sequence lock with a saturating + * shutdown bit (similarly for per-queue "qlocks"), mainly + * protecting updates to the workQueues array, as well as to + * enable shutdown. When used as a lock, it is normally only very + * briefly held, so is nearly always available after at most a + * brief spin, but we use a monitor-based backup strategy to + * block when needed. + * + * Recording WorkQueues. WorkQueues are recorded in the + * "workQueues" array that is created upon first use and expanded + * if necessary. Updates to the array while recording new workers + * and unrecording terminated ones are protected from each other + * by a lock but the array is otherwise concurrently readable, and + * accessed directly. To simplify index-based operations, the + * array size is always a power of two, and all readers must + * tolerate null slots. Worker queues are at odd indices. Shared + * (submission) queues are at even indices, up to a maximum of 64 + * slots, to limit growth even if array needs to expand to add + * more workers. Grouping them together in this way simplifies and + * speeds up task scanning. + * + * All worker thread creation is on-demand, triggered by task + * submissions, replacement of terminated workers, and/or + * compensation for blocked workers. However, all other support + * code is set up to work with other policies. To ensure that we + * do not hold on to worker references that would prevent GC, ALL + * accesses to workQueues are via indices into the workQueues + * array (which is one source of some of the messy code + * constructions here). In essence, the workQueues array serves as + * a weak reference mechanism. Thus for example the wait queue + * field of ctl stores indices, not references. Access to the + * workQueues in associated methods (for example signalWork) must + * both index-check and null-check the IDs. All such accesses + * ignore bad IDs by returning out early from what they are doing, + * since this can only be associated with termination, in which + * case it is OK to give up. All uses of the workQueues array + * also check that it is non-null (even if previously + * non-null). This allows nulling during termination, which is + * currently not necessary, but remains an option for + * resource-revocation-based shutdown schemes. It also helps + * reduce JIT issuance of uncommon-trap code, which tends to + * unnecessarily complicate control flow in some methods. + * + * Event Queuing. Unlike HPC work-stealing frameworks, we cannot + * let workers spin indefinitely scanning for tasks when none can + * be found immediately, and we cannot start/resume workers unless + * there appear to be tasks available. On the other hand, we must + * quickly prod them into action when new tasks are submitted or + * generated. In many usages, ramp-up time to activate workers is + * the main limiting factor in overall performance (this is + * compounded at program start-up by JIT compilation and + * allocation). So we try to streamline this as much as possible. + * We park/unpark workers after placing in an event wait queue + * when they cannot find work. This "queue" is actually a simple + * Treiber stack, headed by the "id" field of ctl, plus a 15bit + * counter value (that reflects the number of times a worker has + * been inactivated) to avoid ABA effects (we need only as many + * version numbers as worker threads). Successors are held in + * field WorkQueue.nextWait. Queuing deals with several intrinsic + * races, mainly that a task-producing thread can miss seeing (and + * signalling) another thread that gave up looking for work but + * has not yet entered the wait queue. We solve this by requiring + * a full sweep of all workers (via repeated calls to method + * scan()) both before and after a newly waiting worker is added + * to the wait queue. During a rescan, the worker might release + * some other queued worker rather than itself, which has the same + * net effect. Because enqueued workers may actually be rescanning + * rather than waiting, we set and clear the "parker" field of + * WorkQueues to reduce unnecessary calls to unpark. (This + * requires a secondary recheck to avoid missed signals.) Note + * the unusual conventions about Thread.interrupts surrounding + * parking and other blocking: Because interrupts are used solely + * to alert threads to check termination, which is checked anyway + * upon blocking, we clear status (using Thread.interrupted) + * before any call to park, so that park does not immediately + * return due to status being set via some other unrelated call to + * interrupt in user code. + * + * Signalling. We create or wake up workers only when there + * appears to be at least one task they might be able to find and + * execute. However, many other threads may notice the same task + * and each signal to wake up a thread that might take it. So in + * general, pools will be over-signalled. When a submission is + * added or another worker adds a task to a queue that has fewer + * than two tasks, they signal waiting workers (or trigger + * creation of new ones if fewer than the given parallelism level + * -- signalWork), and may leave a hint to the unparked worker to + * help signal others upon wakeup). These primary signals are + * buttressed by others (see method helpSignal) whenever other + * threads scan for work or do not have a task to process. On + * most platforms, signalling (unpark) overhead time is noticeably + * long, and the time between signalling a thread and it actually + * making progress can be very noticeably long, so it is worth + * offloading these delays from critical paths as much as + * possible. + * + * Trimming workers. To release resources after periods of lack of + * use, a worker starting to wait when the pool is quiescent will + * time out and terminate if the pool has remained quiescent for a + * given period -- a short period if there are more threads than + * parallelism, longer as the number of threads decreases. This + * will slowly propagate, eventually terminating all workers after + * periods of non-use. + * + * Shutdown and Termination. A call to shutdownNow atomically sets + * a plock bit and then (non-atomically) sets each worker's + * qlock status, cancels all unprocessed tasks, and wakes up + * all waiting workers. Detecting whether termination should + * commence after a non-abrupt shutdown() call requires more work + * and bookkeeping. We need consensus about quiescence (i.e., that + * there is no more work). The active count provides a primary + * indication but non-abrupt shutdown still requires a rechecking + * scan for any workers that are inactive but not queued. + * + * Joining Tasks + * ============= + * + * Any of several actions may be taken when one worker is waiting + * to join a task stolen (or always held) by another. Because we + * are multiplexing many tasks on to a pool of workers, we can't + * just let them block (as in Thread.join). We also cannot just + * reassign the joiner's run-time stack with another and replace + * it later, which would be a form of "continuation", that even if + * possible is not necessarily a good idea since we sometimes need + * both an unblocked task and its continuation to progress. + * Instead we combine two tactics: + * + * Helping: Arranging for the joiner to execute some task that it + * would be running if the steal had not occurred. + * + * Compensating: Unless there are already enough live threads, + * method tryCompensate() may create or re-activate a spare + * thread to compensate for blocked joiners until they unblock. + * + * A third form (implemented in tryRemoveAndExec) amounts to + * helping a hypothetical compensator: If we can readily tell that + * a possible action of a compensator is to steal and execute the + * task being joined, the joining thread can do so directly, + * without the need for a compensation thread (although at the + * expense of larger run-time stacks, but the tradeoff is + * typically worthwhile). + * + * The ManagedBlocker extension API can't use helping so relies + * only on compensation in method awaitBlocker. + * + * The algorithm in tryHelpStealer entails a form of "linear" + * helping: Each worker records (in field currentSteal) the most + * recent task it stole from some other worker. Plus, it records + * (in field currentJoin) the task it is currently actively + * joining. Method tryHelpStealer uses these markers to try to + * find a worker to help (i.e., steal back a task from and execute + * it) that could hasten completion of the actively joined task. + * In essence, the joiner executes a task that would be on its own + * local deque had the to-be-joined task not been stolen. This may + * be seen as a conservative variant of the approach in Wagner & + * Calder "Leapfrogging: a portable technique for implementing + * efficient futures" SIGPLAN Notices, 1993 + * (http://portal.acm.org/citation.cfm?id=155354). It differs in + * that: (1) We only maintain dependency links across workers upon + * steals, rather than use per-task bookkeeping. This sometimes + * requires a linear scan of workQueues array to locate stealers, + * but often doesn't because stealers leave hints (that may become + * stale/wrong) of where to locate them. It is only a hint + * because a worker might have had multiple steals and the hint + * records only one of them (usually the most current). Hinting + * isolates cost to when it is needed, rather than adding to + * per-task overhead. (2) It is "shallow", ignoring nesting and + * potentially cyclic mutual steals. (3) It is intentionally + * racy: field currentJoin is updated only while actively joining, + * which means that we miss links in the chain during long-lived + * tasks, GC stalls etc (which is OK since blocking in such cases + * is usually a good idea). (4) We bound the number of attempts + * to find work (see MAX_HELP) and fall back to suspending the + * worker and if necessary replacing it with another. + * + * Helping actions for CountedCompleters are much simpler: Method + * helpComplete can take and execute any task with the same root + * as the task being waited on. However, this still entails some + * traversal of completer chains, so is less efficient than using + * CountedCompleters without explicit joins. + * + * It is impossible to keep exactly the target parallelism number + * of threads running at any given time. Determining the + * existence of conservatively safe helping targets, the + * availability of already-created spares, and the apparent need + * to create new spares are all racy, so we rely on multiple + * retries of each. Compensation in the apparent absence of + * helping opportunities is challenging to control on JVMs, where + * GC and other activities can stall progress of tasks that in + * turn stall out many other dependent tasks, without us being + * able to determine whether they will ever require compensation. + * Even though work-stealing otherwise encounters little + * degradation in the presence of more threads than cores, + * aggressively adding new threads in such cases entails risk of + * unwanted positive feedback control loops in which more threads + * cause more dependent stalls (as well as delayed progress of + * unblocked threads to the point that we know they are available) + * leading to more situations requiring more threads, and so + * on. This aspect of control can be seen as an (analytically + * intractable) game with an opponent that may choose the worst + * (for us) active thread to stall at any time. We take several + * precautions to bound losses (and thus bound gains), mainly in + * methods tryCompensate and awaitJoin. + * + * Common Pool + * =========== + * + * The static common Pool always exists after static + * initialization. Since it (or any other created pool) need + * never be used, we minimize initial construction overhead and + * footprint to the setup of about a dozen fields, with no nested + * allocation. Most bootstrapping occurs within method + * fullExternalPush during the first submission to the pool. + * + * When external threads submit to the common pool, they can + * perform some subtask processing (see externalHelpJoin and + * related methods). We do not need to record whether these + * submissions are to the common pool -- if not, externalHelpJoin + * returns quickly (at the most helping to signal some common pool + * workers). These submitters would otherwise be blocked waiting + * for completion, so the extra effort (with liberally sprinkled + * task status checks) in inapplicable cases amounts to an odd + * form of limited spin-wait before blocking in ForkJoinTask.join. + * + * Style notes + * =========== + * + * There is a lot of representation-level coupling among classes + * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask. The + * fields of WorkQueue maintain data structures managed by + * ForkJoinPool, so are directly accessed. There is little point * trying to reduce this, since any associated future changes in * representations will need to be accompanied by algorithmic - * changes anyway. - * - * Style notes: There are lots of inline assignments (of form - * "while ((local = field) != 0)") which are usually the simplest - * way to ensure read orderings. Also several occurrences of the - * unusual "do {} while(!cas...)" which is the simplest way to - * force an update of a CAS'ed variable. There are also a few - * other coding oddities that help some methods perform reasonably - * even when interpreted (not compiled). - * - * The order of declarations in this file is: (1) statics (2) - * fields (along with constants used when unpacking some of them) - * (3) internal control methods (4) callbacks and other support - * for ForkJoinTask and ForkJoinWorkerThread classes, (5) exported - * methods (plus a few little helpers). + * changes anyway. Several methods intrinsically sprawl because + * they must accumulate sets of consistent reads of volatiles held + * in local variables. Methods signalWork() and scan() are the + * main bottlenecks, so are especially heavily + * micro-optimized/mangled. There are lots of inline assignments + * (of form "while ((local = field) != 0)") which are usually the + * simplest way to ensure the required read orderings (which are + * sometimes critical). This leads to a "C"-like style of listing + * declarations of these locals at the heads of methods or blocks. + * There are several occurrences of the unusual "do {} while + * (!cas...)" which is the simplest way to force an update of a + * CAS'ed variable. There are also other coding oddities (including + * several unnecessary-looking hoisted null checks) that help + * some methods perform reasonably even when interpreted (not + * compiled). + * + * The order of declarations in this file is: + * (1) Static utility functions + * (2) Nested (static) classes + * (3) Static fields + * (4) Fields, along with constants used when unpacking some of them + * (5) Internal control methods + * (6) Callbacks and other support for ForkJoinTask methods + * (7) Exported methods + * (8) Static block initializing statics in minimally dependent order */ + // Static utilities + + /** + * If there is a security manager, makes sure caller has + * permission to modify threads. + */ + private static void checkPermission() { + SecurityManager security = System.getSecurityManager(); + if (security != null) + security.checkPermission(modifyThreadPermission); + } + + // Nested classes + /** * Factory for creating new {@link ForkJoinWorkerThread}s. * A {@code ForkJoinWorkerThreadFactory} must be defined and used @@ -346,868 +527,1938 @@ public class ForkJoinPool extends Abstra * Default ForkJoinWorkerThreadFactory implementation; creates a * new ForkJoinWorkerThread. */ - static class DefaultForkJoinWorkerThreadFactory + static final class DefaultForkJoinWorkerThreadFactory implements ForkJoinWorkerThreadFactory { - public ForkJoinWorkerThread newThread(ForkJoinPool pool) { + public final ForkJoinWorkerThread newThread(ForkJoinPool pool) { return new ForkJoinWorkerThread(pool); } } /** - * Creates a new ForkJoinWorkerThread. This factory is used unless - * overridden in ForkJoinPool constructors. + * Per-thread records for threads that submit to pools. Currently + * holds only pseudo-random seed / index that is used to choose + * submission queues in method externalPush. In the future, this may + * also incorporate a means to implement different task rejection + * and resubmission policies. + * + * Seeds for submitters and workers/workQueues work in basically + * the same way but are initialized and updated using slightly + * different mechanics. Both are initialized using the same + * approach as in class ThreadLocal, where successive values are + * unlikely to collide with previous values. Seeds are then + * randomly modified upon collisions using xorshifts, which + * requires a non-zero seed. + */ + static final class Submitter { + int seed; + Submitter(int s) { seed = s; } + } + + /** + * Class for artificial tasks that are used to replace the target + * of local joins if they are removed from an interior queue slot + * in WorkQueue.tryRemoveAndExec. We don't need the proxy to + * actually do anything beyond having a unique identity. + */ + static final class EmptyTask extends ForkJoinTask { + private static final long serialVersionUID = -7721805057305804111L; + EmptyTask() { status = ForkJoinTask.NORMAL; } // force done + public final Void getRawResult() { return null; } + public final void setRawResult(Void x) {} + public final boolean exec() { return true; } + } + + /** + * Queues supporting work-stealing as well as external task + * submission. See above for main rationale and algorithms. + * Implementation relies heavily on "Unsafe" intrinsics + * and selective use of "volatile": + * + * Field "base" is the index (mod array.length) of the least valid + * queue slot, which is always the next position to steal (poll) + * from if nonempty. Reads and writes require volatile orderings + * but not CAS, because updates are only performed after slot + * CASes. + * + * Field "top" is the index (mod array.length) of the next queue + * slot to push to or pop from. It is written only by owner thread + * for push, or under lock for external/shared push, and accessed + * by other threads only after reading (volatile) base. Both top + * and base are allowed to wrap around on overflow, but (top - + * base) (or more commonly -(base - top) to force volatile read of + * base before top) still estimates size. The lock ("qlock") is + * forced to -1 on termination, causing all further lock attempts + * to fail. (Note: we don't need CAS for termination state because + * upon pool shutdown, all shared-queues will stop being used + * anyway.) Nearly all lock bodies are set up so that exceptions + * within lock bodies are "impossible" (modulo JVM errors that + * would cause failure anyway.) + * + * The array slots are read and written using the emulation of + * volatiles/atomics provided by Unsafe. Insertions must in + * general use putOrderedObject as a form of releasing store to + * ensure that all writes to the task object are ordered before + * its publication in the queue. All removals entail a CAS to + * null. The array is always a power of two. To ensure safety of + * Unsafe array operations, all accesses perform explicit null + * checks and implicit bounds checks via power-of-two masking. + * + * In addition to basic queuing support, this class contains + * fields described elsewhere to control execution. It turns out + * to work better memory-layout-wise to include them in this class + * rather than a separate class. + * + * Performance on most platforms is very sensitive to placement of + * instances of both WorkQueues and their arrays -- we absolutely + * do not want multiple WorkQueue instances or multiple queue + * arrays sharing cache lines. (It would be best for queue objects + * and their arrays to share, but there is nothing available to + * help arrange that). Unfortunately, because they are recorded + * in a common array, WorkQueue instances are often moved to be + * adjacent by garbage collectors. To reduce impact, we use field + * padding that works OK on common platforms; this effectively + * trades off slightly slower average field access for the sake of + * avoiding really bad worst-case access. (Until better JVM + * support is in place, this padding is dependent on transient + * properties of JVM field layout rules.) We also take care in + * allocating, sizing and resizing the array. Non-shared queue + * arrays are initialized by workers before use. Others are + * allocated on first use. */ - public static final ForkJoinWorkerThreadFactory - defaultForkJoinWorkerThreadFactory = - new DefaultForkJoinWorkerThreadFactory(); + static final class WorkQueue { + /** + * Capacity of work-stealing queue array upon initialization. + * Must be a power of two; at least 4, but should be larger to + * reduce or eliminate cacheline sharing among queues. + * Currently, it is much larger, as a partial workaround for + * the fact that JVMs often place arrays in locations that + * share GC bookkeeping (especially cardmarks) such that + * per-write accesses encounter serious memory contention. + */ + static final int INITIAL_QUEUE_CAPACITY = 1 << 13; - /** - * Permission required for callers of methods that may start or - * kill threads. - */ - private static final RuntimePermission modifyThreadPermission = - new RuntimePermission("modifyThread"); + /** + * Maximum size for queue arrays. Must be a power of two less + * than or equal to 1 << (31 - width of array entry) to ensure + * lack of wraparound of index calculations, but defined to a + * value a bit less than this to help users trap runaway + * programs before saturating systems. + */ + static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M - /** - * If there is a security manager, makes sure caller has - * permission to modify threads. - */ - private static void checkPermission() { - SecurityManager security = System.getSecurityManager(); - if (security != null) - security.checkPermission(modifyThreadPermission); - } + // Heuristic padding to ameliorate unfortunate memory placements + volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06; - /** - * Generator for assigning sequence numbers as pool names. - */ - private static final AtomicInteger poolNumberGenerator = - new AtomicInteger(); + int seed; // for random scanning; initialize nonzero + volatile int eventCount; // encoded inactivation count; < 0 if inactive + int nextWait; // encoded record of next event waiter + int hint; // steal or signal hint (index) + int poolIndex; // index of this queue in pool (or 0) + final int mode; // 0: lifo, > 0: fifo, < 0: shared + int nsteals; // number of steals + volatile int qlock; // 1: locked, -1: terminate; else 0 + volatile int base; // index of next slot for poll + int top; // index of next slot for push + ForkJoinTask[] array; // the elements (initially unallocated) + final ForkJoinPool pool; // the containing pool (may be null) + final ForkJoinWorkerThread owner; // owning thread or null if shared + volatile Thread parker; // == owner during call to park; else null + volatile ForkJoinTask currentJoin; // task being joined in awaitJoin + ForkJoinTask currentSteal; // current non-local task being executed + + volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17; + volatile Object pad18, pad19, pad1a, pad1b, pad1c, pad1d; + + WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode, + int seed) { + this.pool = pool; + this.owner = owner; + this.mode = mode; + this.seed = seed; + // Place indices in the center of array (that is not yet allocated) + base = top = INITIAL_QUEUE_CAPACITY >>> 1; + } - /** - * Absolute bound for parallelism level. Twice this number must - * fit into a 16bit field to enable word-packing for some counts. - */ - private static final int MAX_THREADS = 0x7fff; + /** + * Returns the approximate number of tasks in the queue. + */ + final int queueSize() { + int n = base - top; // non-owner callers must read base first + return (n >= 0) ? 0 : -n; // ignore transient negative + } - /** - * Array holding all worker threads in the pool. Array size must - * be a power of two. Updates and replacements are protected by - * workerLock, but the array is always kept in a consistent enough - * state to be randomly accessed without locking by workers - * performing work-stealing, as well as other traversal-based - * methods in this class. All readers must tolerate that some - * array slots may be null. - */ - volatile ForkJoinWorkerThread[] workers; + /** + * Provides a more accurate estimate of whether this queue has + * any tasks than does queueSize, by checking whether a + * near-empty queue has at least one unclaimed task. + */ + final boolean isEmpty() { + ForkJoinTask[] a; int m, s; + int n = base - (s = top); + return (n >= 0 || + (n == -1 && + ((a = array) == null || + (m = a.length - 1) < 0 || + U.getObject + (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null))); + } - /** - * Queue for external submissions. - */ - private final LinkedTransferQueue> submissionQueue; + /** + * Pushes a task. Call only by owner in unshared queues. (The + * shared-queue version is embedded in method externalPush.) + * + * @param task the task. Caller must ensure non-null. + * @throws RejectedExecutionException if array cannot be resized + */ + final void push(ForkJoinTask task) { + ForkJoinTask[] a; ForkJoinPool p; + int s = top, m, n; + if ((a = array) != null) { // ignore if queue removed + int j = (((m = a.length - 1) & s) << ASHIFT) + ABASE; + U.putOrderedObject(a, j, task); + if ((n = (top = s + 1) - base) <= 2) { + if ((p = pool) != null) + p.signalWork(this); + } + else if (n >= m) + growArray(); + } + } - /** - * Lock protecting updates to workers array. - */ - private final ReentrantLock workerLock; + /** + * Initializes or doubles the capacity of array. Call either + * by owner or with lock held -- it is OK for base, but not + * top, to move while resizings are in progress. + */ + final ForkJoinTask[] growArray() { + ForkJoinTask[] oldA = array; + int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY; + if (size > MAXIMUM_QUEUE_CAPACITY) + throw new RejectedExecutionException("Queue capacity exceeded"); + int oldMask, t, b; + ForkJoinTask[] a = array = new ForkJoinTask[size]; + if (oldA != null && (oldMask = oldA.length - 1) >= 0 && + (t = top) - (b = base) > 0) { + int mask = size - 1; + do { + ForkJoinTask x; + int oldj = ((b & oldMask) << ASHIFT) + ABASE; + int j = ((b & mask) << ASHIFT) + ABASE; + x = (ForkJoinTask)U.getObjectVolatile(oldA, oldj); + if (x != null && + U.compareAndSwapObject(oldA, oldj, x, null)) + U.putObjectVolatile(a, j, x); + } while (++b != t); + } + return a; + } - /** - * Latch released upon termination. - */ - private final CountDownLatch terminationLatch; + /** + * Takes next task, if one exists, in LIFO order. Call only + * by owner in unshared queues. + */ + final ForkJoinTask pop() { + ForkJoinTask[] a; ForkJoinTask t; int m; + if ((a = array) != null && (m = a.length - 1) >= 0) { + for (int s; (s = top - 1) - base >= 0;) { + long j = ((m & s) << ASHIFT) + ABASE; + if ((t = (ForkJoinTask)U.getObject(a, j)) == null) + break; + if (U.compareAndSwapObject(a, j, t, null)) { + top = s; + return t; + } + } + } + return null; + } - /** - * Creation factory for worker threads. - */ - private final ForkJoinWorkerThreadFactory factory; + /** + * Takes a task in FIFO order if b is base of queue and a task + * can be claimed without contention. Specialized versions + * appear in ForkJoinPool methods scan and tryHelpStealer. + */ + final ForkJoinTask pollAt(int b) { + ForkJoinTask t; ForkJoinTask[] a; + if ((a = array) != null) { + int j = (((a.length - 1) & b) << ASHIFT) + ABASE; + if ((t = (ForkJoinTask)U.getObjectVolatile(a, j)) != null && + base == b && + U.compareAndSwapObject(a, j, t, null)) { + base = b + 1; + return t; + } + } + return null; + } - /** - * Sum of per-thread steal counts, updated only when threads are - * idle or terminating. - */ - private volatile long stealCount; + /** + * Takes next task, if one exists, in FIFO order. + */ + final ForkJoinTask poll() { + ForkJoinTask[] a; int b; ForkJoinTask t; + while ((b = base) - top < 0 && (a = array) != null) { + int j = (((a.length - 1) & b) << ASHIFT) + ABASE; + t = (ForkJoinTask)U.getObjectVolatile(a, j); + if (t != null) { + if (base == b && + U.compareAndSwapObject(a, j, t, null)) { + base = b + 1; + return t; + } + } + else if (base == b) { + if (b + 1 == top) + break; + Thread.yield(); // wait for lagging update (very rare) + } + } + return null; + } - /** - * Encoded record of top of treiber stack of threads waiting for - * events. The top 32 bits contain the count being waited for. The - * bottom word contains one plus the pool index of waiting worker - * thread. - */ - private volatile long eventWaiters; + /** + * Takes next task, if one exists, in order specified by mode. + */ + final ForkJoinTask nextLocalTask() { + return mode == 0 ? pop() : poll(); + } - private static final int EVENT_COUNT_SHIFT = 32; - private static final long WAITER_INDEX_MASK = (1L << EVENT_COUNT_SHIFT)-1L; + /** + * Returns next task, if one exists, in order specified by mode. + */ + final ForkJoinTask peek() { + ForkJoinTask[] a = array; int m; + if (a == null || (m = a.length - 1) < 0) + return null; + int i = mode == 0 ? top - 1 : base; + int j = ((i & m) << ASHIFT) + ABASE; + return (ForkJoinTask)U.getObjectVolatile(a, j); + } - /** - * A counter for events that may wake up worker threads: - * - Submission of a new task to the pool - * - A worker pushing a task on an empty queue - * - termination and reconfiguration - */ - private volatile int eventCount; + /** + * Pops the given task only if it is at the current top. + * (A shared version is available only via FJP.tryExternalUnpush) + */ + final boolean tryUnpush(ForkJoinTask t) { + ForkJoinTask[] a; int s; + if ((a = array) != null && (s = top) != base && + U.compareAndSwapObject + (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) { + top = s; + return true; + } + return false; + } - /** - * Lifecycle control. The low word contains the number of workers - * that are (probably) executing tasks. This value is atomically - * incremented before a worker gets a task to run, and decremented - * when worker has no tasks and cannot find any. Bits 16-18 - * contain runLevel value. When all are zero, the pool is - * running. Level transitions are monotonic (running -> shutdown - * -> terminating -> terminated) so each transition adds a bit. - * These are bundled together to ensure consistent read for - * termination checks (i.e., that runLevel is at least SHUTDOWN - * and active threads is zero). - */ - private volatile int runState; + /** + * Removes and cancels all known tasks, ignoring any exceptions. + */ + final void cancelAll() { + ForkJoinTask.cancelIgnoringExceptions(currentJoin); + ForkJoinTask.cancelIgnoringExceptions(currentSteal); + for (ForkJoinTask t; (t = poll()) != null; ) + ForkJoinTask.cancelIgnoringExceptions(t); + } - // Note: The order among run level values matters. - private static final int RUNLEVEL_SHIFT = 16; - private static final int SHUTDOWN = 1 << RUNLEVEL_SHIFT; - private static final int TERMINATING = 1 << (RUNLEVEL_SHIFT + 1); - private static final int TERMINATED = 1 << (RUNLEVEL_SHIFT + 2); - private static final int ACTIVE_COUNT_MASK = (1 << RUNLEVEL_SHIFT) - 1; - private static final int ONE_ACTIVE = 1; // active update delta + /** + * Computes next value for random probes. Scans don't require + * a very high quality generator, but also not a crummy one. + * Marsaglia xor-shift is cheap and works well enough. Note: + * This is manually inlined in its usages in ForkJoinPool to + * avoid writes inside busy scan loops. + */ + final int nextSeed() { + int r = seed; + r ^= r << 13; + r ^= r >>> 17; + return seed = r ^= r << 5; + } - /** - * Holds number of total (i.e., created and not yet terminated) - * and running (i.e., not blocked on joins or other managed sync) - * threads, packed together to ensure consistent snapshot when - * making decisions about creating and suspending spare - * threads. Updated only by CAS. Note that adding a new worker - * requires incrementing both counts, since workers start off in - * running state. This field is also used for memory-fencing - * configuration parameters. - */ - private volatile int workerCounts; + // Specialized execution methods - private static final int TOTAL_COUNT_SHIFT = 16; - private static final int RUNNING_COUNT_MASK = (1 << TOTAL_COUNT_SHIFT) - 1; - private static final int ONE_RUNNING = 1; - private static final int ONE_TOTAL = 1 << TOTAL_COUNT_SHIFT; + /** + * Pops and runs tasks until empty. + */ + private void popAndExecAll() { + // A bit faster than repeated pop calls + ForkJoinTask[] a; int m, s; long j; ForkJoinTask t; + while ((a = array) != null && (m = a.length - 1) >= 0 && + (s = top - 1) - base >= 0 && + (t = ((ForkJoinTask) + U.getObject(a, j = ((m & s) << ASHIFT) + ABASE))) + != null) { + if (U.compareAndSwapObject(a, j, t, null)) { + top = s; + t.doExec(); + } + } + } - /* - * Fields parallelism. maxPoolSize, and maintainsParallelism are - * non-volatile, but external reads/writes use workerCount fences - * to ensure visability. - */ + /** + * Polls and runs tasks until empty. + */ + private void pollAndExecAll() { + for (ForkJoinTask t; (t = poll()) != null;) + t.doExec(); + } + + /** + * If present, removes from queue and executes the given task, + * or any other cancelled task. Returns (true) on any CAS + * or consistency check failure so caller can retry. + * + * @return false if no progress can be made, else true + */ + final boolean tryRemoveAndExec(ForkJoinTask task) { + boolean stat = true, removed = false, empty = true; + ForkJoinTask[] a; int m, s, b, n; + if ((a = array) != null && (m = a.length - 1) >= 0 && + (n = (s = top) - (b = base)) > 0) { + for (ForkJoinTask t;;) { // traverse from s to b + int j = ((--s & m) << ASHIFT) + ABASE; + t = (ForkJoinTask)U.getObjectVolatile(a, j); + if (t == null) // inconsistent length + break; + else if (t == task) { + if (s + 1 == top) { // pop + if (!U.compareAndSwapObject(a, j, task, null)) + break; + top = s; + removed = true; + } + else if (base == b) // replace with proxy + removed = U.compareAndSwapObject(a, j, task, + new EmptyTask()); + break; + } + else if (t.status >= 0) + empty = false; + else if (s + 1 == top) { // pop and throw away + if (U.compareAndSwapObject(a, j, t, null)) + top = s; + break; + } + if (--n == 0) { + if (!empty && base == b) + stat = false; + break; + } + } + } + if (removed) + task.doExec(); + return stat; + } + + /** + * Polls for and executes the given task or any other task in + * its CountedCompleter computation. + */ + final boolean pollAndExecCC(ForkJoinTask root) { + ForkJoinTask[] a; int b; Object o; + outer: while ((b = base) - top < 0 && (a = array) != null) { + long j = (((a.length - 1) & b) << ASHIFT) + ABASE; + if ((o = U.getObject(a, j)) == null || + !(o instanceof CountedCompleter)) + break; + for (CountedCompleter t = (CountedCompleter)o, r = t;;) { + if (r == root) { + if (base == b && + U.compareAndSwapObject(a, j, t, null)) { + base = b + 1; + t.doExec(); + return true; + } + else + break; // restart + } + if ((r = r.completer) == null) + break outer; // not part of root computation + } + } + return false; + } + + /** + * Executes a top-level task and any local tasks remaining + * after execution. + */ + final void runTask(ForkJoinTask t) { + if (t != null) { + (currentSteal = t).doExec(); + currentSteal = null; + ++nsteals; + if (base - top < 0) { // process remaining local tasks + if (mode == 0) + popAndExecAll(); + else + pollAndExecAll(); + } + } + } + + /** + * Executes a non-top-level (stolen) task. + */ + final void runSubtask(ForkJoinTask t) { + if (t != null) { + ForkJoinTask ps = currentSteal; + (currentSteal = t).doExec(); + currentSteal = ps; + } + } + + /** + * Returns true if owned and not known to be blocked. + */ + final boolean isApparentlyUnblocked() { + Thread wt; Thread.State s; + return (eventCount >= 0 && + (wt = owner) != null && + (s = wt.getState()) != Thread.State.BLOCKED && + s != Thread.State.WAITING && + s != Thread.State.TIMED_WAITING); + } + + // Unsafe mechanics + private static final sun.misc.Unsafe U; + private static final long QLOCK; + private static final int ABASE; + private static final int ASHIFT; + static { + try { + U = getUnsafe(); + Class k = WorkQueue.class; + Class ak = ForkJoinTask[].class; + QLOCK = U.objectFieldOffset + (k.getDeclaredField("qlock")); + ABASE = U.arrayBaseOffset(ak); + int scale = U.arrayIndexScale(ak); + if ((scale & (scale - 1)) != 0) + throw new Error("data type scale not a power of two"); + ASHIFT = 31 - Integer.numberOfLeadingZeros(scale); + } catch (Exception e) { + throw new Error(e); + } + } + } + + // static fields (initialized in static initializer below) /** - * The target parallelism level. + * Creates a new ForkJoinWorkerThread. This factory is used unless + * overridden in ForkJoinPool constructors. */ - private int parallelism; + public static final ForkJoinWorkerThreadFactory + defaultForkJoinWorkerThreadFactory; /** - * The maximum allowed pool size. + * Per-thread submission bookkeeping. Shared across all pools + * to reduce ThreadLocal pollution and because random motion + * to avoid contention in one pool is likely to hold for others. + * Lazily initialized on first submission (but null-checked + * in other contexts to avoid unnecessary initialization). */ - private int maxPoolSize; + static final ThreadLocal submitters; /** - * True if use local fifo, not default lifo, for local polling - * Replicated by ForkJoinWorkerThreads + * Permission required for callers of methods that may start or + * kill threads. */ - private volatile boolean locallyFifo; + private static final RuntimePermission modifyThreadPermission; /** - * Controls whether to add spares to maintain parallelism + * Common (static) pool. Non-null for public use unless a static + * construction exception, but internal usages null-check on use + * to paranoically avoid potential initialization circularities + * as well as to simplify generated code. */ - private boolean maintainsParallelism; + static final ForkJoinPool common; /** - * The uncaught exception handler used when any worker - * abruptly terminates + * Common pool parallelism. Must equal common.parallelism. */ - private volatile Thread.UncaughtExceptionHandler ueh; + static final int commonParallelism; /** - * Pool number, just for assigning useful names to worker threads + * Sequence number for creating workerNamePrefix. */ - private final int poolNumber; - - // utilities for updating fields + private static int poolNumberSequence; /** - * Adds delta to running count. Used mainly by ForkJoinTask. + * Returns the next sequence number. We don't expect this to + * ever contend, so use simple builtin sync. */ - final void updateRunningCount(int delta) { - int wc; - do {} while (!UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc = workerCounts, - wc + delta)); + private static final synchronized int nextPoolId() { + return ++poolNumberSequence; } + // static constants + /** - * Decrements running count unless already zero + * Initial timeout value (in nanoseconds) for the thread + * triggering quiescence to park waiting for new work. On timeout, + * the thread will instead try to shrink the number of + * workers. The value should be large enough to avoid overly + * aggressive shrinkage during most transient stalls (long GCs + * etc). */ - final boolean tryDecrementRunningCount() { - int wc = workerCounts; - if ((wc & RUNNING_COUNT_MASK) == 0) - return false; - return UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - ONE_RUNNING); - } + private static final long IDLE_TIMEOUT = 2000L * 1000L * 1000L; // 2sec /** - * Write fence for user modifications of pool parameters - * (parallelism. etc). Note that it doesn't matter if CAS fails. + * Timeout value when there are more threads than parallelism level */ - private void workerCountWriteFence() { - int wc; - UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc = workerCounts, wc); - } + private static final long FAST_IDLE_TIMEOUT = 200L * 1000L * 1000L; /** - * Read fence for external reads of pool parameters - * (parallelism. maxPoolSize, etc). + * Tolerance for idle timeouts, to cope with timer undershoots */ - private void workerCountReadFence() { - int ignore = workerCounts; - } + private static final long TIMEOUT_SLOP = 2000000L; /** - * Tries incrementing active count; fails on contention. - * Called by workers before executing tasks. - * - * @return true on success + * The maximum stolen->joining link depth allowed in method + * tryHelpStealer. Must be a power of two. Depths for legitimate + * chains are unbounded, but we use a fixed constant to avoid + * (otherwise unchecked) cycles and to bound staleness of + * traversal parameters at the expense of sometimes blocking when + * we could be helping. */ - final boolean tryIncrementActiveCount() { - int c; - return UNSAFE.compareAndSwapInt(this, runStateOffset, - c = runState, c + ONE_ACTIVE); - } + private static final int MAX_HELP = 64; /** - * Tries decrementing active count; fails on contention. - * Called when workers cannot find tasks to run. + * Increment for seed generators. See class ThreadLocal for + * explanation. */ - final boolean tryDecrementActiveCount() { - int c; - return UNSAFE.compareAndSwapInt(this, runStateOffset, - c = runState, c - ONE_ACTIVE); - } + private static final int SEED_INCREMENT = 0x61c88647; - /** - * Advances to at least the given level. Returns true if not - * already in at least the given level. + /* + * Bits and masks for control variables + * + * Field ctl is a long packed with: + * AC: Number of active running workers minus target parallelism (16 bits) + * TC: Number of total workers minus target parallelism (16 bits) + * ST: true if pool is terminating (1 bit) + * EC: the wait count of top waiting thread (15 bits) + * ID: poolIndex of top of Treiber stack of waiters (16 bits) + * + * When convenient, we can extract the upper 32 bits of counts and + * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e = + * (int)ctl. The ec field is never accessed alone, but always + * together with id and st. The offsets of counts by the target + * parallelism and the positionings of fields makes it possible to + * perform the most common checks via sign tests of fields: When + * ac is negative, there are not enough active workers, when tc is + * negative, there are not enough total workers, and when e is + * negative, the pool is terminating. To deal with these possibly + * negative fields, we use casts in and out of "short" and/or + * signed shifts to maintain signedness. + * + * When a thread is queued (inactivated), its eventCount field is + * set negative, which is the only way to tell if a worker is + * prevented from executing tasks, even though it must continue to + * scan for them to avoid queuing races. Note however that + * eventCount updates lag releases so usage requires care. + * + * Field plock is an int packed with: + * SHUTDOWN: true if shutdown is enabled (1 bit) + * SEQ: a sequence lock, with PL_LOCK bit set if locked (30 bits) + * SIGNAL: set when threads may be waiting on the lock (1 bit) + * + * The sequence number enables simple consistency checks: + * Staleness of read-only operations on the workQueues array can + * be checked by comparing plock before vs after the reads. + */ + + // bit positions/shifts for fields + private static final int AC_SHIFT = 48; + private static final int TC_SHIFT = 32; + private static final int ST_SHIFT = 31; + private static final int EC_SHIFT = 16; + + // bounds + private static final int SMASK = 0xffff; // short bits + private static final int MAX_CAP = 0x7fff; // max #workers - 1 + private static final int EVENMASK = 0xfffe; // even short bits + private static final int SQMASK = 0x007e; // max 64 (even) slots + private static final int SHORT_SIGN = 1 << 15; + private static final int INT_SIGN = 1 << 31; + + // masks + private static final long STOP_BIT = 0x0001L << ST_SHIFT; + private static final long AC_MASK = ((long)SMASK) << AC_SHIFT; + private static final long TC_MASK = ((long)SMASK) << TC_SHIFT; + + // units for incrementing and decrementing + private static final long TC_UNIT = 1L << TC_SHIFT; + private static final long AC_UNIT = 1L << AC_SHIFT; + + // masks and units for dealing with u = (int)(ctl >>> 32) + private static final int UAC_SHIFT = AC_SHIFT - 32; + private static final int UTC_SHIFT = TC_SHIFT - 32; + private static final int UAC_MASK = SMASK << UAC_SHIFT; + private static final int UTC_MASK = SMASK << UTC_SHIFT; + private static final int UAC_UNIT = 1 << UAC_SHIFT; + private static final int UTC_UNIT = 1 << UTC_SHIFT; + + // masks and units for dealing with e = (int)ctl + private static final int E_MASK = 0x7fffffff; // no STOP_BIT + private static final int E_SEQ = 1 << EC_SHIFT; + + // plock bits + private static final int SHUTDOWN = 1 << 31; + private static final int PL_LOCK = 2; + private static final int PL_SIGNAL = 1; + private static final int PL_SPINS = 1 << 8; + + // access mode for WorkQueue + static final int LIFO_QUEUE = 0; + static final int FIFO_QUEUE = 1; + static final int SHARED_QUEUE = -1; + + // bounds for #steps in scan loop -- must be power 2 minus 1 + private static final int MIN_SCAN = 0x1ff; // cover estimation slop + private static final int MAX_SCAN = 0x1ffff; // 4 * max workers + + // Instance fields + + /* + * Field layout of this class tends to matter more than one would + * like. Runtime layout order is only loosely related to + * declaration order and may differ across JVMs, but the following + * empirically works OK on current JVMs. + */ + + // Heuristic padding to ameliorate unfortunate memory placements + volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06; + + volatile long stealCount; // collects worker counts + volatile long ctl; // main pool control + volatile int plock; // shutdown status and seqLock + volatile int indexSeed; // worker/submitter index seed + final int config; // mode and parallelism level + WorkQueue[] workQueues; // main registry + final ForkJoinWorkerThreadFactory factory; + final Thread.UncaughtExceptionHandler ueh; // per-worker UEH + final String workerNamePrefix; // to create worker name string + + volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17; + volatile Object pad18, pad19, pad1a, pad1b; + + /** + * Acquires the plock lock to protect worker array and related + * updates. This method is called only if an initial CAS on plock + * fails. This acts as a spinlock for normal cases, but falls back + * to builtin monitor to block when (rarely) needed. This would be + * a terrible idea for a highly contended lock, but works fine as + * a more conservative alternative to a pure spinlock. */ - private boolean advanceRunLevel(int level) { + private int acquirePlock() { + int spins = PL_SPINS, r = 0, ps, nps; for (;;) { - int s = runState; - if ((s & level) != 0) - return false; - if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, s | level)) - return true; + if (((ps = plock) & PL_LOCK) == 0 && + U.compareAndSwapInt(this, PLOCK, ps, nps = ps + PL_LOCK)) + return nps; + else if (r == 0) { // randomize spins if possible + Thread t = Thread.currentThread(); WorkQueue w; Submitter z; + if ((t instanceof ForkJoinWorkerThread) && + (w = ((ForkJoinWorkerThread)t).workQueue) != null) + r = w.seed; + else if ((z = submitters.get()) != null) + r = z.seed; + else + r = 1; + } + else if (spins >= 0) { + r ^= r << 1; r ^= r >>> 3; r ^= r << 10; // xorshift + if (r >= 0) + --spins; + } + else if (U.compareAndSwapInt(this, PLOCK, ps, ps | PL_SIGNAL)) { + synchronized (this) { + if ((plock & PL_SIGNAL) != 0) { + try { + wait(); + } catch (InterruptedException ie) { + try { + Thread.currentThread().interrupt(); + } catch (SecurityException ignore) { + } + } + } + else + notifyAll(); + } + } } } - // workers array maintenance - /** - * Records and returns a workers array index for new worker. + * Unlocks and signals any thread waiting for plock. Called only + * when CAS of seq value for unlock fails. */ - private int recordWorker(ForkJoinWorkerThread w) { - // Try using slot totalCount-1. If not available, scan and/or resize - int k = (workerCounts >>> TOTAL_COUNT_SHIFT) - 1; - final ReentrantLock lock = this.workerLock; - lock.lock(); - try { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - if (k < 0 || k >= nws || ws[k] != null) { - for (k = 0; k < nws && ws[k] != null; ++k) - ; - if (k == nws) - ws = Arrays.copyOf(ws, nws << 1); - } - ws[k] = w; - workers = ws; // volatile array write ensures slot visibility - } finally { - lock.unlock(); - } - return k; + private void releasePlock(int ps) { + plock = ps; + synchronized (this) { notifyAll(); } } /** - * Nulls out record of worker in workers array - */ - private void forgetWorker(ForkJoinWorkerThread w) { - int idx = w.poolIndex; - // Locking helps method recordWorker avoid unecessary expansion - final ReentrantLock lock = this.workerLock; - lock.lock(); - try { - ForkJoinWorkerThread[] ws = workers; - if (idx >= 0 && idx < ws.length && ws[idx] == w) // verify - ws[idx] = null; - } finally { - lock.unlock(); + * Tries to create and start one worker if fewer than target + * parallelism level exist. Adjusts counts etc on failure. + */ + private void tryAddWorker() { + long c; int u; + while ((u = (int)((c = ctl) >>> 32)) < 0 && + (u & SHORT_SIGN) != 0 && (int)c == 0) { + long nc = (long)(((u + UTC_UNIT) & UTC_MASK) | + ((u + UAC_UNIT) & UAC_MASK)) << 32; + if (U.compareAndSwapLong(this, CTL, c, nc)) { + ForkJoinWorkerThreadFactory fac; + Throwable ex = null; + ForkJoinWorkerThread wt = null; + try { + if ((fac = factory) != null && + (wt = fac.newThread(this)) != null) { + wt.start(); + break; + } + } catch (Throwable e) { + ex = e; + } + deregisterWorker(wt, ex); + break; + } } } - // adding and removing workers + // Registering and deregistering workers /** - * Tries to create and add new worker. Assumes that worker counts - * are already updated to accommodate the worker, so adjusts on - * failure. - * - * @return new worker or null if creation failed - */ - private ForkJoinWorkerThread addWorker() { - ForkJoinWorkerThread w = null; + * Callback from ForkJoinWorkerThread to establish and record its + * WorkQueue. To avoid scanning bias due to packing entries in + * front of the workQueues array, we treat the array as a simple + * power-of-two hash table using per-thread seed as hash, + * expanding as needed. + * + * @param wt the worker thread + * @return the worker's queue + */ + final WorkQueue registerWorker(ForkJoinWorkerThread wt) { + Thread.UncaughtExceptionHandler handler; WorkQueue[] ws; int s, ps; + wt.setDaemon(true); + if ((handler = ueh) != null) + wt.setUncaughtExceptionHandler(handler); + do {} while (!U.compareAndSwapInt(this, INDEXSEED, s = indexSeed, + s += SEED_INCREMENT) || + s == 0); // skip 0 + WorkQueue w = new WorkQueue(this, wt, config >>> 16, s); + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); try { - w = factory.newThread(this); - } finally { // Adjust on either null or exceptional factory return - if (w == null) { - onWorkerCreationFailure(); - return null; + if ((ws = workQueues) != null) { // skip if shutting down + int n = ws.length, m = n - 1; + int r = (s << 1) | 1; // use odd-numbered indices + if (ws[r &= m] != null) { // collision + int probes = 0; // step by approx half size + int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2; + while (ws[r = (r + step) & m] != null) { + if (++probes >= n) { + workQueues = ws = Arrays.copyOf(ws, n <<= 1); + m = n - 1; + probes = 0; + } + } + } + w.eventCount = w.poolIndex = r; // volatile write orders + ws[r] = w; } + } finally { + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); } - w.start(recordWorker(w), locallyFifo, ueh); + wt.setName(workerNamePrefix.concat(Integer.toString(w.poolIndex))); return w; } /** - * Adjusts counts upon failure to create worker - */ - private void onWorkerCreationFailure() { - for (;;) { - int wc = workerCounts; - if ((wc >>> TOTAL_COUNT_SHIFT) > 0 && - UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - (ONE_RUNNING|ONE_TOTAL))) - break; + * Final callback from terminating worker, as well as upon failure + * to construct or start a worker. Removes record of worker from + * array, and adjusts counts. If pool is shutting down, tries to + * complete termination. + * + * @param wt the worker thread or null if construction failed + * @param ex the exception causing failure, or null if none + */ + final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) { + WorkQueue w = null; + if (wt != null && (w = wt.workQueue) != null) { + int ps; + w.qlock = -1; // ensure set + long ns = w.nsteals, sc; // collect steal count + do {} while (!U.compareAndSwapLong(this, STEALCOUNT, + sc = stealCount, sc + ns)); + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); + try { + int idx = w.poolIndex; + WorkQueue[] ws = workQueues; + if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w) + ws[idx] = null; + } finally { + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } } - tryTerminate(false); // in case of failure during shutdown - } - /** - * Create enough total workers to establish target parallelism, - * giving up if terminating or addWorker fails - */ - private void ensureEnoughTotalWorkers() { - int wc; - while (((wc = workerCounts) >>> TOTAL_COUNT_SHIFT) < parallelism && - runState < TERMINATING) { - if ((UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc + (ONE_RUNNING|ONE_TOTAL)) && - addWorker() == null)) - break; + long c; // adjust ctl counts + do {} while (!U.compareAndSwapLong + (this, CTL, c = ctl, (((c - AC_UNIT) & AC_MASK) | + ((c - TC_UNIT) & TC_MASK) | + (c & ~(AC_MASK|TC_MASK))))); + + if (!tryTerminate(false, false) && w != null && w.array != null) { + w.cancelAll(); // cancel remaining tasks + WorkQueue[] ws; WorkQueue v; Thread p; int u, i, e; + while ((u = (int)((c = ctl) >>> 32)) < 0 && (e = (int)c) >= 0) { + if (e > 0) { // activate or create replacement + if ((ws = workQueues) == null || + (i = e & SMASK) >= ws.length || + (v = ws[i]) == null) + break; + long nc = (((long)(v.nextWait & E_MASK)) | + ((long)(u + UAC_UNIT) << 32)); + if (v.eventCount != (e | INT_SIGN)) + break; + if (U.compareAndSwapLong(this, CTL, c, nc)) { + v.eventCount = (e + E_SEQ) & E_MASK; + if ((p = v.parker) != null) + U.unpark(p); + break; + } + } + else { + if ((short)u < 0) + tryAddWorker(); + break; + } + } } + if (ex == null) // help clean refs on way out + ForkJoinTask.helpExpungeStaleExceptions(); + else // rethrow + ForkJoinTask.rethrow(ex); + } + + // Submissions + + /** + * Unless shutting down, adds the given task to a submission queue + * at submitter's current queue index (modulo submission + * range). Only the most common path is directly handled in this + * method. All others are relayed to fullExternalPush. + * + * @param task the task. Caller must ensure non-null. + */ + final void externalPush(ForkJoinTask task) { + WorkQueue[] ws; WorkQueue q; Submitter z; int m; ForkJoinTask[] a; + if ((z = submitters.get()) != null && plock > 0 && + (ws = workQueues) != null && (m = (ws.length - 1)) >= 0 && + (q = ws[m & z.seed & SQMASK]) != null && + U.compareAndSwapInt(q, QLOCK, 0, 1)) { // lock + int b = q.base, s = q.top, n, an; + if ((a = q.array) != null && (an = a.length) > (n = s + 1 - b)) { + int j = (((an - 1) & s) << ASHIFT) + ABASE; + U.putOrderedObject(a, j, task); + q.top = s + 1; // push on to deque + q.qlock = 0; + if (n <= 2) + signalWork(q); + return; + } + q.qlock = 0; + } + fullExternalPush(task); } /** - * Final callback from terminating worker. Removes record of - * worker from array, and adjusts counts. If pool is shutting - * down, tries to complete terminatation, else possibly replaces - * the worker. - * - * @param w the worker - */ - final void workerTerminated(ForkJoinWorkerThread w) { - if (w.active) { // force inactive - w.active = false; - do {} while (!tryDecrementActiveCount()); - } - forgetWorker(w); - - // Decrement total count, and if was running, running count - // Spin (waiting for other updates) if either would be negative - int nr = w.isTrimmed() ? 0 : ONE_RUNNING; - int unit = ONE_TOTAL + nr; - for (;;) { - int wc = workerCounts; - int rc = wc & RUNNING_COUNT_MASK; - if (rc - nr < 0 || (wc >>> TOTAL_COUNT_SHIFT) == 0) - Thread.yield(); // back off if waiting for other updates - else if (UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - unit)) - break; + * Full version of externalPush. This method is called, among + * other times, upon the first submission of the first task to the + * pool, so must perform secondary initialization. It also + * detects first submission by an external thread by looking up + * its ThreadLocal, and creates a new shared queue if the one at + * index if empty or contended. The plock lock body must be + * exception-free (so no try/finally) so we optimistically + * allocate new queues outside the lock and throw them away if + * (very rarely) not needed. + * + * Secondary initialization occurs when plock is zero, to create + * workQueue array and set plock to a valid value. This lock body + * must also be exception-free. Because the plock seq value can + * eventually wrap around zero, this method harmlessly fails to + * reinitialize if workQueues exists, while still advancing plock. + */ + private void fullExternalPush(ForkJoinTask task) { + int r = 0; // random index seed + for (Submitter z = submitters.get();;) { + WorkQueue[] ws; WorkQueue q; int ps, m, k; + if (z == null) { + if (U.compareAndSwapInt(this, INDEXSEED, r = indexSeed, + r += SEED_INCREMENT) && r != 0) + submitters.set(z = new Submitter(r)); + } + else if (r == 0) { // move to a different index + r = z.seed; + r ^= r << 13; // same xorshift as WorkQueues + r ^= r >>> 17; + z.seed = r ^ (r << 5); + } + else if ((ps = plock) < 0) + throw new RejectedExecutionException(); + else if (ps == 0 || (ws = workQueues) == null || + (m = ws.length - 1) < 0) { // initialize workQueues + int p = config & SMASK; // find power of two table size + int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots + n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; + n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1; + WorkQueue[] nws = ((ws = workQueues) == null || ws.length == 0 ? + new WorkQueue[n] : null); + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + if (((ws = workQueues) == null || ws.length == 0) && nws != null) + workQueues = nws; + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + else if ((q = ws[k = r & m & SQMASK]) != null) { + if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) { + ForkJoinTask[] a = q.array; + int s = q.top; + boolean submitted = false; + try { // locked version of push + if ((a != null && a.length > s + 1 - q.base) || + (a = q.growArray()) != null) { // must presize + int j = (((a.length - 1) & s) << ASHIFT) + ABASE; + U.putOrderedObject(a, j, task); + q.top = s + 1; + submitted = true; + } + } finally { + q.qlock = 0; // unlock + } + if (submitted) { + signalWork(q); + return; + } + } + r = 0; // move on failure + } + else if (((ps = plock) & PL_LOCK) == 0) { // create new queue + q = new WorkQueue(this, null, SHARED_QUEUE, r); + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + if ((ws = workQueues) != null && k < ws.length && ws[k] == null) + ws[k] = q; + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + else + r = 0; // try elsewhere while lock held } - - accumulateStealCount(w); // collect final count - if (!tryTerminate(false)) - ensureEnoughTotalWorkers(); } - // Waiting for and signalling events + // Maintaining ctl counts /** - * Ensures eventCount on exit is different (mod 2^32) than on - * entry. CAS failures are OK -- any change in count suffices. + * Increments active count; mainly called upon return from blocking. */ - private void advanceEventCount() { - int c; - UNSAFE.compareAndSwapInt(this, eventCountOffset, c = eventCount, c+1); + final void incrementActiveCount() { + long c; + do {} while (!U.compareAndSwapLong(this, CTL, c = ctl, c + AC_UNIT)); } /** - * Releases workers blocked on a count not equal to current count. + * Tries to create or activate a worker if too few are active. + * + * @param q the (non-null) queue holding tasks to be signalled */ - final void releaseWaiters() { - long top; - int id; - while ((id = (int)((top = eventWaiters) & WAITER_INDEX_MASK)) > 0 && - (int)(top >>> EVENT_COUNT_SHIFT) != eventCount) { - ForkJoinWorkerThread[] ws = workers; - ForkJoinWorkerThread w; - if (ws.length >= id && (w = ws[id - 1]) != null && - UNSAFE.compareAndSwapLong(this, eventWaitersOffset, - top, w.nextWaiter)) - LockSupport.unpark(w); + final void signalWork(WorkQueue q) { + int hint = q.poolIndex; + long c; int e, u, i, n; WorkQueue[] ws; WorkQueue w; Thread p; + while ((u = (int)((c = ctl) >>> 32)) < 0) { + if ((e = (int)c) > 0) { + if ((ws = workQueues) != null && ws.length > (i = e & SMASK) && + (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) { + long nc = (((long)(w.nextWait & E_MASK)) | + ((long)(u + UAC_UNIT) << 32)); + if (U.compareAndSwapLong(this, CTL, c, nc)) { + w.hint = hint; + w.eventCount = (e + E_SEQ) & E_MASK; + if ((p = w.parker) != null) + U.unpark(p); + break; + } + if (q.top - q.base <= 0) + break; + } + else + break; + } + else { + if ((short)u < 0) + tryAddWorker(); + break; + } } } + // Scanning for tasks + /** - * Advances eventCount and releases waiters until interference by - * other releasing threads is detected. + * Top-level runloop for workers, called by ForkJoinWorkerThread.run. */ - final void signalWork() { - int ec; - UNSAFE.compareAndSwapInt(this, eventCountOffset, ec=eventCount, ec+1); - outer:for (;;) { - long top = eventWaiters; - ec = eventCount; - for (;;) { - ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w; - int id = (int)(top & WAITER_INDEX_MASK); - if (id <= 0 || (int)(top >>> EVENT_COUNT_SHIFT) == ec) - return; - if ((ws = workers).length < id || (w = ws[id - 1]) == null || - !UNSAFE.compareAndSwapLong(this, eventWaitersOffset, - top, top = w.nextWaiter)) - continue outer; // possibly stale; reread - LockSupport.unpark(w); - if (top != eventWaiters) // let someone else take over - return; + final void runWorker(WorkQueue w) { + w.growArray(); // allocate queue + do { w.runTask(scan(w)); } while (w.qlock >= 0); + } + + /** + * Scans for and, if found, returns one task, else possibly + * inactivates the worker. This method operates on single reads of + * volatile state and is designed to be re-invoked continuously, + * in part because it returns upon detecting inconsistencies, + * contention, or state changes that indicate possible success on + * re-invocation. + * + * The scan searches for tasks across queues (starting at a random + * index, and relying on registerWorker to irregularly scatter + * them within array to avoid bias), checking each at least twice. + * The scan terminates upon either finding a non-empty queue, or + * completing the sweep. If the worker is not inactivated, it + * takes and returns a task from this queue. Otherwise, if not + * activated, it signals workers (that may include itself) and + * returns so caller can retry. Also returns for true if the + * worker array may have changed during an empty scan. On failure + * to find a task, we take one of the following actions, after + * which the caller will retry calling this method unless + * terminated. + * + * * If pool is terminating, terminate the worker. + * + * * If not already enqueued, try to inactivate and enqueue the + * worker on wait queue. Or, if inactivating has caused the pool + * to be quiescent, relay to idleAwaitWork to possibly shrink + * pool. + * + * * If already enqueued and none of the above apply, possibly + * park awaiting signal, else lingering to help scan and signal. + * + * * If a non-empty queue discovered or left as a hint, + * help wake up other workers before return. + * + * @param w the worker (via its WorkQueue) + * @return a task or null if none found + */ + private final ForkJoinTask scan(WorkQueue w) { + WorkQueue[] ws; int m; + int ps = plock; // read plock before ws + if (w != null && (ws = workQueues) != null && (m = ws.length - 1) >= 0) { + int ec = w.eventCount; // ec is negative if inactive + int r = w.seed; r ^= r << 13; r ^= r >>> 17; w.seed = r ^= r << 5; + w.hint = -1; // update seed and clear hint + int j = ((m + m + 1) | MIN_SCAN) & MAX_SCAN; + do { + WorkQueue q; ForkJoinTask[] a; int b; + if ((q = ws[(r + j) & m]) != null && (b = q.base) - q.top < 0 && + (a = q.array) != null) { // probably nonempty + int i = (((a.length - 1) & b) << ASHIFT) + ABASE; + ForkJoinTask t = (ForkJoinTask) + U.getObjectVolatile(a, i); + if (q.base == b && ec >= 0 && t != null && + U.compareAndSwapObject(a, i, t, null)) { + if ((q.base = b + 1) - q.top < 0) + signalWork(q); + return t; // taken + } + else if ((ec < 0 || j < m) && (int)(ctl >> AC_SHIFT) <= 0) { + w.hint = (r + j) & m; // help signal below + break; // cannot take + } + } + } while (--j >= 0); + + int h, e, ns; long c, sc; WorkQueue q; + if ((ns = w.nsteals) != 0) { + if (U.compareAndSwapLong(this, STEALCOUNT, + sc = stealCount, sc + ns)) + w.nsteals = 0; // collect steals and rescan + } + else if (plock != ps) // consistency check + ; // skip + else if ((e = (int)(c = ctl)) < 0) + w.qlock = -1; // pool is terminating + else { + if ((h = w.hint) < 0) { + if (ec >= 0) { // try to enqueue/inactivate + long nc = (((long)ec | + ((c - AC_UNIT) & (AC_MASK|TC_MASK)))); + w.nextWait = e; // link and mark inactive + w.eventCount = ec | INT_SIGN; + if (ctl != c || !U.compareAndSwapLong(this, CTL, c, nc)) + w.eventCount = ec; // unmark on CAS failure + else if ((int)(c >> AC_SHIFT) == 1 - (config & SMASK)) + idleAwaitWork(w, nc, c); + } + else if (w.eventCount < 0 && ctl == c) { + Thread wt = Thread.currentThread(); + Thread.interrupted(); // clear status + U.putObject(wt, PARKBLOCKER, this); + w.parker = wt; // emulate LockSupport.park + if (w.eventCount < 0) // recheck + U.park(false, 0L); // block + w.parker = null; + U.putObject(wt, PARKBLOCKER, null); + } + } + if ((h >= 0 || (h = w.hint) >= 0) && + (ws = workQueues) != null && h < ws.length && + (q = ws[h]) != null) { // signal others before retry + WorkQueue v; Thread p; int u, i, s; + for (int n = (config & SMASK) - 1;;) { + int idleCount = (w.eventCount < 0) ? 0 : -1; + if (((s = idleCount - q.base + q.top) <= n && + (n = s) <= 0) || + (u = (int)((c = ctl) >>> 32)) >= 0 || + (e = (int)c) <= 0 || m < (i = e & SMASK) || + (v = ws[i]) == null) + break; + long nc = (((long)(v.nextWait & E_MASK)) | + ((long)(u + UAC_UNIT) << 32)); + if (v.eventCount != (e | INT_SIGN) || + !U.compareAndSwapLong(this, CTL, c, nc)) + break; + v.hint = h; + v.eventCount = (e + E_SEQ) & E_MASK; + if ((p = v.parker) != null) + U.unpark(p); + if (--n <= 0) + break; + } + } } } + return null; } /** - * If worker is inactive, blocks until terminating or event count - * advances from last value held by worker; in any case helps - * release others. - * - * @param w the calling worker thread - */ - private void eventSync(ForkJoinWorkerThread w) { - if (!w.active) { - int prev = w.lastEventCount; - long nextTop = (((long)prev << EVENT_COUNT_SHIFT) | - ((long)(w.poolIndex + 1))); - long top; - while ((runState < SHUTDOWN || !tryTerminate(false)) && - (((int)(top = eventWaiters) & WAITER_INDEX_MASK) == 0 || - (int)(top >>> EVENT_COUNT_SHIFT) == prev) && - eventCount == prev) { - if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset, - w.nextWaiter = top, nextTop)) { - accumulateStealCount(w); // transfer steals while idle - Thread.interrupted(); // clear/ignore interrupt - while (eventCount == prev) - w.doPark(); + * If inactivating worker w has caused the pool to become + * quiescent, checks for pool termination, and, so long as this is + * not the only worker, waits for event for up to a given + * duration. On timeout, if ctl has not changed, terminates the + * worker, which will in turn wake up another worker to possibly + * repeat this process. + * + * @param w the calling worker + * @param currentCtl the ctl value triggering possible quiescence + * @param prevCtl the ctl value to restore if thread is terminated + */ + private void idleAwaitWork(WorkQueue w, long currentCtl, long prevCtl) { + if (w != null && w.eventCount < 0 && + !tryTerminate(false, false) && (int)prevCtl != 0 && + ctl == currentCtl) { + int dc = -(short)(currentCtl >>> TC_SHIFT); + long parkTime = dc < 0 ? FAST_IDLE_TIMEOUT: (dc + 1) * IDLE_TIMEOUT; + long deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP; + Thread wt = Thread.currentThread(); + while (ctl == currentCtl) { + Thread.interrupted(); // timed variant of version in scan() + U.putObject(wt, PARKBLOCKER, this); + w.parker = wt; + if (ctl == currentCtl) + U.park(false, parkTime); + w.parker = null; + U.putObject(wt, PARKBLOCKER, null); + if (ctl != currentCtl) + break; + if (deadline - System.nanoTime() <= 0L && + U.compareAndSwapLong(this, CTL, currentCtl, prevCtl)) { + w.eventCount = (w.eventCount + E_SEQ) | E_MASK; + w.hint = -1; + w.qlock = -1; // shrink break; } } - w.lastEventCount = eventCount; } - releaseWaiters(); } /** - * Callback from workers invoked upon each top-level action (i.e., - * stealing a task or taking a submission and running - * it). Performs one or both of the following: - * - * * If the worker cannot find work, updates its active status to - * inactive and updates activeCount unless there is contention, in - * which case it may try again (either in this or a subsequent - * call). Additionally, awaits the next task event and/or helps - * wake up other releasable waiters. - * - * * If there are too many running threads, suspends this worker - * (first forcing inactivation if necessary). If it is not - * resumed before a keepAlive elapses, the worker may be "trimmed" - * -- killed while suspended within suspendAsSpare. Otherwise, - * upon resume it rechecks to make sure that it is still needed. - * - * @param w the worker - * @param worked false if the worker scanned for work but didn't - * find any (in which case it may block waiting for work). - */ - final void preStep(ForkJoinWorkerThread w, boolean worked) { - boolean active = w.active; - boolean inactivate = !worked & active; - for (;;) { - if (inactivate) { - int c = runState; - if (UNSAFE.compareAndSwapInt(this, runStateOffset, - c, c - ONE_ACTIVE)) - inactivate = active = w.active = false; - } - int wc = workerCounts; - if ((wc & RUNNING_COUNT_MASK) <= parallelism) { - if (!worked) - eventSync(w); - return; + * Scans through queues looking for work while joining a task; if + * any present, signals. May return early if more signalling is + * detectably unneeded. + * + * @param task return early if done + * @param origin an index to start scan + */ + private void helpSignal(ForkJoinTask task, int origin) { + WorkQueue[] ws; WorkQueue w; Thread p; long c; int m, u, e, i, s; + if (task != null && task.status >= 0 && + (u = (int)(ctl >>> 32)) < 0 && (u >> UAC_SHIFT) < 0 && + (ws = workQueues) != null && (m = ws.length - 1) >= 0) { + outer: for (int k = origin, j = m; j >= 0; --j) { + WorkQueue q = ws[k++ & m]; + for (int n = m;;) { // limit to at most m signals + if (task.status < 0) + break outer; + if (q == null || + ((s = -q.base + q.top) <= n && (n = s) <= 0)) + break; + if ((u = (int)((c = ctl) >>> 32)) >= 0 || + (e = (int)c) <= 0 || m < (i = e & SMASK) || + (w = ws[i]) == null) + break outer; + long nc = (((long)(w.nextWait & E_MASK)) | + ((long)(u + UAC_UNIT) << 32)); + if (w.eventCount != (e | INT_SIGN)) + break outer; + if (U.compareAndSwapLong(this, CTL, c, nc)) { + w.eventCount = (e + E_SEQ) & E_MASK; + if ((p = w.parker) != null) + U.unpark(p); + if (--n <= 0) + break; + } + } } - if (!(inactivate |= active) && // must inactivate to suspend - UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - ONE_RUNNING) && - !w.suspendAsSpare()) // false if trimmed - return; } } /** - * Adjusts counts and creates or resumes compensating threads for - * a worker blocking on task joinMe. First tries resuming an - * existing spare (which usually also avoids any count - * adjustment), but must then decrement running count to determine - * whether a new thread is needed. See above for fuller - * explanation. This code is sprawled out non-modularly mainly - * because adaptive spinning works best if the entire method is - * either interpreted or compiled vs having only some pieces of it - * compiled. - * - * @param joinMe the task to join - * @return task status on exit (to simplify usage by callers) - */ - final int awaitJoin(ForkJoinTask joinMe) { - int pc = parallelism; - boolean adj = false; // true when running count adjusted - int scans = 0; - - while (joinMe.status >= 0) { - ForkJoinWorkerThread spare = null; - if ((workerCounts & RUNNING_COUNT_MASK) < pc) { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null && w.isSuspended()) { - spare = w; - break; + * Tries to locate and execute tasks for a stealer of the given + * task, or in turn one of its stealers, Traces currentSteal -> + * currentJoin links looking for a thread working on a descendant + * of the given task and with a non-empty queue to steal back and + * execute tasks from. The first call to this method upon a + * waiting join will often entail scanning/search, (which is OK + * because the joiner has nothing better to do), but this method + * leaves hints in workers to speed up subsequent calls. The + * implementation is very branchy to cope with potential + * inconsistencies or loops encountering chains that are stale, + * unknown, or so long that they are likely cyclic. + * + * @param joiner the joining worker + * @param task the task to join + * @return 0 if no progress can be made, negative if task + * known complete, else positive + */ + private int tryHelpStealer(WorkQueue joiner, ForkJoinTask task) { + int stat = 0, steps = 0; // bound to avoid cycles + if (joiner != null && task != null) { // hoist null checks + restart: for (;;) { + ForkJoinTask subtask = task; // current target + for (WorkQueue j = joiner, v;;) { // v is stealer of subtask + WorkQueue[] ws; int m, s, h; + if ((s = task.status) < 0) { + stat = s; + break restart; } - } - if (joinMe.status < 0) - break; - } - int wc = workerCounts; - int rc = wc & RUNNING_COUNT_MASK; - int dc = pc - rc; - if (dc > 0 && spare != null && spare.tryUnsuspend()) { - if (adj) { - int c; - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); - } - adj = true; - LockSupport.unpark(spare); - } - else if (adj) { - if (dc <= 0) - break; - int tc = wc >>> TOTAL_COUNT_SHIFT; - if (scans > tc) { - int ts = (tc - pc) * pc; - if (rc != 0 && (dc * dc < ts || !maintainsParallelism)) - break; - if (scans > ts && tc < maxPoolSize && - UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc, - wc+(ONE_RUNNING|ONE_TOTAL))){ - addWorker(); - break; + if ((ws = workQueues) == null || (m = ws.length - 1) <= 0) + break restart; // shutting down + if ((v = ws[h = (j.hint | 1) & m]) == null || + v.currentSteal != subtask) { + for (int origin = h;;) { // find stealer + if (((h = (h + 2) & m) & 15) == 1 && + (subtask.status < 0 || j.currentJoin != subtask)) + continue restart; // occasional staleness check + if ((v = ws[h]) != null && + v.currentSteal == subtask) { + j.hint = h; // save hint + break; + } + if (h == origin) + break restart; // cannot find stealer + } + } + for (;;) { // help stealer or descend to its stealer + ForkJoinTask[] a; int b; + if (subtask.status < 0) // surround probes with + continue restart; // consistency checks + if ((b = v.base) - v.top < 0 && (a = v.array) != null) { + int i = (((a.length - 1) & b) << ASHIFT) + ABASE; + ForkJoinTask t = + (ForkJoinTask)U.getObjectVolatile(a, i); + if (subtask.status < 0 || j.currentJoin != subtask || + v.currentSteal != subtask) + continue restart; // stale + stat = 1; // apparent progress + if (t != null && v.base == b && + U.compareAndSwapObject(a, i, t, null)) { + v.base = b + 1; // help stealer + joiner.runSubtask(t); + } + else if (v.base == b && ++steps == MAX_HELP) + break restart; // v apparently stalled + } + else { // empty -- try to descend + ForkJoinTask next = v.currentJoin; + if (subtask.status < 0 || j.currentJoin != subtask || + v.currentSteal != subtask) + continue restart; // stale + else if (next == null || ++steps == MAX_HELP) + break restart; // dead-end or maybe cyclic + else { + subtask = next; + j = v; + break; + } + } } } } - else if (rc != 0) - adj = UNSAFE.compareAndSwapInt (this, workerCountsOffset, - wc, wc - ONE_RUNNING); - if ((scans++ & 1) == 0) - releaseWaiters(); // help others progress - else - Thread.yield(); // avoid starving productive threads } - - if (adj) { - joinMe.internalAwaitDone(); - int c; - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); - } - return joinMe.status; + return stat; } /** - * Same idea as awaitJoin - */ - final void awaitBlocker(ManagedBlocker blocker, boolean maintainPar) - throws InterruptedException { - maintainPar &= maintainsParallelism; - int pc = parallelism; - boolean adj = false; // true when running count adjusted - int scans = 0; - boolean done; - - for (;;) { - if (done = blocker.isReleasable()) - break; - ForkJoinWorkerThread spare = null; - if ((workerCounts & RUNNING_COUNT_MASK) < pc) { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null && w.isSuspended()) { - spare = w; + * Analog of tryHelpStealer for CountedCompleters. Tries to steal + * and run tasks within the target's computation. + * + * @param task the task to join + * @param mode if shared, exit upon completing any task + * if all workers are active + */ + private int helpComplete(ForkJoinTask task, int mode) { + WorkQueue[] ws; WorkQueue q; int m, n, s, u; + if (task != null && (ws = workQueues) != null && + (m = ws.length - 1) >= 0) { + for (int j = 1, origin = j;;) { + if ((s = task.status) < 0) + return s; + if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) { + origin = j; + if (mode == SHARED_QUEUE && + ((u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0)) break; - } } - if (done = blocker.isReleasable()) + else if ((j = (j + 2) & m) == origin) break; } - int wc = workerCounts; - int rc = wc & RUNNING_COUNT_MASK; - int dc = pc - rc; - if (dc > 0 && spare != null && spare.tryUnsuspend()) { - if (adj) { - int c; - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); + } + return 0; + } + + /** + * Tries to decrement active count (sometimes implicitly) and + * possibly release or create a compensating worker in preparation + * for blocking. Fails on contention or termination. Otherwise, + * adds a new thread if no idle workers are available and pool + * may become starved. + */ + final boolean tryCompensate() { + int pc = config & SMASK, e, i, tc; long c; + WorkQueue[] ws; WorkQueue w; Thread p; + if ((ws = workQueues) != null && (e = (int)(c = ctl)) >= 0) { + if (e != 0 && (i = e & SMASK) < ws.length && + (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) { + long nc = ((long)(w.nextWait & E_MASK) | + (c & (AC_MASK|TC_MASK))); + if (U.compareAndSwapLong(this, CTL, c, nc)) { + w.eventCount = (e + E_SEQ) & E_MASK; + if ((p = w.parker) != null) + U.unpark(p); + return true; // replace with idle worker } - adj = true; - LockSupport.unpark(spare); } - else if (adj) { - if (dc <= 0) - break; - int tc = wc >>> TOTAL_COUNT_SHIFT; - if (scans > tc) { - int ts = (tc - pc) * pc; - if (rc != 0 && (dc * dc < ts || !maintainPar)) - break; - if (scans > ts && tc < maxPoolSize && - UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc, - wc+(ONE_RUNNING|ONE_TOTAL))){ - addWorker(); - break; + else if ((tc = (short)(c >>> TC_SHIFT)) >= 0 && + (int)(c >> AC_SHIFT) + pc > 1) { + long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK); + if (U.compareAndSwapLong(this, CTL, c, nc)) + return true; // no compensation + } + else if (tc + pc < MAX_CAP) { + long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK); + if (U.compareAndSwapLong(this, CTL, c, nc)) { + ForkJoinWorkerThreadFactory fac; + Throwable ex = null; + ForkJoinWorkerThread wt = null; + try { + if ((fac = factory) != null && + (wt = fac.newThread(this)) != null) { + wt.start(); + return true; + } + } catch (Throwable rex) { + ex = rex; } + deregisterWorker(wt, ex); // clean up and return false } } - else if (rc != 0) - adj = UNSAFE.compareAndSwapInt (this, workerCountsOffset, - wc, wc - ONE_RUNNING); - if ((++scans & 1) == 0) - releaseWaiters(); // help others progress - else - Thread.yield(); // avoid starving productive threads - } - - try { - if (!done) - do {} while (!blocker.isReleasable() && !blocker.block()); - } finally { - if (adj) { - int c; - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); - } } + return false; } /** - * Unless there are not enough other running threads, adjusts - * counts and blocks a worker performing helpJoin that cannot find - * any work. + * Helps and/or blocks until the given task is done. * - * @return true if joinMe now done + * @param joiner the joining worker + * @param task the task + * @return task status on exit */ - final boolean tryAwaitBusyJoin(ForkJoinTask joinMe) { - int pc = parallelism; - outer:for (;;) { - releaseWaiters(); - if ((workerCounts & RUNNING_COUNT_MASK) < pc) { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null && w.isSuspended()) { - if (joinMe.status < 0) - return true; - if ((workerCounts & RUNNING_COUNT_MASK) > pc) - break; - if (w.tryUnsuspend()) { - LockSupport.unpark(w); - break outer; + final int awaitJoin(WorkQueue joiner, ForkJoinTask task) { + int s = 0; + if (joiner != null && task != null && (s = task.status) >= 0) { + ForkJoinTask prevJoin = joiner.currentJoin; + joiner.currentJoin = task; + do {} while ((s = task.status) >= 0 && !joiner.isEmpty() && + joiner.tryRemoveAndExec(task)); // process local tasks + if (s >= 0 && (s = task.status) >= 0) { + helpSignal(task, joiner.poolIndex); + if ((s = task.status) >= 0 && + (task instanceof CountedCompleter)) + s = helpComplete(task, LIFO_QUEUE); + } + while (s >= 0 && (s = task.status) >= 0) { + if ((!joiner.isEmpty() || // try helping + (s = tryHelpStealer(joiner, task)) == 0) && + (s = task.status) >= 0) { + helpSignal(task, joiner.poolIndex); + if ((s = task.status) >= 0 && tryCompensate()) { + if (task.trySetSignal() && (s = task.status) >= 0) { + synchronized (task) { + if (task.status >= 0) { + try { // see ForkJoinTask + task.wait(); // for explanation + } catch (InterruptedException ie) { + } + } + else + task.notifyAll(); + } } - continue outer; + long c; // re-activate + do {} while (!U.compareAndSwapLong + (this, CTL, c = ctl, c + AC_UNIT)); } } } - if (joinMe.status < 0) - return true; - int wc = workerCounts; - if ((wc & RUNNING_COUNT_MASK) <= 2 || - (wc >>> TOTAL_COUNT_SHIFT) < pc) - return false; // keep this thread alive - if (UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - ONE_RUNNING)) - break; + joiner.currentJoin = prevJoin; } - - joinMe.internalAwaitDone(); - int c; - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); - return true; + return s; } /** - * Possibly initiates and/or completes termination. + * Stripped-down variant of awaitJoin used by timed joins. Tries + * to help join only while there is continuous progress. (Caller + * will then enter a timed wait.) * - * @param now if true, unconditionally terminate, else only - * if shutdown and empty queue and no active workers - * @return true if now terminating or terminated - */ - private boolean tryTerminate(boolean now) { - if (now) - advanceRunLevel(SHUTDOWN); // ensure at least SHUTDOWN - else if (runState < SHUTDOWN || - !submissionQueue.isEmpty() || - (runState & ACTIVE_COUNT_MASK) != 0) - return false; - - if (advanceRunLevel(TERMINATING)) - startTerminating(); - - // Finish now if all threads terminated; else in some subsequent call - if ((workerCounts >>> TOTAL_COUNT_SHIFT) == 0) { - advanceRunLevel(TERMINATED); - terminationLatch.countDown(); - } - return true; - } - - /** - * Actions on transition to TERMINATING + * @param joiner the joining worker + * @param task the task */ - private void startTerminating() { - for (int i = 0; i < 2; ++i) { // twice to mop up newly created workers - cancelSubmissions(); - shutdownWorkers(); - cancelWorkerTasks(); - advanceEventCount(); - releaseWaiters(); - interruptWorkers(); + final void helpJoinOnce(WorkQueue joiner, ForkJoinTask task) { + int s; + if (joiner != null && task != null && (s = task.status) >= 0) { + ForkJoinTask prevJoin = joiner.currentJoin; + joiner.currentJoin = task; + do {} while ((s = task.status) >= 0 && !joiner.isEmpty() && + joiner.tryRemoveAndExec(task)); + if (s >= 0 && (s = task.status) >= 0) { + helpSignal(task, joiner.poolIndex); + if ((s = task.status) >= 0 && + (task instanceof CountedCompleter)) + s = helpComplete(task, LIFO_QUEUE); + } + if (s >= 0 && joiner.isEmpty()) { + do {} while (task.status >= 0 && + tryHelpStealer(joiner, task) > 0); + } + joiner.currentJoin = prevJoin; } } /** - * Clear out and cancel submissions, ignoring exceptions + * Returns a (probably) non-empty steal queue, if one is found + * during a scan, else null. This method must be retried by + * caller if, by the time it tries to use the queue, it is empty. + * @param r a (random) seed for scanning */ - private void cancelSubmissions() { - ForkJoinTask task; - while ((task = submissionQueue.poll()) != null) { - try { - task.cancel(false); - } catch (Throwable ignore) { + private WorkQueue findNonEmptyStealQueue(int r) { + for (;;) { + int ps = plock, m; WorkQueue[] ws; WorkQueue q; + if ((ws = workQueues) != null && (m = ws.length - 1) >= 0) { + for (int j = (m + 1) << 2; j >= 0; --j) { + if ((q = ws[(((r + j) << 1) | 1) & m]) != null && + q.base - q.top < 0) + return q; + } } + if (plock == ps) + return null; } } /** - * Sets all worker run states to at least shutdown, - * also resuming suspended workers - */ - private void shutdownWorkers() { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null) - w.shutdown(); + * Runs tasks until {@code isQuiescent()}. We piggyback on + * active count ctl maintenance, but rather than blocking + * when tasks cannot be found, we rescan until all others cannot + * find tasks either. + */ + final void helpQuiescePool(WorkQueue w) { + for (boolean active = true;;) { + long c; WorkQueue q; ForkJoinTask t; int b; + while ((t = w.nextLocalTask()) != null) { + if (w.base - w.top < 0) + signalWork(w); + t.doExec(); + } + if ((q = findNonEmptyStealQueue(w.nextSeed())) != null) { + if (!active) { // re-establish active count + active = true; + do {} while (!U.compareAndSwapLong + (this, CTL, c = ctl, c + AC_UNIT)); + } + if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) { + if (q.base - q.top < 0) + signalWork(q); + w.runSubtask(t); + } + } + else if (active) { // decrement active count without queuing + long nc = (c = ctl) - AC_UNIT; + if ((int)(nc >> AC_SHIFT) + (config & SMASK) == 0) + return; // bypass decrement-then-increment + if (U.compareAndSwapLong(this, CTL, c, nc)) + active = false; + } + else if ((int)((c = ctl) >> AC_SHIFT) + (config & SMASK) == 0 && + U.compareAndSwapLong(this, CTL, c, c + AC_UNIT)) + return; } } /** - * Clears out and cancels all locally queued tasks + * Gets and removes a local or stolen task for the given worker. + * + * @return a task, if available */ - private void cancelWorkerTasks() { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null) - w.cancelTasks(); + final ForkJoinTask nextTaskFor(WorkQueue w) { + for (ForkJoinTask t;;) { + WorkQueue q; int b; + if ((t = w.nextLocalTask()) != null) + return t; + if ((q = findNonEmptyStealQueue(w.nextSeed())) == null) + return null; + if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) { + if (q.base - q.top < 0) + signalWork(q); + return t; + } } } /** - * Unsticks all workers blocked on joins etc + * Returns a cheap heuristic guide for task partitioning when + * programmers, frameworks, tools, or languages have little or no + * idea about task granularity. In essence by offering this + * method, we ask users only about tradeoffs in overhead vs + * expected throughput and its variance, rather than how finely to + * partition tasks. + * + * In a steady state strict (tree-structured) computation, each + * thread makes available for stealing enough tasks for other + * threads to remain active. Inductively, if all threads play by + * the same rules, each thread should make available only a + * constant number of tasks. + * + * The minimum useful constant is just 1. But using a value of 1 + * would require immediate replenishment upon each steal to + * maintain enough tasks, which is infeasible. Further, + * partitionings/granularities of offered tasks should minimize + * steal rates, which in general means that threads nearer the top + * of computation tree should generate more than those nearer the + * bottom. In perfect steady state, each thread is at + * approximately the same level of computation tree. However, + * producing extra tasks amortizes the uncertainty of progress and + * diffusion assumptions. + * + * So, users will want to use values larger (but not much larger) + * than 1 to both smooth over transient shortages and hedge + * against uneven progress; as traded off against the cost of + * extra task overhead. We leave the user to pick a threshold + * value to compare with the results of this call to guide + * decisions, but recommend values such as 3. + * + * When all threads are active, it is on average OK to estimate + * surplus strictly locally. In steady-state, if one thread is + * maintaining say 2 surplus tasks, then so are others. So we can + * just use estimated queue length. However, this strategy alone + * leads to serious mis-estimates in some non-steady-state + * conditions (ramp-up, ramp-down, other stalls). We can detect + * many of these by further considering the number of "idle" + * threads, that are known to have zero queued tasks, so + * compensate by a factor of (#idle/#active) threads. + * + * Note: The approximation of #busy workers as #active workers is + * not very good under current signalling scheme, and should be + * improved. + */ + static int getSurplusQueuedTaskCount() { + Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q; + if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) { + int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK; + int n = (q = wt.workQueue).top - q.base; + int a = (int)(pool.ctl >> AC_SHIFT) + p; + return n - (a > (p >>>= 1) ? 0 : + a > (p >>>= 1) ? 1 : + a > (p >>>= 1) ? 2 : + a > (p >>>= 1) ? 4 : + 8); + } + return 0; + } + + // Termination + + /** + * Possibly initiates and/or completes termination. The caller + * triggering termination runs three passes through workQueues: + * (0) Setting termination status, followed by wakeups of queued + * workers; (1) cancelling all tasks; (2) interrupting lagging + * threads (likely in external tasks, but possibly also blocked in + * joins). Each pass repeats previous steps because of potential + * lagging thread creation. + * + * @param now if true, unconditionally terminate, else only + * if no work and no active workers + * @param enable if true, enable shutdown when next possible + * @return true if now terminating or terminated */ - private void interruptWorkers() { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null && !w.isTerminated()) { - try { - w.interrupt(); - } catch (SecurityException ignore) { + private boolean tryTerminate(boolean now, boolean enable) { + int ps; + if (this == common) // cannot shut down + return false; + if ((ps = plock) >= 0) { // enable by setting plock + if (!enable) + return false; + if ((ps & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + int nps = ((ps + PL_LOCK) & ~SHUTDOWN) | SHUTDOWN; + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + for (long c;;) { + if (((c = ctl) & STOP_BIT) != 0) { // already terminating + if ((short)(c >>> TC_SHIFT) == -(config & SMASK)) { + synchronized (this) { + notifyAll(); // signal when 0 workers + } + } + return true; + } + if (!now) { // check if idle & no tasks + WorkQueue[] ws; WorkQueue w; + if ((int)(c >> AC_SHIFT) != -(config & SMASK)) + return false; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; ++i) { + if ((w = ws[i]) != null) { + if (!w.isEmpty()) { // signal unprocessed tasks + signalWork(w); + return false; + } + if ((i & 1) != 0 && w.eventCount >= 0) + return false; // unqueued inactive worker + } + } + } + } + if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT)) { + for (int pass = 0; pass < 3; ++pass) { + WorkQueue[] ws; WorkQueue w; Thread wt; + if ((ws = workQueues) != null) { + int n = ws.length; + for (int i = 0; i < n; ++i) { + if ((w = ws[i]) != null) { + w.qlock = -1; + if (pass > 0) { + w.cancelAll(); + if (pass > 1 && (wt = w.owner) != null) { + if (!wt.isInterrupted()) { + try { + wt.interrupt(); + } catch (Throwable ignore) { + } + } + U.unpark(wt); + } + } + } + } + // Wake up workers parked on event queue + int i, e; long cc; Thread p; + while ((e = (int)(cc = ctl) & E_MASK) != 0 && + (i = e & SMASK) < n && i >= 0 && + (w = ws[i]) != null) { + long nc = ((long)(w.nextWait & E_MASK) | + ((cc + AC_UNIT) & AC_MASK) | + (cc & (TC_MASK|STOP_BIT))); + if (w.eventCount == (e | INT_SIGN) && + U.compareAndSwapLong(this, CTL, cc, nc)) { + w.eventCount = (e + E_SEQ) & E_MASK; + w.qlock = -1; + if ((p = w.parker) != null) + U.unpark(p); + } + } + } } } } } - // misc support for ForkJoinWorkerThread + // external operations on common pool /** - * Returns pool number + * Returns common pool queue for a thread that has submitted at + * least one task. */ - final int getPoolNumber() { - return poolNumber; + static WorkQueue commonSubmitterQueue() { + ForkJoinPool p; WorkQueue[] ws; int m; Submitter z; + return ((z = submitters.get()) != null && + (p = common) != null && + (ws = p.workQueues) != null && + (m = ws.length - 1) >= 0) ? + ws[m & z.seed & SQMASK] : null; } /** - * Accumulates steal count from a worker, clearing - * the worker's value + * Tries to pop the given task from submitter's queue in common pool. + */ + static boolean tryExternalUnpush(ForkJoinTask t) { + ForkJoinPool p; WorkQueue[] ws; WorkQueue q; Submitter z; + ForkJoinTask[] a; int m, s; + if (t != null && + (z = submitters.get()) != null && + (p = common) != null && + (ws = p.workQueues) != null && + (m = ws.length - 1) >= 0 && + (q = ws[m & z.seed & SQMASK]) != null && + (s = q.top) != q.base && + (a = q.array) != null) { + long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE; + if (U.getObject(a, j) == t && + U.compareAndSwapInt(q, QLOCK, 0, 1)) { + if (q.array == a && q.top == s && // recheck + U.compareAndSwapObject(a, j, t, null)) { + q.top = s - 1; + q.qlock = 0; + return true; + } + q.qlock = 0; + } + } + return false; + } + + /** + * Tries to pop and run local tasks within the same computation + * as the given root. On failure, tries to help complete from + * other queues via helpComplete. */ - final void accumulateStealCount(ForkJoinWorkerThread w) { - int sc = w.stealCount; - if (sc != 0) { - long c; - w.stealCount = 0; - do {} while (!UNSAFE.compareAndSwapLong(this, stealCountOffset, - c = stealCount, c + sc)); + private void externalHelpComplete(WorkQueue q, ForkJoinTask root) { + ForkJoinTask[] a; int m; + if (q != null && (a = q.array) != null && (m = (a.length - 1)) >= 0 && + root != null && root.status >= 0) { + for (;;) { + int s, u; Object o; CountedCompleter task = null; + if ((s = q.top) - q.base > 0) { + long j = ((m & (s - 1)) << ASHIFT) + ABASE; + if ((o = U.getObject(a, j)) != null && + (o instanceof CountedCompleter)) { + CountedCompleter t = (CountedCompleter)o, r = t; + do { + if (r == root) { + if (U.compareAndSwapInt(q, QLOCK, 0, 1)) { + if (q.array == a && q.top == s && + U.compareAndSwapObject(a, j, t, null)) { + q.top = s - 1; + task = t; + } + q.qlock = 0; + } + break; + } + } while ((r = r.completer) != null); + } + } + if (task != null) + task.doExec(); + if (root.status < 0 || + (u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0) + break; + if (task == null) { + helpSignal(root, q.poolIndex); + if (root.status >= 0) + helpComplete(root, SHARED_QUEUE); + break; + } + } } } /** - * Returns the approximate (non-atomic) number of idle threads per - * active thread. + * Tries to help execute or signal availability of the given task + * from submitter's queue in common pool. */ - final int idlePerActive() { - int ac = runState; // no mask -- artifically boosts during shutdown - int pc = parallelism; // use targeted parallelism, not rc - // Use exact results for small values, saturate past 4 - return pc <= ac? 0 : pc >>> 1 <= ac? 1 : pc >>> 2 <= ac? 3 : pc >>> 3; + static void externalHelpJoin(ForkJoinTask t) { + // Some hard-to-avoid overlap with tryExternalUnpush + ForkJoinPool p; WorkQueue[] ws; WorkQueue q, w; Submitter z; + ForkJoinTask[] a; int m, s, n; + if (t != null && + (z = submitters.get()) != null && + (p = common) != null && + (ws = p.workQueues) != null && + (m = ws.length - 1) >= 0 && + (q = ws[m & z.seed & SQMASK]) != null && + (a = q.array) != null) { + int am = a.length - 1; + if ((s = q.top) != q.base) { + long j = ((am & (s - 1)) << ASHIFT) + ABASE; + if (U.getObject(a, j) == t && + U.compareAndSwapInt(q, QLOCK, 0, 1)) { + if (q.array == a && q.top == s && + U.compareAndSwapObject(a, j, t, null)) { + q.top = s - 1; + q.qlock = 0; + t.doExec(); + } + else + q.qlock = 0; + } + } + if (t.status >= 0) { + if (t instanceof CountedCompleter) + p.externalHelpComplete(q, t); + else + p.helpSignal(t, q.poolIndex); + } + } } - // Public and protected methods + // Exported methods // Constructors /** * Creates a {@code ForkJoinPool} with parallelism equal to {@link - * java.lang.Runtime#availableProcessors}, and using the {@linkplain - * #defaultForkJoinWorkerThreadFactory default thread factory}. + * java.lang.Runtime#availableProcessors}, using the {@linkplain + * #defaultForkJoinWorkerThreadFactory default thread factory}, + * no UncaughtExceptionHandler, and non-async LIFO processing mode. * * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads @@ -1215,14 +2466,15 @@ public class ForkJoinPool extends Abstra * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool() { - this(Runtime.getRuntime().availableProcessors(), - defaultForkJoinWorkerThreadFactory); + this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()), + defaultForkJoinWorkerThreadFactory, null, false); } /** * Creates a {@code ForkJoinPool} with the indicated parallelism - * level and using the {@linkplain - * #defaultForkJoinWorkerThreadFactory default thread factory}. + * level, the {@linkplain + * #defaultForkJoinWorkerThreadFactory default thread factory}, + * no UncaughtExceptionHandler, and non-async LIFO processing mode. * * @param parallelism the parallelism level * @throws IllegalArgumentException if parallelism less than or @@ -1233,31 +2485,25 @@ public class ForkJoinPool extends Abstra * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool(int parallelism) { - this(parallelism, defaultForkJoinWorkerThreadFactory); - } - - /** - * Creates a {@code ForkJoinPool} with parallelism equal to {@link - * java.lang.Runtime#availableProcessors}, and using the given - * thread factory. - * - * @param factory the factory for creating new threads - * @throws NullPointerException if the factory is null - * @throws SecurityException if a security manager exists and - * the caller is not permitted to modify threads - * because it does not hold {@link - * java.lang.RuntimePermission}{@code ("modifyThread")} - */ - public ForkJoinPool(ForkJoinWorkerThreadFactory factory) { - this(Runtime.getRuntime().availableProcessors(), factory); + this(parallelism, defaultForkJoinWorkerThreadFactory, null, false); } /** - * Creates a {@code ForkJoinPool} with the given parallelism and - * thread factory. + * Creates a {@code ForkJoinPool} with the given parameters. * - * @param parallelism the parallelism level - * @param factory the factory for creating new threads + * @param parallelism the parallelism level. For default value, + * use {@link java.lang.Runtime#availableProcessors}. + * @param factory the factory for creating new threads. For default value, + * use {@link #defaultForkJoinWorkerThreadFactory}. + * @param handler the handler for internal worker threads that + * terminate due to unrecoverable errors encountered while executing + * tasks. For default value, use {@code null}. + * @param asyncMode if true, + * establishes local first-in-first-out scheduling mode for forked + * tasks that are never joined. This mode may be more appropriate + * than default locally stack-based mode in applications in which + * worker threads only process event-style asynchronous tasks. + * For default value, use {@code false}. * @throws IllegalArgumentException if parallelism less than or * equal to zero, or greater than implementation limit * @throws NullPointerException if the factory is null @@ -1266,56 +2512,70 @@ public class ForkJoinPool extends Abstra * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ - public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory) { + public ForkJoinPool(int parallelism, + ForkJoinWorkerThreadFactory factory, + Thread.UncaughtExceptionHandler handler, + boolean asyncMode) { checkPermission(); if (factory == null) throw new NullPointerException(); - if (parallelism <= 0 || parallelism > MAX_THREADS) + if (parallelism <= 0 || parallelism > MAX_CAP) throw new IllegalArgumentException(); - this.poolNumber = poolNumberGenerator.incrementAndGet(); - int arraySize = initialArraySizeFor(parallelism); - this.parallelism = parallelism; this.factory = factory; - this.maxPoolSize = MAX_THREADS; - this.maintainsParallelism = true; - this.workers = new ForkJoinWorkerThread[arraySize]; - this.submissionQueue = new LinkedTransferQueue>(); - this.workerLock = new ReentrantLock(); - this.terminationLatch = new CountDownLatch(1); + this.ueh = handler; + this.config = parallelism | (asyncMode ? (FIFO_QUEUE << 16) : 0); + long np = (long)(-parallelism); // offset ctl counts + this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); + int pn = nextPoolId(); + StringBuilder sb = new StringBuilder("ForkJoinPool-"); + sb.append(Integer.toString(pn)); + sb.append("-worker-"); + this.workerNamePrefix = sb.toString(); } /** - * Returns initial power of two size for workers array. - * @param pc the initial parallelism level - */ - private static int initialArraySizeFor(int pc) { - // See Hackers Delight, sec 3.2. We know MAX_THREADS < (1 >>> 16) - int size = pc < MAX_THREADS ? pc + 1 : MAX_THREADS; - size |= size >>> 1; - size |= size >>> 2; - size |= size >>> 4; - size |= size >>> 8; - return size + 1; + * Constructor for common pool, suitable only for static initialization. + * Basically the same as above, but uses smallest possible initial footprint. + */ + ForkJoinPool(int parallelism, long ctl, + ForkJoinWorkerThreadFactory factory, + Thread.UncaughtExceptionHandler handler) { + this.config = parallelism; + this.ctl = ctl; + this.factory = factory; + this.ueh = handler; + this.workerNamePrefix = "ForkJoinPool.commonPool-worker-"; } - // Execution methods - /** - * Common code for execute, invoke and submit - */ - private void doSubmit(ForkJoinTask task) { - if (task == null) - throw new NullPointerException(); - if (runState >= SHUTDOWN) - throw new RejectedExecutionException(); - submissionQueue.offer(task); - advanceEventCount(); - releaseWaiters(); - ensureEnoughTotalWorkers(); + * Returns the common pool instance. This pool is statically + * constructed; its run state is unaffected by attempts to {@link + * #shutdown} or {@link #shutdownNow}. However this pool and any + * ongoing processing are automatically terminated upon program + * {@link System#exit}. Any program that relies on asynchronous + * task processing to complete before program termination should + * invoke {@code commonPool().}{@link #awaitQuiescence}, before + * exit. + * + * @return the common pool instance + * @since 1.8 + */ + public static ForkJoinPool commonPool() { + // assert common != null : "static init error"; + return common; } + // Execution methods + /** * Performs the given task, returning its result upon completion. + * If the computation encounters an unchecked Exception or Error, + * it is rethrown as the outcome of this invocation. Rethrown + * exceptions behave in the same way as regular exceptions, but, + * when possible, contain stack traces (as displayed for example + * using {@code ex.printStackTrace()}) of both the current thread + * as well as the thread actually encountering the exception; + * minimally only the latter. * * @param task the task * @return the task's result @@ -1324,7 +2584,9 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public T invoke(ForkJoinTask task) { - doSubmit(task); + if (task == null) + throw new NullPointerException(); + externalPush(task); return task.join(); } @@ -1337,7 +2599,9 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public void execute(ForkJoinTask task) { - doSubmit(task); + if (task == null) + throw new NullPointerException(); + externalPush(task); } // AbstractExecutorService methods @@ -1348,12 +2612,30 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public void execute(Runnable task) { + if (task == null) + throw new NullPointerException(); ForkJoinTask job; if (task instanceof ForkJoinTask) // avoid re-wrap job = (ForkJoinTask) task; else - job = ForkJoinTask.adapt(task, null); - doSubmit(job); + job = new ForkJoinTask.AdaptedRunnableAction(task); + externalPush(job); + } + + /** + * Submits a ForkJoinTask for execution. + * + * @param task the task to submit + * @return the task + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public ForkJoinTask submit(ForkJoinTask task) { + if (task == null) + throw new NullPointerException(); + externalPush(task); + return task; } /** @@ -1362,8 +2644,8 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public ForkJoinTask submit(Callable task) { - ForkJoinTask job = ForkJoinTask.adapt(task); - doSubmit(job); + ForkJoinTask job = new ForkJoinTask.AdaptedCallable(task); + externalPush(job); return job; } @@ -1373,8 +2655,8 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public ForkJoinTask submit(Runnable task, T result) { - ForkJoinTask job = ForkJoinTask.adapt(task, result); - doSubmit(job); + ForkJoinTask job = new ForkJoinTask.AdaptedRunnable(task, result); + externalPush(job); return job; } @@ -1384,53 +2666,43 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public ForkJoinTask submit(Runnable task) { + if (task == null) + throw new NullPointerException(); ForkJoinTask job; if (task instanceof ForkJoinTask) // avoid re-wrap job = (ForkJoinTask) task; else - job = ForkJoinTask.adapt(task, null); - doSubmit(job); + job = new ForkJoinTask.AdaptedRunnableAction(task); + externalPush(job); return job; } /** - * Submits a ForkJoinTask for execution. - * - * @param task the task to submit - * @return the task - * @throws NullPointerException if the task is null - * @throws RejectedExecutionException if the task cannot be - * scheduled for execution - */ - public ForkJoinTask submit(ForkJoinTask task) { - doSubmit(task); - return task; - } - - /** * @throws NullPointerException {@inheritDoc} * @throws RejectedExecutionException {@inheritDoc} */ public List> invokeAll(Collection> tasks) { - ArrayList> forkJoinTasks = - new ArrayList>(tasks.size()); - for (Callable task : tasks) - forkJoinTasks.add(ForkJoinTask.adapt(task)); - invoke(new InvokeAll(forkJoinTasks)); - - @SuppressWarnings({"unchecked", "rawtypes"}) - List> futures = (List>) (List) forkJoinTasks; - return futures; - } - - static final class InvokeAll extends RecursiveAction { - final ArrayList> tasks; - InvokeAll(ArrayList> tasks) { this.tasks = tasks; } - public void compute() { - try { invokeAll(tasks); } - catch (Exception ignore) {} + // In previous versions of this class, this method constructed + // a task to run ForkJoinTask.invokeAll, but now external + // invocation of multiple tasks is at least as efficient. + ArrayList> futures = new ArrayList>(tasks.size()); + + boolean done = false; + try { + for (Callable t : tasks) { + ForkJoinTask f = new ForkJoinTask.AdaptedCallable(t); + futures.add(f); + externalPush(f); + } + for (int i = 0, size = futures.size(); i < size; i++) + ((ForkJoinTask)futures.get(i)).quietlyJoin(); + done = true; + return futures; + } finally { + if (!done) + for (int i = 0, size = futures.size(); i < size; i++) + futures.get(i).cancel(false); } - private static final long serialVersionUID = -7914297376763021607L; } /** @@ -1449,188 +2721,38 @@ public class ForkJoinPool extends Abstra * @return the handler, or {@code null} if none */ public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() { - workerCountReadFence(); return ueh; } /** - * Sets the handler for internal worker threads that terminate due - * to unrecoverable errors encountered while executing tasks. - * Unless set, the current default or ThreadGroup handler is used - * as handler. - * - * @param h the new handler - * @return the old handler, or {@code null} if none - * @throws SecurityException if a security manager exists and - * the caller is not permitted to modify threads - * because it does not hold {@link - * java.lang.RuntimePermission}{@code ("modifyThread")} - */ - public Thread.UncaughtExceptionHandler - setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler h) { - checkPermission(); - Thread.UncaughtExceptionHandler old = ueh; - if (h != old) { - ueh = h; - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null) - w.setUncaughtExceptionHandler(h); - } - } - return old; - } - - /** - * Sets the target parallelism level of this pool. + * Returns the targeted parallelism level of this pool. * - * @param parallelism the target parallelism - * @throws IllegalArgumentException if parallelism less than or - * equal to zero or greater than maximum size bounds - * @throws SecurityException if a security manager exists and - * the caller is not permitted to modify threads - * because it does not hold {@link - * java.lang.RuntimePermission}{@code ("modifyThread")} + * @return the targeted parallelism level of this pool */ - public void setParallelism(int parallelism) { - checkPermission(); - if (parallelism <= 0 || parallelism > maxPoolSize) - throw new IllegalArgumentException(); - workerCountReadFence(); - int pc = this.parallelism; - if (pc != parallelism) { - this.parallelism = parallelism; - workerCountWriteFence(); - // Release spares. If too many, some will die after re-suspend - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null && w.tryUnsuspend()) { - int c; - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); - LockSupport.unpark(w); - } - } - ensureEnoughTotalWorkers(); - advanceEventCount(); - releaseWaiters(); // force config recheck by existing workers - } + public int getParallelism() { + return config & SMASK; } /** - * Returns the targeted parallelism level of this pool. + * Returns the targeted parallelism level of the common pool. * - * @return the targeted parallelism level of this pool + * @return the targeted parallelism level of the common pool + * @since 1.8 */ - public int getParallelism() { - // workerCountReadFence(); // inlined below - int ignore = workerCounts; - return parallelism; + public static int getCommonPoolParallelism() { + return commonParallelism; } /** * Returns the number of worker threads that have started but not - * yet terminated. This result returned by this method may differ + * yet terminated. The result returned by this method may differ * from {@link #getParallelism} when threads are created to * maintain parallelism when others are cooperatively blocked. * * @return the number of worker threads */ public int getPoolSize() { - return workerCounts >>> TOTAL_COUNT_SHIFT; - } - - /** - * Returns the maximum number of threads allowed to exist in the - * pool. Unless set using {@link #setMaximumPoolSize}, the - * maximum is an implementation-defined value designed only to - * prevent runaway growth. - * - * @return the maximum - */ - public int getMaximumPoolSize() { - workerCountReadFence(); - return maxPoolSize; - } - - /** - * Sets the maximum number of threads allowed to exist in the - * pool. The given value should normally be greater than or equal - * to the {@link #getParallelism parallelism} level. Setting this - * value has no effect on current pool size. It controls - * construction of new threads. The use of this method may cause - * tasks that intrinsically require extra threads for dependent - * computations to indefinitely stall. If you are instead trying - * to minimize internal thread creation, consider setting {@link - * #setMaintainsParallelism} as false. - * - * @throws IllegalArgumentException if negative or greater than - * internal implementation limit - */ - public void setMaximumPoolSize(int newMax) { - if (newMax < 0 || newMax > MAX_THREADS) - throw new IllegalArgumentException(); - maxPoolSize = newMax; - workerCountWriteFence(); - } - - /** - * Returns {@code true} if this pool dynamically maintains its - * target parallelism level. If false, new threads are added only - * to avoid possible starvation. This setting is by default true. - * - * @return {@code true} if maintains parallelism - */ - public boolean getMaintainsParallelism() { - workerCountReadFence(); - return maintainsParallelism; - } - - /** - * Sets whether this pool dynamically maintains its target - * parallelism level. If false, new threads are added only to - * avoid possible starvation. - * - * @param enable {@code true} to maintain parallelism - */ - public void setMaintainsParallelism(boolean enable) { - maintainsParallelism = enable; - workerCountWriteFence(); - } - - /** - * Establishes local first-in-first-out scheduling mode for forked - * tasks that are never joined. This mode may be more appropriate - * than default locally stack-based mode in applications in which - * worker threads only process asynchronous tasks. This method is - * designed to be invoked only when the pool is quiescent, and - * typically only before any tasks are submitted. The effects of - * invocations at other times may be unpredictable. - * - * @param async if {@code true}, use locally FIFO scheduling - * @return the previous mode - * @see #getAsyncMode - */ - public boolean setAsyncMode(boolean async) { - workerCountReadFence(); - boolean oldMode = locallyFifo; - if (oldMode != async) { - locallyFifo = async; - workerCountWriteFence(); - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null) - w.setAsyncMode(async); - } - } - return oldMode; + return (config & SMASK) + (short)(ctl >>> TC_SHIFT); } /** @@ -1638,11 +2760,9 @@ public class ForkJoinPool extends Abstra * scheduling mode for forked tasks that are never joined. * * @return {@code true} if this pool uses async mode - * @see #setAsyncMode */ public boolean getAsyncMode() { - workerCountReadFence(); - return locallyFifo; + return (config >>> 16) == FIFO_QUEUE; } /** @@ -1654,7 +2774,15 @@ public class ForkJoinPool extends Abstra * @return the number of worker threads */ public int getRunningThreadCount() { - return workerCounts & RUNNING_COUNT_MASK; + int rc = 0; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 1; i < ws.length; i += 2) { + if ((w = ws[i]) != null && w.isApparentlyUnblocked()) + ++rc; + } + } + return rc; } /** @@ -1665,7 +2793,8 @@ public class ForkJoinPool extends Abstra * @return the number of active threads */ public int getActiveThreadCount() { - return runState & ACTIVE_COUNT_MASK; + int r = (config & SMASK) + (int)(ctl >> AC_SHIFT); + return (r <= 0) ? 0 : r; // suppress momentarily negative values } /** @@ -1680,7 +2809,7 @@ public class ForkJoinPool extends Abstra * @return {@code true} if all threads are currently idle */ public boolean isQuiescent() { - return (runState & ACTIVE_COUNT_MASK) == 0; + return (int)(ctl >> AC_SHIFT) + (config & SMASK) == 0; } /** @@ -1695,7 +2824,15 @@ public class ForkJoinPool extends Abstra * @return the number of steals */ public long getStealCount() { - return stealCount; + long count = stealCount; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 1; i < ws.length; i += 2) { + if ((w = ws[i]) != null) + count += w.nsteals; + } + } + return count; } /** @@ -1710,25 +2847,33 @@ public class ForkJoinPool extends Abstra */ public long getQueuedTaskCount() { long count = 0; - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null) - count += w.getQueueSize(); + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 1; i < ws.length; i += 2) { + if ((w = ws[i]) != null) + count += w.queueSize(); + } } return count; } /** * Returns an estimate of the number of tasks submitted to this - * pool that have not yet begun executing. This method takes time - * proportional to the number of submissions. + * pool that have not yet begun executing. This method may take + * time proportional to the number of submissions. * * @return the number of queued submissions */ public int getQueuedSubmissionCount() { - return submissionQueue.size(); + int count = 0; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; i += 2) { + if ((w = ws[i]) != null) + count += w.queueSize(); + } + } + return count; } /** @@ -1738,7 +2883,14 @@ public class ForkJoinPool extends Abstra * @return {@code true} if there are any queued submissions */ public boolean hasQueuedSubmissions() { - return !submissionQueue.isEmpty(); + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; i += 2) { + if ((w = ws[i]) != null && !w.isEmpty()) + return true; + } + } + return false; } /** @@ -1749,7 +2901,14 @@ public class ForkJoinPool extends Abstra * @return the next submission, or {@code null} if none */ protected ForkJoinTask pollSubmission() { - return submissionQueue.poll(); + WorkQueue[] ws; WorkQueue w; ForkJoinTask t; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; i += 2) { + if ((w = ws[i]) != null && (t = w.poll()) != null) + return t; + } + } + return null; } /** @@ -1770,15 +2929,19 @@ public class ForkJoinPool extends Abstra * @return the number of elements transferred */ protected int drainTasksTo(Collection> c) { - int n = submissionQueue.drainTo(c); - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null) - n += w.drainTasksTo(c); + int count = 0; + WorkQueue[] ws; WorkQueue w; ForkJoinTask t; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; ++i) { + if ((w = ws[i]) != null) { + while ((t = w.poll()) != null) { + c.add(t); + ++count; + } + } + } } - return n; + return count; } /** @@ -1789,17 +2952,38 @@ public class ForkJoinPool extends Abstra * @return a string identifying this pool, as well as its state */ public String toString() { - long st = getStealCount(); - long qt = getQueuedTaskCount(); - long qs = getQueuedSubmissionCount(); - int wc = workerCounts; - int tc = wc >>> TOTAL_COUNT_SHIFT; - int rc = wc & RUNNING_COUNT_MASK; - int pc = parallelism; - int rs = runState; - int ac = rs & ACTIVE_COUNT_MASK; + // Use a single pass through workQueues to collect counts + long qt = 0L, qs = 0L; int rc = 0; + long st = stealCount; + long c = ctl; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; ++i) { + if ((w = ws[i]) != null) { + int size = w.queueSize(); + if ((i & 1) == 0) + qs += size; + else { + qt += size; + st += w.nsteals; + if (w.isApparentlyUnblocked()) + ++rc; + } + } + } + } + int pc = (config & SMASK); + int tc = pc + (short)(c >>> TC_SHIFT); + int ac = pc + (int)(c >> AC_SHIFT); + if (ac < 0) // ignore transient negative + ac = 0; + String level; + if ((c & STOP_BIT) != 0) + level = (tc == 0) ? "Terminated" : "Terminating"; + else + level = plock < 0 ? "Shutting down" : "Running"; return super.toString() + - "[" + runLevelToString(rs) + + "[" + level + ", parallelism = " + pc + ", size = " + tc + ", active = " + ac + @@ -1810,19 +2994,14 @@ public class ForkJoinPool extends Abstra "]"; } - private static String runLevelToString(int s) { - return ((s & TERMINATED) != 0 ? "Terminated" : - ((s & TERMINATING) != 0 ? "Terminating" : - ((s & SHUTDOWN) != 0 ? "Shutting down" : - "Running"))); - } - /** - * Initiates an orderly shutdown in which previously submitted - * tasks are executed, but no new tasks will be accepted. - * Invocation has no additional effect if already shut down. - * Tasks that are in the process of being submitted concurrently - * during the course of this method may or may not be rejected. + * Possibly initiates an orderly shutdown in which previously + * submitted tasks are executed, but no new tasks will be + * accepted. Invocation has no effect on execution state if this + * is the {@link #commonPool()}, and no additional effect if + * already shut down. Tasks that are in the process of being + * submitted concurrently during the course of this method may or + * may not be rejected. * * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads @@ -1831,19 +3010,20 @@ public class ForkJoinPool extends Abstra */ public void shutdown() { checkPermission(); - advanceRunLevel(SHUTDOWN); - tryTerminate(false); + tryTerminate(false, true); } /** - * Attempts to cancel and/or stop all tasks, and reject all - * subsequently submitted tasks. Tasks that are in the process of - * being submitted or executed concurrently during the course of - * this method may or may not be rejected. This method cancels - * both existing and unexecuted tasks, in order to permit - * termination in the presence of task dependencies. So the method - * always returns an empty list (unlike the case for some other - * Executors). + * Possibly attempts to cancel and/or stop all tasks, and reject + * all subsequently submitted tasks. Invocation has no effect on + * execution state if this is the {@link #commonPool()}, and no + * additional effect if already shut down. Otherwise, tasks that + * are in the process of being submitted or executed concurrently + * during the course of this method may or may not be + * rejected. This method cancels both existing and unexecuted + * tasks, in order to permit termination in the presence of task + * dependencies. So the method always returns an empty list + * (unlike the case for some other Executors). * * @return an empty list * @throws SecurityException if a security manager exists and @@ -1853,7 +3033,7 @@ public class ForkJoinPool extends Abstra */ public List shutdownNow() { checkPermission(); - tryTerminate(true); + tryTerminate(true, true); return Collections.emptyList(); } @@ -1863,7 +3043,9 @@ public class ForkJoinPool extends Abstra * @return {@code true} if all tasks have completed following shut down */ public boolean isTerminated() { - return runState >= TERMINATED; + long c = ctl; + return ((c & STOP_BIT) != 0L && + (short)(c >>> TC_SHIFT) == -(config & SMASK)); } /** @@ -1871,13 +3053,18 @@ public class ForkJoinPool extends Abstra * commenced but not yet completed. This method may be useful for * debugging. A return of {@code true} reported a sufficient * period after shutdown may indicate that submitted tasks have - * ignored or suppressed interruption, causing this executor not - * to properly terminate. + * ignored or suppressed interruption, or are waiting for I/O, + * causing this executor not to properly terminate. (See the + * advisory notes for class {@link ForkJoinTask} stating that + * tasks should not normally entail blocking operations. But if + * they do, they must abort them on interrupt.) * * @return {@code true} if terminating but not yet terminated */ public boolean isTerminating() { - return (runState & (TERMINATING|TERMINATED)) == TERMINATING; + long c = ctl; + return ((c & STOP_BIT) != 0L && + (short)(c >>> TC_SHIFT) != -(config & SMASK)); } /** @@ -1886,13 +3073,16 @@ public class ForkJoinPool extends Abstra * @return {@code true} if this pool has been shut down */ public boolean isShutdown() { - return runState >= SHUTDOWN; + return plock < 0; } /** - * Blocks until all tasks have completed execution after a shutdown - * request, or the timeout occurs, or the current thread is - * interrupted, whichever happens first. + * Blocks until all tasks have completed execution after a + * shutdown request, or the timeout occurs, or the current thread + * is interrupted, whichever happens first. Because the {@link + * #commonPool()} never terminates until program shutdown, when + * applied to the common pool, this method is equivalent to {@link + * #awaitQuiescence} but always returns {@code false}. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument @@ -1902,18 +3092,103 @@ public class ForkJoinPool extends Abstra */ public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException { - return terminationLatch.await(timeout, unit); + if (Thread.interrupted()) + throw new InterruptedException(); + if (this == common) { + awaitQuiescence(timeout, unit); + return false; + } + long nanos = unit.toNanos(timeout); + if (isTerminated()) + return true; + long startTime = System.nanoTime(); + boolean terminated = false; + synchronized (this) { + for (long waitTime = nanos, millis = 0L;;) { + if (terminated = isTerminated() || + waitTime <= 0L || + (millis = unit.toMillis(waitTime)) <= 0L) + break; + wait(millis); + waitTime = nanos - (System.nanoTime() - startTime); + } + } + return terminated; + } + + /** + * If called by a ForkJoinTask operating in this pool, equivalent + * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise, + * waits and/or attempts to assist performing tasks until this + * pool {@link #isQuiescent} or the indicated timeout elapses. + * + * @param timeout the maximum time to wait + * @param unit the time unit of the timeout argument + * @return {@code true} if quiescent; {@code false} if the + * timeout elapsed. + */ + public boolean awaitQuiescence(long timeout, TimeUnit unit) { + long nanos = unit.toNanos(timeout); + ForkJoinWorkerThread wt; + Thread thread = Thread.currentThread(); + if ((thread instanceof ForkJoinWorkerThread) && + (wt = (ForkJoinWorkerThread)thread).pool == this) { + helpQuiescePool(wt.workQueue); + return true; + } + long startTime = System.nanoTime(); + WorkQueue[] ws; + int r = 0, m; + boolean found = true; + while (!isQuiescent() && (ws = workQueues) != null && + (m = ws.length - 1) >= 0) { + if (!found) { + if ((System.nanoTime() - startTime) > nanos) + return false; + Thread.yield(); // cannot block + } + found = false; + for (int j = (m + 1) << 2; j >= 0; --j) { + ForkJoinTask t; WorkQueue q; int b; + if ((q = ws[r++ & m]) != null && (b = q.base) - q.top < 0) { + found = true; + if ((t = q.pollAt(b)) != null) { + if (q.base - q.top < 0) + signalWork(q); + t.doExec(); + } + break; + } + } + } + return true; + } + + /** + * Waits and/or attempts to assist performing tasks indefinitely + * until the {@link #commonPool()} {@link #isQuiescent}. + */ + static void quiesceCommonPool() { + common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS); } /** * Interface for extending managed parallelism for tasks running * in {@link ForkJoinPool}s. * - *

A {@code ManagedBlocker} provides two methods. - * Method {@code isReleasable} must return {@code true} if - * blocking is not necessary. Method {@code block} blocks the - * current thread if necessary (perhaps internally invoking - * {@code isReleasable} before actually blocking). + *

A {@code ManagedBlocker} provides two methods. Method + * {@code isReleasable} must return {@code true} if blocking is + * not necessary. Method {@code block} blocks the current thread + * if necessary (perhaps internally invoking {@code isReleasable} + * before actually blocking). These actions are performed by any + * thread invoking {@link ForkJoinPool#managedBlock}. The + * unusual methods in this API accommodate synchronizers that may, + * but don't usually, block for long periods. Similarly, they + * allow more efficient internal handling of cases in which + * additional workers may be, but usually are not, needed to + * ensure sufficient parallelism. Toward this end, + * implementations of method {@code isReleasable} must be amenable + * to repeated invocation. * *

For example, here is a ManagedBlocker based on a * ReentrantLock: @@ -1931,6 +3206,26 @@ public class ForkJoinPool extends Abstra * return hasLock || (hasLock = lock.tryLock()); * } * }} + * + *

Here is a class that possibly blocks waiting for an + * item on a given queue: + * 

 {@code
+     * class QueueTaker implements ManagedBlocker {
+     *   final BlockingQueue queue;
+     *   volatile E item = null;
+     *   QueueTaker(BlockingQueue q) { this.queue = q; }
+     *   public boolean block() throws InterruptedException {
+     *     if (item == null)
+     *       item = queue.take();
+     *     return true;
+     *   }
+     *   public boolean isReleasable() {
+     *     return item != null || (item = queue.poll()) != null;
+     *   }
+     *   public E getItem() { // call after pool.managedBlock completes
+     *     return item;
+     *   }
+     * }}
*/ public static interface ManagedBlocker { /** @@ -1954,14 +3249,7 @@ public class ForkJoinPool extends Abstra * Blocks in accord with the given blocker. If the current thread * is a {@link ForkJoinWorkerThread}, this method possibly * arranges for a spare thread to be activated if necessary to - * ensure parallelism while the current thread is blocked. - * - *

If {@code maintainParallelism} is {@code true} and the pool - * supports it ({@link #getMaintainsParallelism}), this method - * attempts to maintain the pool's nominal parallelism. Otherwise - * it activates a thread only if necessary to avoid complete - * starvation. This option may be preferable when blockages use - * timeouts, or are almost always brief. + * ensure sufficient parallelism while the current thread is blocked. * *

If the caller is not a {@link ForkJoinTask}, this method is * behaviorally equivalent to @@ -1975,29 +3263,42 @@ public class ForkJoinPool extends Abstra * first be expanded to ensure parallelism, and later adjusted. * * @param blocker the blocker - * @param maintainParallelism if {@code true} and supported by - * this pool, attempt to maintain the pool's nominal parallelism; - * otherwise activate a thread only if necessary to avoid - * complete starvation. * @throws InterruptedException if blocker.block did so */ - public static void managedBlock(ManagedBlocker blocker, - boolean maintainParallelism) + public static void managedBlock(ManagedBlocker blocker) throws InterruptedException { Thread t = Thread.currentThread(); - if (t instanceof ForkJoinWorkerThread) - ((ForkJoinWorkerThread) t).pool. - awaitBlocker(blocker, maintainParallelism); - else - awaitBlocker(blocker); - } - - /** - * Performs Non-FJ blocking - */ - private static void awaitBlocker(ManagedBlocker blocker) - throws InterruptedException { - do {} while (!blocker.isReleasable() && !blocker.block()); + if (t instanceof ForkJoinWorkerThread) { + ForkJoinPool p = ((ForkJoinWorkerThread)t).pool; + while (!blocker.isReleasable()) { // variant of helpSignal + WorkQueue[] ws; WorkQueue q; int m, u; + if ((ws = p.workQueues) != null && (m = ws.length - 1) >= 0) { + for (int i = 0; i <= m; ++i) { + if (blocker.isReleasable()) + return; + if ((q = ws[i]) != null && q.base - q.top < 0) { + p.signalWork(q); + if ((u = (int)(p.ctl >>> 32)) >= 0 || + (u >> UAC_SHIFT) >= 0) + break; + } + } + } + if (p.tryCompensate()) { + try { + do {} while (!blocker.isReleasable() && + !blocker.block()); + } finally { + p.incrementActiveCount(); + } + break; + } + } + } + else { + do {} while (!blocker.isReleasable() && + !blocker.block()); + } } // AbstractExecutorService overrides. These rely on undocumented @@ -2005,37 +3306,93 @@ public class ForkJoinPool extends Abstra // implement RunnableFuture. protected RunnableFuture newTaskFor(Runnable runnable, T value) { - return (RunnableFuture) ForkJoinTask.adapt(runnable, value); + return new ForkJoinTask.AdaptedRunnable(runnable, value); } protected RunnableFuture newTaskFor(Callable callable) { - return (RunnableFuture) ForkJoinTask.adapt(callable); + return new ForkJoinTask.AdaptedCallable(callable); } // Unsafe mechanics + private static final sun.misc.Unsafe U; + private static final long CTL; + private static final long PARKBLOCKER; + private static final int ABASE; + private static final int ASHIFT; + private static final long STEALCOUNT; + private static final long PLOCK; + private static final long INDEXSEED; + private static final long QLOCK; - private static final sun.misc.Unsafe UNSAFE = getUnsafe(); - private static final long workerCountsOffset = - objectFieldOffset("workerCounts", ForkJoinPool.class); - private static final long runStateOffset = - objectFieldOffset("runState", ForkJoinPool.class); - private static final long eventCountOffset = - objectFieldOffset("eventCount", ForkJoinPool.class); - private static final long eventWaitersOffset = - objectFieldOffset("eventWaiters",ForkJoinPool.class); - private static final long stealCountOffset = - objectFieldOffset("stealCount",ForkJoinPool.class); + static { + // initialize field offsets for CAS etc + try { + U = getUnsafe(); + Class k = ForkJoinPool.class; + CTL = U.objectFieldOffset + (k.getDeclaredField("ctl")); + STEALCOUNT = U.objectFieldOffset + (k.getDeclaredField("stealCount")); + PLOCK = U.objectFieldOffset + (k.getDeclaredField("plock")); + INDEXSEED = U.objectFieldOffset + (k.getDeclaredField("indexSeed")); + Class tk = Thread.class; + PARKBLOCKER = U.objectFieldOffset + (tk.getDeclaredField("parkBlocker")); + Class wk = WorkQueue.class; + QLOCK = U.objectFieldOffset + (wk.getDeclaredField("qlock")); + Class ak = ForkJoinTask[].class; + ABASE = U.arrayBaseOffset(ak); + int scale = U.arrayIndexScale(ak); + if ((scale & (scale - 1)) != 0) + throw new Error("data type scale not a power of two"); + ASHIFT = 31 - Integer.numberOfLeadingZeros(scale); + } catch (Exception e) { + throw new Error(e); + } + + submitters = new ThreadLocal(); + ForkJoinWorkerThreadFactory fac = defaultForkJoinWorkerThreadFactory = + new DefaultForkJoinWorkerThreadFactory(); + modifyThreadPermission = new RuntimePermission("modifyThread"); + + /* + * Establish common pool parameters. For extra caution, + * computations to set up common pool state are here; the + * constructor just assigns these values to fields. + */ + int par = 0; + Thread.UncaughtExceptionHandler handler = null; + try { // TBD: limit or report ignored exceptions? + String pp = System.getProperty + ("java.util.concurrent.ForkJoinPool.common.parallelism"); + String hp = System.getProperty + ("java.util.concurrent.ForkJoinPool.common.exceptionHandler"); + String fp = System.getProperty + ("java.util.concurrent.ForkJoinPool.common.threadFactory"); + if (fp != null) + fac = ((ForkJoinWorkerThreadFactory)ClassLoader. + getSystemClassLoader().loadClass(fp).newInstance()); + if (hp != null) + handler = ((Thread.UncaughtExceptionHandler)ClassLoader. + getSystemClassLoader().loadClass(hp).newInstance()); + if (pp != null) + par = Integer.parseInt(pp); + } catch (Exception ignore) { + } + + if (par <= 0) + par = Runtime.getRuntime().availableProcessors(); + if (par > MAX_CAP) + par = MAX_CAP; + commonParallelism = par; + long np = (long)(-par); // precompute initial ctl value + long ct = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); - private static long objectFieldOffset(String field, Class klazz) { - try { - return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); - } catch (NoSuchFieldException e) { - // Convert Exception to corresponding Error - NoSuchFieldError error = new NoSuchFieldError(field); - error.initCause(e); - throw error; - } + common = new ForkJoinPool(par, ct, fac, handler); } /** @@ -2048,21 +3405,23 @@ public class ForkJoinPool extends Abstra private static sun.misc.Unsafe getUnsafe() { try { return sun.misc.Unsafe.getUnsafe(); - } catch (SecurityException se) { - try { - return java.security.AccessController.doPrivileged - (new java.security - .PrivilegedExceptionAction() { - public sun.misc.Unsafe run() throws Exception { - java.lang.reflect.Field f = sun.misc - .Unsafe.class.getDeclaredField("theUnsafe"); - f.setAccessible(true); - return (sun.misc.Unsafe) f.get(null); - }}); - } catch (java.security.PrivilegedActionException e) { - throw new RuntimeException("Could not initialize intrinsics", - e.getCause()); - } + } catch (SecurityException tryReflectionInstead) {} + try { + return java.security.AccessController.doPrivileged + (new java.security.PrivilegedExceptionAction() { + public sun.misc.Unsafe run() throws Exception { + Class k = sun.misc.Unsafe.class; + for (java.lang.reflect.Field f : k.getDeclaredFields()) { + f.setAccessible(true); + Object x = f.get(null); + if (k.isInstance(x)) + return k.cast(x); + } + throw new NoSuchFieldError("the Unsafe"); + }}); + } catch (java.security.PrivilegedActionException e) { + throw new RuntimeException("Could not initialize intrinsics", + e.getCause()); } } }