--- jsr166/src/jsr166y/ForkJoinPool.java 2010/07/23 13:07:43 1.58 +++ jsr166/src/jsr166y/ForkJoinPool.java 2011/03/04 13:29:39 1.95 @@ -6,17 +6,24 @@ 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.Random; +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; +import java.util.concurrent.TimeoutException; +import java.util.concurrent.atomic.AtomicInteger; 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.locks.Condition; /** * An {@link ExecutorService} for running {@link ForkJoinTask}s. @@ -52,7 +59,7 @@ import java.util.concurrent.CountDownLat * convenient form for informal monitoring. * *

As is the case with other ExecutorServices, there are three - * main task execution methods summarized in the follwoing + * main task execution methods summarized in the following * table. These are designed to be used 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 @@ -60,7 +67,7 @@ import java.util.concurrent.CountDownLat * Runnable}- or {@code Callable}- based activities as well. However, * tasks that are already executing in a pool should normally * NOT use these pool execution methods, but instead use the - * within-computation forms listed in the table. + * within-computation forms listed in the table. * * * @@ -69,7 +76,7 @@ import java.util.concurrent.CountDownLat * * * - * + * * * * @@ -84,7 +91,7 @@ import java.util.concurrent.CountDownLat * * *
Call from within fork/join computations
Arange async execution Arrange async execution {@link #execute(ForkJoinTask)} {@link ForkJoinTask#fork}
{@link ForkJoinTask#fork} (ForkJoinTasks are Futures)
- * + * *

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 @@ -110,7 +117,7 @@ import java.util.concurrent.CountDownLat * *

This implementation rejects submitted tasks (that is, by throwing * {@link RejectedExecutionException}) only when the pool is shut down - * or internal resources have been exhuasted. + * or internal resources have been exhausted. * * @since 1.7 * @author Doug Lea @@ -124,228 +131,208 @@ public class ForkJoinPool extends Abstra * 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.) - * - * Beyond work-stealing support and essential bookkeeping, the - * main responsibility of this framework is to arrange tactics for - * when one worker is waiting to join a task stolen (or always - * held by) another. Becauae 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. Given that the creation costs of most - * threads on most systems mainly surrounds setting up runtime - * stacks, thread creation and switching is usually not much more - * expensive than stack creation and switching, and is more - * flexible). Instead we combine two tactics: - * - * 1. Arranging for the joiner to execute some task that it - * would be running if the steal had not occurred. Method - * ForkJoinWorkerThread.helpJoinTask tracks joining->stealing - * links to try to find such a task. - * - * 2. Unless there are already enough live threads, creating or - * or re-activating a spare thread to compensate for the - * (blocked) joiner until it unblocks. Spares then suspend - * at their next opportunity or eventually die if unused for - * too long. See below and the internal documentation - * for tryAwaitJoin for more details about compensation - * rules. - * - * Because the determining existence of conservatively safe - * helping targets, the availability of already-created spares, - * and the apparent need to create new spares are all racy and - * require heuristic guidance, joins (in - * ForkJoinWorkerThread.joinTask) interleave these options until - * successful. Creating a new spare always succeeds, but also - * increases application footprint, so we try to avoid it, within - * reason. - * - * The ManagedBlocker extension API can't use option (1) so uses a - * special version of (2) in method awaitBlocker. + * 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 worker queues, and + * lastly to new submissions. * * 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 other management responsibilities. 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 accommodate - * unbalanced increments and decrements) 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 + * 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 a single 64bit volatile + * variable ("ctl"). This variable is read on the order of 10-100 + * times as often as it is modified (always via CAS). (There is + * some additional control state, for example variable "shutdown" + * for which we can cope with uncoordinated updates.) This + * streamlines synchronization and control at the expense of messy + * constructions needed to repack status bits upon updates. + * Updates tend not to contend with each other except during + * bursts while submitted tasks begin or end. In some cases when + * they do contend, threads can instead do something else + * (usually, scan for tasks) until contention subsides. + * + * To enable 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. + * + * Recording Workers. Workers are recorded in the "workers" array + * that is created upon pool construction and expanded if (rarely) + * necessary. 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, + * terminated ones are protected from each other by a seqLock + * (scanGuard) 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 - * aim to approximately maintain the given level of parallelism. - * 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. Note however that the - * correspondance of these counts to reality is not guaranteed. In - * particular updates for unblocked threads may lag until they - * actually wake up. - * - * 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. 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 tryAwaitJoin 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 would become zero and - * all workers are busy. However, this is not easy to detect - * reliably in the presence of transients so we use retries and - * allow slack (in tryAwaitJoin) to reduce false alarms. These - * effectively reduce 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. + * readers must tolerate null slots. To avoid flailing during + * start-up, the array is presized to hold twice #parallelism + * workers (which is unlikely to need further resizing during + * execution). But to avoid dealing with so many null slots, + * variable scanGuard includes a mask for the nearest power of two + * that contains all current workers. 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 workers are + * via indices into the workers array (which is one source of some + * of the messy code constructions here). In essence, the workers + * array serves as a weak reference mechanism. Thus for example + * the wait queue field of ctl stores worker indices, not worker + * references. Access to the workers 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 workers array, as well as queue arrays, check + * that the array 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. + * + * Wait 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. 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 to both wake up waiters (by advancing their + * count) and avoid ABA effects. Successors are held in worker + * field "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 both before (in scan()) and after + * (in tryAwaitWork()) 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 "parked" field of + * ForkJoinWorkerThread to reduce unnecessary calls to unpark. + * (Use of the parked field requires a secondary recheck to avoid + * missed signals.) + * + * 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. When a submission is added or another worker adds a + * task to a queue that previously had two or fewer tasks, they + * signal waiting workers (or trigger creation of new ones if + * fewer than the given parallelism level -- see signalWork). + * These primary signals are buttressed by signals during rescans + * as well as those performed when a worker steals a task and + * notices that there are more tasks too; together these cover the + * signals needed in cases when more than two tasks are pushed + * but untaken. + * + * 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 + * SHRINK_RATE nanosecs. This will slowly propagate, eventually + * terminating all workers after long periods of non-use. + * + * Submissions. External submissions are maintained in an + * array-based queue that is structured identically to + * ForkJoinWorkerThread queues except for the use of + * submissionLock in method addSubmission. Unlike the case for + * worker queues, multiple external threads can add new + * submissions, so adding requires a lock. + * + * Compensation. Beyond work-stealing support and lifecycle + * control, the main responsibility of this framework is to take + * actions 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. Method + * ForkJoinWorkerThread.joinTask tracks joining->stealing + * links to try to find such a task. + * + * Compensating: Unless there are already enough live threads, + * method tryPreBlock() may create or re-activate a spare + * thread to compensate for blocked joiners until they + * unblock. + * + * The ManagedBlocker extension API can't use helping so relies + * only on compensation in method awaitBlocker. + * + * 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 and require heuristic + * guidance, so we rely on multiple retries of each. Currently, + * in keeping with on-demand signalling policy, we compensate only + * if blocking would leave less than one active (non-waiting, + * non-blocked) worker. Additionally, to avoid some false alarms + * due to GC, lagging counters, system activity, etc, compensated + * blocking for joins is only attempted after rechecks stabilize + * (retries are interspersed with Thread.yield, for good + * citizenship). The variable blockedCount, incremented before + * blocking and decremented after, is sometimes needed to + * distinguish cases of waiting for work vs blocking on joins or + * other managed sync. Both cases are equivalent for most pool + * control, so we can update non-atomically. (Additionally, + * contention on blockedCount alleviates some contention on ctl). + * + * Shutdown and Termination. A call to shutdownNow atomically sets + * the ctl stop bit and then (non-atomically) sets each workers + * "terminate" 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 quiesence (i.e., that + * there is no more work) which is reflected in active counts so + * long as there are no current blockers, as well as possible + * re-evaluations during independent changes in blocking or + * quiescing workers. * - * Beware that there is a lot of representation-level coupling + * Style notes: There is a lot of representation-level coupling * among classes ForkJoinPool, ForkJoinWorkerThread, and - * ForkJoinTask. For example, direct access to "workers" array by + * ForkJoinTask. Most fields of ForkJoinWorkerThread maintain + * data structures managed by ForkJoinPool, so are directly + * accessed. Conversely we allow access to "workers" array by * workers, and direct access to ForkJoinTask.status by both * ForkJoinPool and ForkJoinWorkerThread. 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 other + * changes anyway. All together, these low-level implementation + * choices produce as much as a factor of 4 performance + * improvement compared to naive implementations, and enable the + * processing of billions of tasks per second, at the expense of + * some ugliness. + * + * 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 that help some methods perform reasonably even - * when interpreted (not compiled), at the expense of messiness. + * 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). + * The order of declarations in this file is: (1) declarations of + * statics (2) fields (along with constants used when unpacking + * some of them), listed in an order that tends to reduce + * contention among them a bit under most JVMs. (3) internal + * control methods (4) callbacks and other support for + * ForkJoinTask and ForkJoinWorkerThread classes, (5) exported + * methods (plus a few little helpers). (6) static block + * initializing all statics in a minimally dependent order. */ /** @@ -380,15 +367,13 @@ public class ForkJoinPool extends Abstra * overridden in ForkJoinPool constructors. */ public static final ForkJoinWorkerThreadFactory - defaultForkJoinWorkerThreadFactory = - new DefaultForkJoinWorkerThreadFactory(); + defaultForkJoinWorkerThreadFactory; /** * Permission required for callers of methods that may start or * kill threads. */ - private static final RuntimePermission modifyThreadPermission = - new RuntimePermission("modifyThread"); + private static final RuntimePermission modifyThreadPermission; /** * If there is a security manager, makes sure caller has @@ -403,40 +388,59 @@ public class ForkJoinPool extends Abstra /** * Generator for assigning sequence numbers as pool names. */ - private static final AtomicInteger poolNumberGenerator = - new AtomicInteger(); + private static final AtomicInteger poolNumberGenerator; + + /** + * Generator for initial random seeds for worker victim + * selection. This is used only to create initial seeds. Random + * steals use a cheaper xorshift generator per steal attempt. We + * don't expect much contention on seedGenerator, so just use a + * plain Random. + */ + static final Random workerSeedGenerator; + + /** + * Array holding all worker threads in the pool. Initialized upon + * construction. Array size must be a power of two. Updates and + * replacements are protected by scanGuard, 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, so long + * as reads memory-acquire by first reading ctl. All readers must + * tolerate that some array slots may be null. + */ + ForkJoinWorkerThread[] workers; /** - * Absolute bound for parallelism level. Twice this number must - * fit into a 16bit field to enable word-packing for some counts. + * Initial size for submission queue array. Must be a power of + * two. In many applications, these always stay small so we use a + * small initial cap. */ - private static final int MAX_THREADS = 0x7fff; + private static final int INITIAL_QUEUE_CAPACITY = 8; /** - * 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. + * Maximum size for submission queue array. Must be a power of two + * less than or equal to 1 << (31 - width of array entry) to + * ensure lack of index wraparound, but is capped at a lower + * value to help users trap runaway computations. */ - volatile ForkJoinWorkerThread[] workers; + private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M /** - * Queue for external submissions. + * Array serving as submission queue. Initialized upon construction. */ - private final LinkedTransferQueue> submissionQueue; + private ForkJoinTask[] submissionQueue; /** - * Lock protecting updates to workers array. + * Lock protecting submissions array for addSubmission */ - private final ReentrantLock workerLock; + private final ReentrantLock submissionLock; /** - * Latch released upon termination. + * Condition for awaitTermination, using submissionLock for + * convenience. */ - private final Phaser termination; + private final Condition termination; /** * Creation factory for worker threads. @@ -444,661 +448,769 @@ public class ForkJoinPool extends Abstra private final ForkJoinWorkerThreadFactory factory; /** + * The uncaught exception handler used when any worker abruptly + * terminates. + */ + final Thread.UncaughtExceptionHandler ueh; + + /** + * Prefix for assigning names to worker threads + */ + private final String workerNamePrefix; + + /** * Sum of per-thread steal counts, updated only when threads are * idle or terminating. */ private volatile long stealCount; /** - * 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; - - private static final int EVENT_COUNT_SHIFT = 32; - private static final long WAITER_ID_MASK = (1L << EVENT_COUNT_SHIFT)-1L; - - /** - * 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; - - /** - * 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; - - // 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 - - /** - * 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; - - 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; + * Main pool control -- a long packed with: + * AC: Number of active running workers minus target parallelism (16 bits) + * TC: Number of total workers minus target parallelism (16bits) + * 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 waiting threads (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, when id is + * negative, there is at least one waiting worker, 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. + */ + volatile long ctl; + + // 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 MAX_ID = 0x7fff; // max poolIndex + private static final int SMASK = 0xffff; // mask short bits + 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 EC_UNIT = 1 << EC_SHIFT; /** * The target parallelism level. - * Accessed directly by ForkJoinWorkerThreads. */ final int parallelism; /** + * Index (mod submission queue length) of next element to take + * from submission queue. Usage is identical to that for + * per-worker queues -- see ForkJoinWorkerThread internal + * documentation. + */ + volatile int queueBase; + + /** + * Index (mod submission queue length) of next element to add + * in submission queue. Usage is identical to that for + * per-worker queues -- see ForkJoinWorkerThread internal + * documentation. + */ + int queueTop; + + /** + * True when shutdown() has been called. + */ + volatile boolean shutdown; + + /** * True if use local fifo, not default lifo, for local polling * Read by, and replicated by ForkJoinWorkerThreads */ final boolean locallyFifo; /** - * The uncaught exception handler used when any worker abruptly - * terminates. + * The number of threads in ForkJoinWorkerThreads.helpQuiescePool. + * When non-zero, suppresses automatic shutdown when active + * counts become zero. */ - private final Thread.UncaughtExceptionHandler ueh; + volatile int quiescerCount; /** - * Pool number, just for assigning useful names to worker threads + * The number of threads blocked in join. */ - private final int poolNumber; + volatile int blockedCount; - // Utilities for CASing fields. Note that several of these - // are manually inlined by callers + /** + * Counter for worker Thread names (unrelated to their poolIndex) + */ + private volatile int nextWorkerNumber; /** - * Increments running count. Also used by ForkJoinTask. + * The index for the next created worker. Accessed under scanGuard. */ - final void incrementRunningCount() { - int c; - do {} while (!UNSAFE.compareAndSwapInt(this, workerCountsOffset, - c = workerCounts, - c + ONE_RUNNING)); - } + private int nextWorkerIndex; /** - * Tries to decrement running count unless already zero + * SeqLock and index masking for updates to workers array. Locked + * when SG_UNIT is set. Unlocking clears bit by adding + * SG_UNIT. Staleness of read-only operations can be checked by + * comparing scanGuard to value before the reads. The low 16 bits + * (i.e, anding with SMASK) hold (the smallest power of two + * covering all worker indices, minus one, and is used to avoid + * dealing with large numbers of null slots when the workers array + * is overallocated. */ - final boolean tryDecrementRunningCount() { - int wc = workerCounts; - if ((wc & RUNNING_COUNT_MASK) == 0) - return false; - return UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - ONE_RUNNING); - } + volatile int scanGuard; + + private static final int SG_UNIT = 1 << 16; /** - * Tries to increment running count + * The wakeup interval (in nanoseconds) for a worker waiting for a + * task when the pool is quiescent to instead try to shrink the + * number of workers. The exact value does not matter too + * much. It must be short enough to release resources during + * sustained periods of idleness, but not so short that threads + * are continually re-created. */ - final boolean tryIncrementRunningCount() { - int wc; - return UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc = workerCounts, wc + ONE_RUNNING); - } + private static final long SHRINK_RATE = + 4L * 1000L * 1000L * 1000L; // 4 seconds /** - * Tries incrementing active count; fails on contention. - * Called by workers before executing tasks. + * Top-level loop for worker threads: On each step: if the + * previous step swept through all queues and found no tasks, or + * there are excess threads, then possibly blocks. Otherwise, + * scans for and, if found, executes a task. Returns when pool + * and/or worker terminate. * - * @return true on success + * @param w the worker */ - final boolean tryIncrementActiveCount() { - int c; - return UNSAFE.compareAndSwapInt(this, runStateOffset, - c = runState, c + ONE_ACTIVE); + final void work(ForkJoinWorkerThread w) { + boolean swept = false; // true on empty scans + long c; + while (!w.terminate && (int)(c = ctl) >= 0) { + int a; // active count + if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0) + swept = scan(w, a); + else if (tryAwaitWork(w, c)) + swept = false; + } } + // Signalling + /** - * Tries decrementing active count; fails on contention. - * Called when workers cannot find tasks to run. + * Wakes up or creates a worker. */ - final boolean tryDecrementActiveCount() { - int c; - return UNSAFE.compareAndSwapInt(this, runStateOffset, - c = runState, c - ONE_ACTIVE); + final void signalWork() { + /* + * The while condition is true if: (there is are too few total + * workers OR there is at least one waiter) AND (there are too + * few active workers OR the pool is terminating). The value + * of e distinguishes the remaining cases: zero (no waiters) + * for create, negative if terminating (in which case do + * nothing), else release a waiter. The secondary checks for + * release (non-null array etc) can fail if the pool begins + * terminating after the test, and don't impose any added cost + * because JVMs must perform null and bounds checks anyway. + */ + long c; int e, u; + while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) & + (INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) { + if (e > 0) { // release a waiting worker + int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws; + if ((ws = workers) == null || + (i = ~e & SMASK) >= ws.length || + (w = ws[i]) == null) + break; + long nc = (((long)(w.nextWait & E_MASK)) | + ((long)(u + UAC_UNIT) << 32)); + if (w.eventCount == e && + UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { + w.eventCount = (e + EC_UNIT) & E_MASK; + if (w.parked) + UNSAFE.unpark(w); + break; + } + } + else if (UNSAFE.compareAndSwapLong + (this, ctlOffset, c, + (long)(((u + UTC_UNIT) & UTC_MASK) | + ((u + UAC_UNIT) & UAC_MASK)) << 32)) { + addWorker(); + break; + } + } } /** - * Advances to at least the given level. Returns true if not - * already in at least the given level. + * Variant of signalWork to help release waiters on rescans. + * Tries once to release a waiter if active count < 0. + * + * @return false if failed due to contention, else true */ - private boolean advanceRunLevel(int level) { - for (;;) { - int s = runState; - if ((s & level) != 0) + private boolean tryReleaseWaiter() { + long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws; + if ((e = (int)(c = ctl)) > 0 && + (int)(c >> AC_SHIFT) < 0 && + (ws = workers) != null && + (i = ~e & SMASK) < ws.length && + (w = ws[i]) != null) { + long nc = ((long)(w.nextWait & E_MASK) | + ((c + AC_UNIT) & (AC_MASK|TC_MASK))); + if (w.eventCount != e || + !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) return false; - if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, s | level)) - return true; + w.eventCount = (e + EC_UNIT) & E_MASK; + if (w.parked) + UNSAFE.unpark(w); } + return true; } - // workers array maintenance + // Scanning for tasks /** - * Records and returns a workers array index for new worker. + * Scans for and, if found, executes one task. Scans start at a + * random index of workers array, and randomly select the first + * (2*#workers)-1 probes, and then, if all empty, resort to 2 + * circular sweeps, which is necessary to check quiescence. and + * taking a submission only if no stealable tasks were found. The + * steal code inside the loop is a specialized form of + * ForkJoinWorkerThread.deqTask, followed bookkeeping to support + * helpJoinTask and signal propagation. The code for submission + * queues is almost identical. On each steal, the worker completes + * not only the task, but also all local tasks that this task may + * have generated. On detecting staleness or contention when + * trying to take a task, this method returns without finishing + * sweep, which allows global state rechecks before retry. + * + * @param w the worker + * @param a the number of active workers + * @return true if swept all queues without finding a task */ - 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); + private boolean scan(ForkJoinWorkerThread w, int a) { + int g = scanGuard; // mask 0 avoids useless scans if only one active + int m = parallelism == 1 - a? 0 : g & SMASK; + ForkJoinWorkerThread[] ws = workers; + if (ws == null || ws.length <= m) // staleness check + return false; + for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) { + ForkJoinTask t; ForkJoinTask[] q; int b, i; + ForkJoinWorkerThread v = ws[k & m]; + if (v != null && (b = v.queueBase) != v.queueTop && + (q = v.queue) != null && (i = (q.length - 1) & b) >= 0) { + long u = (i << ASHIFT) + ABASE; + if ((t = q[i]) != null && v.queueBase == b && + UNSAFE.compareAndSwapObject(q, u, t, null)) { + int d = (v.queueBase = b + 1) - v.queueTop; + v.stealHint = w.poolIndex; + if (d != 0) + signalWork(); // propagate if nonempty + w.execTask(t); + } + r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5); + return false; // store next seed } - ws[k] = w; - workers = ws; // volatile array write ensures slot visibility - } finally { - lock.unlock(); + else if (j < 0) { // xorshift + r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5; + } + else + ++k; + } + if (scanGuard != g) // staleness check + return false; + else { // try to take submission + ForkJoinTask t; ForkJoinTask[] q; int b, i; + if ((b = queueBase) != queueTop && + (q = submissionQueue) != null && + (i = (q.length - 1) & b) >= 0) { + long u = (i << ASHIFT) + ABASE; + if ((t = q[i]) != null && queueBase == b && + UNSAFE.compareAndSwapObject(q, u, t, null)) { + queueBase = b + 1; + w.execTask(t); + } + return false; + } + return true; // all queues empty } - return k; } /** - * 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 enqueue worker w in wait queue and await change in + * worker's eventCount. If the pool is quiescent, possibly + * terminates worker upon exit. Otherwise, before blocking, + * rescans queues to avoid missed signals. Upon finding work, + * releases at least one worker (which may be the current + * worker). Rescans restart upon detected staleness or failure to + * release due to contention. Note the unusual conventions about + * Thread.interrupt here and elsewhere: Because interrupts are + * used solely to alert threads to check termination, which is + * checked here anyway, 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. + * + * @param w the calling worker + * @param c the ctl value on entry + * @return true if waited or another thread was released upon enq + */ + private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) { + int v = w.eventCount; + w.nextWait = (int)c; // w's successor record + long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK)); + if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { + long d = ctl; // return true if lost to a deq, to force scan + return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L; + } + for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount + long s = stealCount; + if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc)) + sc = w.stealCount = 0; + else if (w.eventCount != v) + return true; // update next time + } + if (parallelism + (int)(nc >> AC_SHIFT) == 0 && + blockedCount == 0 && quiescerCount == 0) + idleAwaitWork(w, nc, c, v); // quiescent + for (boolean rescanned = false;;) { + if (w.eventCount != v) + return true; + if (!rescanned) { + int g = scanGuard, m = g & SMASK; + ForkJoinWorkerThread[] ws = workers; + if (ws != null && m < ws.length) { + rescanned = true; + for (int i = 0; i <= m; ++i) { + ForkJoinWorkerThread u = ws[i]; + if (u != null) { + if (u.queueBase != u.queueTop && + !tryReleaseWaiter()) + rescanned = false; // contended + if (w.eventCount != v) + return true; + } + } + } + if (scanGuard != g || // stale + (queueBase != queueTop && !tryReleaseWaiter())) + rescanned = false; + if (!rescanned) + Thread.yield(); // reduce contention + else + Thread.interrupted(); // clear before park + } + else { + w.parked = true; // must recheck + if (w.eventCount != v) { + w.parked = false; + return true; + } + LockSupport.park(this); + rescanned = w.parked = false; + } + } + } + + /** + * If inactivating worker w has caused pool to become + * quiescent, check for pool termination, and wait for event + * for up to SHRINK_RATE nanosecs (rescans are unnecessary in + * this case because quiescence reflects consensus about lack + * of work). On timeout, if ctl has not changed, terminate the + * worker. Upon its termination (see deregisterWorker), it may + * wake up another worker to possibly repeat this process. + * + * @param w the calling worker + * @param currentCtl the ctl value after enqueuing w + * @param prevCtl the ctl value if w terminated + * @param v the eventCount w awaits change + */ + private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl, + long prevCtl, int v) { + if (w.eventCount == v) { + if (shutdown) + tryTerminate(false); + ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs + while (ctl == currentCtl) { + long startTime = System.nanoTime(); + w.parked = true; + if (w.eventCount == v) // must recheck + LockSupport.parkNanos(this, SHRINK_RATE); + w.parked = false; + if (w.eventCount != v) + break; + else if (System.nanoTime() - startTime < SHRINK_RATE) + Thread.interrupted(); // spurious wakeup + else if (UNSAFE.compareAndSwapLong(this, ctlOffset, + currentCtl, prevCtl)) { + w.terminate = true; // restore previous + w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK; + break; + } + } } } - // adding and removing workers + // Submissions /** - * Tries to create and add new worker. Assumes that worker counts - * are already updated to accommodate the worker, so adjusts on - * failure. + * Enqueues the given task in the submissionQueue. Same idea as + * ForkJoinWorkerThread.pushTask except for use of submissionLock. * - * @return new worker or null if creation failed + * @param t the task */ - private ForkJoinWorkerThread addWorker() { - ForkJoinWorkerThread w = null; + private void addSubmission(ForkJoinTask t) { + final ReentrantLock lock = this.submissionLock; + lock.lock(); try { - w = factory.newThread(this); - } finally { // Adjust on either null or exceptional factory return - if (w == null) { - onWorkerCreationFailure(); - return null; + ForkJoinTask[] q; int s, m; + if ((q = submissionQueue) != null) { // ignore if queue removed + long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE; + UNSAFE.putOrderedObject(q, u, t); + queueTop = s + 1; + if (s - queueBase == m) + growSubmissionQueue(); } + } finally { + lock.unlock(); } - w.start(recordWorker(w), ueh); - return w; + signalWork(); } + // (pollSubmission is defined below with exported methods) + /** - * Adjusts counts upon failure to create worker + * Creates or doubles submissionQueue array. + * Basically identical to ForkJoinWorkerThread version. */ - private void onWorkerCreationFailure() { - for (;;) { - int wc = workerCounts; - if ((wc >>> TOTAL_COUNT_SHIFT) == 0) - Thread.yield(); // wait for other counts to settle - else if (UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc, - wc - (ONE_RUNNING|ONE_TOTAL))) - break; + private void growSubmissionQueue() { + ForkJoinTask[] oldQ = submissionQueue; + int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY; + if (size > MAXIMUM_QUEUE_CAPACITY) + throw new RejectedExecutionException("Queue capacity exceeded"); + if (size < INITIAL_QUEUE_CAPACITY) + size = INITIAL_QUEUE_CAPACITY; + ForkJoinTask[] q = submissionQueue = new ForkJoinTask[size]; + int mask = size - 1; + int top = queueTop; + int oldMask; + if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) { + for (int b = queueBase; b != top; ++b) { + long u = ((b & oldMask) << ASHIFT) + ABASE; + Object x = UNSAFE.getObjectVolatile(oldQ, u); + if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null)) + UNSAFE.putObjectVolatile + (q, ((b & mask) << ASHIFT) + ABASE, x); + } } - tryTerminate(false); // in case of failure during shutdown } + // Blocking support + /** - * Creates and/or resumes enough workers to establish target - * parallelism, giving up if terminating or addWorker fails + * Tries to increment blockedCount, decrement active count + * (sometimes implicitly) and possibly release or create a + * compensating worker in preparation for blocking. Fails + * on contention or termination. * - * TODO: recast this to support lazier creation and automated - * parallelism maintenance + * @return true if the caller can block, else should recheck and retry */ - private void ensureEnoughWorkers() { - for (;;) { + private boolean tryPreBlock() { + int b = blockedCount; + if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) { int pc = parallelism; - int wc = workerCounts; - int rc = wc & RUNNING_COUNT_MASK; - int tc = wc >>> TOTAL_COUNT_SHIFT; - if (tc < pc) { - if (runState == TERMINATING || - (UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - wc, wc + (ONE_RUNNING|ONE_TOTAL)) && - addWorker() == null)) - break; - } - else if (tc > pc && rc < pc && - tc > (runState & ACTIVE_COUNT_MASK)) { - ForkJoinWorkerThread spare = null; - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null && w.isSuspended()) { - if ((workerCounts & RUNNING_COUNT_MASK) > pc || - runState == TERMINATING) - return; - if (w.tryResumeSpare()) - incrementRunningCount(); - break; + do { + ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w; + int e, ac, tc, rc, i; + long c = ctl; + int u = (int)(c >>> 32); + if ((e = (int)c) < 0) { + // skip -- terminating + } + else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 && + (ws = workers) != null && + (i = ~e & SMASK) < ws.length && + (w = ws[i]) != null) { + long nc = ((long)(w.nextWait & E_MASK) | + (c & (AC_MASK|TC_MASK))); + if (w.eventCount == e && + UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { + w.eventCount = (e + EC_UNIT) & E_MASK; + if (w.parked) + UNSAFE.unpark(w); + return true; // release an idle worker } } - } - else - break; + else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) { + long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK); + if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) + return true; // no compensation needed + } + else if (tc + pc < MAX_ID) { + long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK); + if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { + addWorker(); + return true; // create a replacement + } + } + // try to back out on any failure and let caller retry + } while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset, + b = blockedCount, b - 1)); } + return false; } /** - * 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 + * Decrements blockedCount and increments active count */ - 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; - } - - accumulateStealCount(w); // collect final count - if (!tryTerminate(false)) - ensureEnoughWorkers(); + private void postBlock() { + long c; + do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask + c = ctl, c + AC_UNIT)); + int b; + do {} while(!UNSAFE.compareAndSwapInt(this, blockedCountOffset, + b = blockedCount, b - 1)); } - // Waiting for and signalling events - /** - * Releases workers blocked on a count not equal to current count. - * @return true if any released - */ - private void releaseWaiters() { - long top; - while ((top = eventWaiters) != 0L) { - ForkJoinWorkerThread[] ws = workers; - int n = ws.length; - for (;;) { - int i = ((int)(top & WAITER_ID_MASK)) - 1; - if (i < 0 || (int)(top >>> EVENT_COUNT_SHIFT) == eventCount) - return; - ForkJoinWorkerThread w; - if (i < n && (w = ws[i]) != null && - UNSAFE.compareAndSwapLong(this, eventWaitersOffset, - top, w.nextWaiter)) { - LockSupport.unpark(w); - top = eventWaiters; - } - else - break; // possibly stale; reread + * Possibly blocks waiting for the given task to complete, or + * cancels the task if terminating. Fails to wait if contended. + * + * @param joinMe the task + */ + final void tryAwaitJoin(ForkJoinTask joinMe) { + int s; + Thread.interrupted(); // clear interrupts before checking termination + if (joinMe.status >= 0) { + if (tryPreBlock()) { + joinMe.tryAwaitDone(0L); + postBlock(); } + else if ((ctl & STOP_BIT) != 0L) + joinMe.cancelIgnoringExceptions(); } } /** - * Ensures eventCount on exit is different (mod 2^32) than on - * entry and wakes up all waiters + * Possibly blocks the given worker waiting for joinMe to + * complete or timeout + * + * @param joinMe the task + * @param millis the wait time for underlying Object.wait */ - private void signalEvent() { - int c; - do {} while (!UNSAFE.compareAndSwapInt(this, eventCountOffset, - c = eventCount, c+1)); - releaseWaiters(); + final void timedAwaitJoin(ForkJoinTask joinMe, long nanos) { + while (joinMe.status >= 0) { + Thread.interrupted(); + if ((ctl & STOP_BIT) != 0L) { + joinMe.cancelIgnoringExceptions(); + break; + } + if (tryPreBlock()) { + long last = System.nanoTime(); + while (joinMe.status >= 0) { + long millis = TimeUnit.NANOSECONDS.toMillis(nanos); + if (millis <= 0) + break; + joinMe.tryAwaitDone(millis); + if (joinMe.status < 0) + break; + if ((ctl & STOP_BIT) != 0L) { + joinMe.cancelIgnoringExceptions(); + break; + } + long now = System.nanoTime(); + nanos -= now - last; + last = now; + } + postBlock(); + break; + } + } } /** - * Advances eventCount and releases waiters until interference by - * other releasing threads is detected. + * If necessary, compensates for blocker, and blocks */ - final void signalWork() { - int c; - UNSAFE.compareAndSwapInt(this, eventCountOffset, c=eventCount, c+1); - long top; - while ((top = eventWaiters) != 0L) { - int ec = eventCount; - ForkJoinWorkerThread[] ws = workers; - int n = ws.length; - for (;;) { - int i = ((int)(top & WAITER_ID_MASK)) - 1; - if (i < 0 || (int)(top >>> EVENT_COUNT_SHIFT) == ec) - return; - ForkJoinWorkerThread w; - if (i < n && (w = ws[i]) != null && - UNSAFE.compareAndSwapLong(this, eventWaitersOffset, - top, top = w.nextWaiter)) { - LockSupport.unpark(w); - if (top != eventWaiters) // let someone else take over - return; + private void awaitBlocker(ManagedBlocker blocker) + throws InterruptedException { + while (!blocker.isReleasable()) { + if (tryPreBlock()) { + try { + do {} while (!blocker.isReleasable() && !blocker.block()); + } finally { + postBlock(); } - else - break; // possibly stale; reread + break; } } } + // Creating, registering and deregistring workers + /** - * 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 - * @param retries the number of scans by caller failing to find work - * @return false if now too many threads running - */ - private boolean eventSync(ForkJoinWorkerThread w, int retries) { - int wec = w.lastEventCount; - if (retries > 1) { // can only block after 2nd miss - long nextTop = (((long)wec << EVENT_COUNT_SHIFT) | - ((long)(w.poolIndex + 1))); - long top; - while ((runState < SHUTDOWN || !tryTerminate(false)) && - (((int)(top = eventWaiters) & WAITER_ID_MASK) == 0 || - (int)(top >>> EVENT_COUNT_SHIFT) == wec) && - eventCount == wec) { - if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset, - w.nextWaiter = top, nextTop)) { - accumulateStealCount(w); // transfer steals while idle - Thread.interrupted(); // clear/ignore interrupt - while (eventCount == wec) - w.doPark(); - break; - } - } - wec = eventCount; - } - releaseWaiters(); - int wc = workerCounts; - if ((wc & RUNNING_COUNT_MASK) <= parallelism) { - w.lastEventCount = wec; - return true; + * Tries to create and start a worker; minimally rolls back counts + * on failure. + */ + private void addWorker() { + Throwable ex = null; + ForkJoinWorkerThread t = null; + try { + t = factory.newThread(this); + } catch (Throwable e) { + ex = e; + } + if (t == null) { // null or exceptional factory return + long c; // adjust counts + do {} while (!UNSAFE.compareAndSwapLong + (this, ctlOffset, c = ctl, + (((c - AC_UNIT) & AC_MASK) | + ((c - TC_UNIT) & TC_MASK) | + (c & ~(AC_MASK|TC_MASK))))); + // Propagate exception if originating from an external caller + if (!tryTerminate(false) && ex != null && + !(Thread.currentThread() instanceof ForkJoinWorkerThread)) + UNSAFE.throwException(ex); } - if (wec != w.lastEventCount) // back up if may re-wait - w.lastEventCount = wec - (wc >>> TOTAL_COUNT_SHIFT); - return false; + else + t.start(); } /** - * 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 retries the number of scans by caller failing to find work - * find any (in which case it may block waiting for work). + * Callback from ForkJoinWorkerThread constructor to assign a + * public name */ - final void preStep(ForkJoinWorkerThread w, int retries) { - boolean active = w.active; - boolean inactivate = active && retries != 0; - for (;;) { - int rs, wc; - if (inactivate && - UNSAFE.compareAndSwapInt(this, runStateOffset, - rs = runState, rs - ONE_ACTIVE)) - inactivate = active = w.active = false; - if (((wc = workerCounts) & RUNNING_COUNT_MASK) <= parallelism) { - if (active || eventSync(w, retries)) - break; - } - else if (!(inactivate |= active) && // must inactivate to suspend - UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - ONE_RUNNING) && - !w.suspendAsSpare()) // false if trimmed - break; + final String nextWorkerName() { + for (int n;;) { + if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset, + n = nextWorkerNumber, ++n)) + return workerNamePrefix + n; } } /** - * Awaits join of the given task if enough threads, or can resume - * or create a spare. Fails (in which case the given task might - * not be done) upon contention or lack of decision about - * blocking. Returns void because caller must check - * task status on return anyway. - * - * We allow blocking if: - * - * 1. There would still be at least as many running threads as - * parallelism level if this thread blocks. - * - * 2. A spare is resumed to replace this worker. We tolerate - * slop in the decision to replace if a spare is found without - * first decrementing run count. This may release too many, - * but if so, the superfluous ones will re-suspend via - * preStep(). - * - * 3. After #spares repeated checks, there are no fewer than #spare - * threads not running. We allow this slack to avoid hysteresis - * and as a hedge against lag/uncertainty of running count - * estimates when signalling or unblocking stalls. - * - * 4. All existing workers are busy (as rechecked via repeated - * retries by caller) and a new spare is created. - * - * If none of the above hold, we try to escape out by - * re-incrementing count and returning to caller, which can retry - * later. - * - * @param joinMe the task to join - * @param retries if negative, then serve only as a precheck - * that the thread can be replaced by a spare. Otherwise, - * the number of repeated calls to this method returning busy - * @return true if the call must be retried because there - * none of the blocking checks hold + * Callback from ForkJoinWorkerThread constructor to + * determine its poolIndex and record in workers array. + * + * @param w the worker + * @return the worker's pool index */ - final boolean tryAwaitJoin(ForkJoinTask joinMe, int retries) { - if (joinMe.status < 0) // precheck to prime loop - return false; - int pc = parallelism; - boolean running = true; // false when running count decremented - outer:for (;;) { - int wc = workerCounts; - int rc = wc & RUNNING_COUNT_MASK; - int tc = wc >>> TOTAL_COUNT_SHIFT; - if (running) { // replace with spare or decrement count - if (rc <= pc && tc > pc && - (retries > 0 || tc > (runState & ACTIVE_COUNT_MASK))) { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { // search for spare - ForkJoinWorkerThread w = ws[i]; - if (w != null) { - if (joinMe.status < 0) - return false; - if (w.isSuspended()) { - if ((workerCounts & RUNNING_COUNT_MASK)>=pc && - w.tryResumeSpare()) { - running = false; - break outer; - } - continue outer; // rescan - } + final int registerWorker(ForkJoinWorkerThread w) { + /* + * In the typical case, a new worker acquires the lock, uses + * next available index and returns quickly. Since we should + * not block callers (ultimately from signalWork or + * tryPreBlock) waiting for the lock needed to do this, we + * instead help release other workers while waiting for the + * lock. + */ + for (int g;;) { + ForkJoinWorkerThread[] ws; + if (((g = scanGuard) & SG_UNIT) == 0 && + UNSAFE.compareAndSwapInt(this, scanGuardOffset, + g, g | SG_UNIT)) { + int k = nextWorkerIndex; + try { + if ((ws = workers) != null) { // ignore on shutdown + int n = ws.length; + if (k < 0 || k >= n || ws[k] != null) { + for (k = 0; k < n && ws[k] != null; ++k) + ; + if (k == n) + ws = workers = Arrays.copyOf(ws, n << 1); } + ws[k] = w; + nextWorkerIndex = k + 1; + int m = g & SMASK; + g = k >= m? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1); } + } finally { + scanGuard = g; } - if (retries < 0 || // < 0 means replacement check only - rc == 0 || joinMe.status < 0 || workerCounts != wc || - !UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - ONE_RUNNING)) - return false; // done or inconsistent or contended - running = false; - if (rc > pc) - break; + return k; } - else { // allow blocking if enough threads - if (rc >= pc || joinMe.status < 0) - break; - int sc = tc - pc + 1; // = spare threads, plus the one to add - if (retries > sc) { - if (rc > 0 && rc >= pc - sc) // allow slack - break; - if (tc < MAX_THREADS && - tc == (runState & ACTIVE_COUNT_MASK) && - workerCounts == wc && - UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc, - wc+(ONE_RUNNING|ONE_TOTAL))) { - addWorker(); - break; + else if ((ws = workers) != null) { // help release others + for (ForkJoinWorkerThread u : ws) { + if (u != null && u.queueBase != u.queueTop) { + if (tryReleaseWaiter()) + break; } } - if (workerCounts == wc && // back out to allow rescan - UNSAFE.compareAndSwapInt (this, workerCountsOffset, - wc, wc + ONE_RUNNING)) { - releaseWaiters(); // help others progress - return true; // let caller retry - } } } - // arrive here if can block - joinMe.internalAwaitDone(); - int c; // to inline incrementRunningCount - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); - return false; } /** - * Same idea as (and shares many code snippets with) tryAwaitJoin, - * but self-contained because there are no caller retries. - * TODO: Rework to use simpler API. + * Final callback from terminating worker. Removes record of + * worker from array, and adjusts counts. If pool is shutting + * down, tries to complete termination. + * + * @param w the worker */ - final void awaitBlocker(ManagedBlocker blocker) - throws InterruptedException { - boolean done; - if (done = blocker.isReleasable()) - return; - int pc = parallelism; - int retries = 0; - boolean running = true; // false when running count decremented - outer:for (;;) { - int wc = workerCounts; - int rc = wc & RUNNING_COUNT_MASK; - int tc = wc >>> TOTAL_COUNT_SHIFT; - if (running) { - if (rc <= pc && tc > pc && - (retries > 0 || tc > (runState & ACTIVE_COUNT_MASK))) { - ForkJoinWorkerThread[] ws = workers; - int nws = ws.length; - for (int i = 0; i < nws; ++i) { - ForkJoinWorkerThread w = ws[i]; - if (w != null) { - if (done = blocker.isReleasable()) - return; - if (w.isSuspended()) { - if ((workerCounts & RUNNING_COUNT_MASK)>=pc && - w.tryResumeSpare()) { - running = false; - break outer; - } - continue outer; // rescan - } - } - } - } - if (done = blocker.isReleasable()) - return; - if (rc == 0 || workerCounts != wc || - !UNSAFE.compareAndSwapInt(this, workerCountsOffset, - wc, wc - ONE_RUNNING)) - continue; - running = false; - if (rc > pc) - break; - } - else { - if (rc >= pc || (done = blocker.isReleasable())) - break; - int sc = tc - pc + 1; - if (retries++ > sc) { - if (rc > 0 && rc >= pc - sc) - break; - if (tc < MAX_THREADS && - tc == (runState & ACTIVE_COUNT_MASK) && - workerCounts == wc && - UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc, - wc+(ONE_RUNNING|ONE_TOTAL))) { - addWorker(); - break; - } - } - Thread.yield(); - } - } - - try { - if (!done) - do {} while (!blocker.isReleasable() && !blocker.block()); - } finally { - if (!running) { - int c; - do {} while (!UNSAFE.compareAndSwapInt - (this, workerCountsOffset, - c = workerCounts, c + ONE_RUNNING)); + final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) { + int idx = w.poolIndex; + int sc = w.stealCount; + int steps = 0; + // Remove from array, adjust worker counts and collect steal count. + // We can intermix failed removes or adjusts with steal updates + do { + long s, c; + int g; + if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 && + UNSAFE.compareAndSwapInt(this, scanGuardOffset, + g, g |= SG_UNIT)) { + ForkJoinWorkerThread[] ws = workers; + if (ws != null && idx >= 0 && + idx < ws.length && ws[idx] == w) + ws[idx] = null; // verify + nextWorkerIndex = idx; + scanGuard = g + SG_UNIT; + steps = 1; } + if (steps == 1 && + UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl, + (((c - AC_UNIT) & AC_MASK) | + ((c - TC_UNIT) & TC_MASK) | + (c & ~(AC_MASK|TC_MASK))))) + steps = 2; + if (sc != 0 && + UNSAFE.compareAndSwapLong(this, stealCountOffset, + s = stealCount, s + sc)) + sc = 0; + } while (steps != 2 || sc != 0); + if (!tryTerminate(false)) { + if (ex != null) // possibly replace if died abnormally + signalWork(); + else + tryReleaseWaiter(); } - } + } + + // Shutdown and termination /** * Possibly initiates and/or completes termination. @@ -1108,115 +1220,132 @@ public class ForkJoinPool extends Abstra * @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); - termination.arrive(); + long c; + while (((c = ctl) & STOP_BIT) == 0) { + if (!now) { + if ((int)(c >> AC_SHIFT) != -parallelism) + return false; + if (!shutdown || blockedCount != 0 || quiescerCount != 0 || + queueBase != queueTop) { + if (ctl == c) // staleness check + return false; + continue; + } + } + if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT)) + startTerminating(); + } + if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers + final ReentrantLock lock = this.submissionLock; + lock.lock(); + try { + termination.signalAll(); + } finally { + lock.unlock(); + } } return true; } /** - * Actions on transition to TERMINATING + * Runs up to three passes through workers: (0) Setting + * termination status for each worker, followed by wakeups up to + * queued workers; (1) helping cancel 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. */ private void startTerminating() { - for (int i = 0; i < 2; ++i) { // twice to mop up newly created workers - cancelSubmissions(); - shutdownWorkers(); - cancelWorkerTasks(); - signalEvent(); - interruptWorkers(); + cancelSubmissions(); + for (int pass = 0; pass < 3; ++pass) { + ForkJoinWorkerThread[] ws = workers; + if (ws != null) { + for (ForkJoinWorkerThread w : ws) { + if (w != null) { + w.terminate = true; + if (pass > 0) { + w.cancelTasks(); + if (pass > 1 && !w.isInterrupted()) { + try { + w.interrupt(); + } catch (SecurityException ignore) { + } + } + } + } + } + terminateWaiters(); + } } } /** - * Clear out and cancel submissions, ignoring exceptions + * Polls and cancels all submissions. Called only during termination. */ private void cancelSubmissions() { - ForkJoinTask task; - while ((task = submissionQueue.poll()) != null) { - try { - task.cancel(false); - } catch (Throwable ignore) { + while (queueBase != queueTop) { + ForkJoinTask task = pollSubmission(); + if (task != null) { + try { + task.cancel(false); + } catch (Throwable ignore) { + } } } } /** - * 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(); - } - } - - /** - * Clears out and cancels all locally queued tasks - */ - 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(); - } - } - - /** - * Unsticks all workers blocked on joins etc + * Tries to set the termination status of waiting workers, and + * then wakes them up (after which they will terminate). */ - private void interruptWorkers() { + private void terminateWaiters() { 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) { + if (ws != null) { + ForkJoinWorkerThread w; long c; int i, e; + int n = ws.length; + while ((i = ~(e = (int)(c = ctl)) & SMASK) < n && + (w = ws[i]) != null && w.eventCount == (e & E_MASK)) { + if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, + (long)(w.nextWait & E_MASK) | + ((c + AC_UNIT) & AC_MASK) | + (c & (TC_MASK|STOP_BIT)))) { + w.terminate = true; + w.eventCount = e + EC_UNIT; + if (w.parked) + UNSAFE.unpark(w); } } } } - // misc support for ForkJoinWorkerThread + // misc ForkJoinWorkerThread support /** - * Returns pool number + * Increment or decrement quiescerCount. Needed only to prevent + * triggering shutdown if a worker is transiently inactive while + * checking quiescence. + * + * @param delta 1 for increment, -1 for decrement */ - final int getPoolNumber() { - return poolNumber; + final void addQuiescerCount(int delta) { + int c; + do {} while(!UNSAFE.compareAndSwapInt(this, quiescerCountOffset, + c = quiescerCount, c + delta)); } /** - * Accumulates steal count from a worker, clearing - * the worker's value + * Directly increment or decrement active count without + * queuing. This method is used to transiently assert inactivation + * while checking quiescence. + * + * @param delta 1 for increment, -1 for decrement */ - 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)); - } + final void addActiveCount(int delta) { + long d = delta < 0 ? -AC_UNIT : AC_UNIT; + long c; + do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl, + ((c + d) & AC_MASK) | + (c & ~AC_MASK))); } /** @@ -1224,13 +1353,17 @@ public class ForkJoinPool extends Abstra * active thread. */ final int idlePerActive() { - int pc = parallelism; // use parallelism, not rc - int ac = runState; // no mask -- artifically boosts during shutdown - // Use exact results for small values, saturate past 4 - return pc <= ac? 0 : pc >>> 1 <= ac? 1 : pc >>> 2 <= ac? 3 : pc >>> 3; + // Approximate at powers of two for small values, saturate past 4 + int p = parallelism; + int a = p + (int)(ctl >> AC_SHIFT); + return (a > (p >>>= 1) ? 0 : + a > (p >>>= 1) ? 1 : + a > (p >>>= 1) ? 2 : + a > (p >>>= 1) ? 4 : + 8); } - // Public and protected methods + // Exported methods // Constructors @@ -1275,15 +1408,15 @@ public class ForkJoinPool extends Abstra * 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 null. - * @param asyncMode if true, + * @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 false. + * 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 @@ -1292,61 +1425,49 @@ public class ForkJoinPool extends Abstra * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ - public ForkJoinPool(int parallelism, + 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_ID) throw new IllegalArgumentException(); this.parallelism = parallelism; this.factory = factory; this.ueh = handler; this.locallyFifo = asyncMode; - int arraySize = initialArraySizeFor(parallelism); - this.workers = new ForkJoinWorkerThread[arraySize]; - this.submissionQueue = new LinkedTransferQueue>(); - this.workerLock = new ReentrantLock(); - this.termination = new Phaser(1); - this.poolNumber = poolNumberGenerator.incrementAndGet(); - } - - /** - * 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; + long np = (long)(-parallelism); // offset ctl counts + this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); + this.submissionQueue = new ForkJoinTask[INITIAL_QUEUE_CAPACITY]; + // initialize workers array with room for 2*parallelism if possible + int n = parallelism << 1; + if (n >= MAX_ID) + n = MAX_ID; + else { // See Hackers Delight, sec 3.2, where n < (1 << 16) + n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; + } + workers = new ForkJoinWorkerThread[n + 1]; + this.submissionLock = new ReentrantLock(); + this.termination = submissionLock.newCondition(); + StringBuilder sb = new StringBuilder("ForkJoinPool-"); + sb.append(poolNumberGenerator.incrementAndGet()); + sb.append("-worker-"); + this.workerNamePrefix = sb.toString(); } // 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); - signalEvent(); - ensureEnoughWorkers(); - } - - /** * Performs the given task, returning its result upon completion. - * If the caller is already engaged in a fork/join computation in - * the current pool, this method is equivalent in effect to - * {@link ForkJoinTask#invoke}. + * 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 @@ -1355,15 +1476,38 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public T invoke(ForkJoinTask task) { - doSubmit(task); - return task.join(); + Thread t = Thread.currentThread(); + if (task == null) + throw new NullPointerException(); + if (shutdown) + throw new RejectedExecutionException(); + if ((t instanceof ForkJoinWorkerThread) && + ((ForkJoinWorkerThread)t).pool == this) + return task.invoke(); // bypass submit if in same pool + else { + addSubmission(task); + return task.join(); + } + } + + /** + * Unless terminating, forks task if within an ongoing FJ + * computation in the current pool, else submits as external task. + */ + private void forkOrSubmit(ForkJoinTask task) { + ForkJoinWorkerThread w; + Thread t = Thread.currentThread(); + if (shutdown) + throw new RejectedExecutionException(); + if ((t instanceof ForkJoinWorkerThread) && + (w = (ForkJoinWorkerThread)t).pool == this) + w.pushTask(task); + else + addSubmission(task); } /** * Arranges for (asynchronous) execution of the given task. - * If the caller is already engaged in a fork/join computation in - * the current pool, this method is equivalent in effect to - * {@link ForkJoinTask#fork}. * * @param task the task * @throws NullPointerException if the task is null @@ -1371,7 +1515,9 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public void execute(ForkJoinTask task) { - doSubmit(task); + if (task == null) + throw new NullPointerException(); + forkOrSubmit(task); } // AbstractExecutorService methods @@ -1382,19 +1528,18 @@ 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); + forkOrSubmit(job); } /** * Submits a ForkJoinTask for execution. - * If the caller is already engaged in a fork/join computation in - * the current pool, this method is equivalent in effect to - * {@link ForkJoinTask#fork}. * * @param task the task to submit * @return the task @@ -1403,7 +1548,9 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public ForkJoinTask submit(ForkJoinTask task) { - doSubmit(task); + if (task == null) + throw new NullPointerException(); + forkOrSubmit(task); return task; } @@ -1413,8 +1560,10 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public ForkJoinTask submit(Callable task) { + if (task == null) + throw new NullPointerException(); ForkJoinTask job = ForkJoinTask.adapt(task); - doSubmit(job); + forkOrSubmit(job); return job; } @@ -1424,8 +1573,10 @@ public class ForkJoinPool extends Abstra * scheduled for execution */ public ForkJoinTask submit(Runnable task, T result) { + if (task == null) + throw new NullPointerException(); ForkJoinTask job = ForkJoinTask.adapt(task, result); - doSubmit(job); + forkOrSubmit(job); return job; } @@ -1435,12 +1586,14 @@ 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); + forkOrSubmit(job); return job; } @@ -1500,14 +1653,14 @@ public class ForkJoinPool extends Abstra /** * 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; + return parallelism + (short)(ctl >>> TC_SHIFT); } /** @@ -1529,7 +1682,8 @@ public class ForkJoinPool extends Abstra * @return the number of worker threads */ public int getRunningThreadCount() { - return workerCounts & RUNNING_COUNT_MASK; + int r = parallelism + (int)(ctl >> AC_SHIFT); + return r <= 0? 0 : r; // suppress momentarily negative values } /** @@ -1540,7 +1694,8 @@ public class ForkJoinPool extends Abstra * @return the number of active threads */ public int getActiveThreadCount() { - return runState & ACTIVE_COUNT_MASK; + int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount; + return r <= 0? 0 : r; // suppress momentarily negative values } /** @@ -1555,7 +1710,7 @@ public class ForkJoinPool extends Abstra * @return {@code true} if all threads are currently idle */ public boolean isQuiescent() { - return (runState & ACTIVE_COUNT_MASK) == 0; + return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0; } /** @@ -1585,25 +1740,25 @@ 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(); + ForkJoinWorkerThread[] ws; + if ((short)(ctl >>> TC_SHIFT) > -parallelism && + (ws = workers) != null) { + for (ForkJoinWorkerThread w : ws) + if (w != null) + count -= w.queueBase - w.queueTop; // must read base first } 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(); + return -queueBase + queueTop; } /** @@ -1613,7 +1768,7 @@ public class ForkJoinPool extends Abstra * @return {@code true} if there are any queued submissions */ public boolean hasQueuedSubmissions() { - return !submissionQueue.isEmpty(); + return queueBase != queueTop; } /** @@ -1624,7 +1779,19 @@ public class ForkJoinPool extends Abstra * @return the next submission, or {@code null} if none */ protected ForkJoinTask pollSubmission() { - return submissionQueue.poll(); + ForkJoinTask t; ForkJoinTask[] q; int b, i; + while ((b = queueBase) != queueTop && + (q = submissionQueue) != null && + (i = (q.length - 1) & b) >= 0) { + long u = (i << ASHIFT) + ABASE; + if ((t = q[i]) != null && + queueBase == b && + UNSAFE.compareAndSwapObject(q, u, t, null)) { + queueBase = b + 1; + return t; + } + } + return null; } /** @@ -1645,29 +1812,20 @@ 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); - } - return n; - } - - /** - * Returns count of total parks by existing workers. - * Used during development only since not meaningful to users. - */ - private int collectParkCount() { int 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.parkCount; + while (queueBase != queueTop) { + ForkJoinTask t = pollSubmission(); + if (t != null) { + c.add(t); + ++count; + } + } + ForkJoinWorkerThread[] ws; + if ((short)(ctl >>> TC_SHIFT) > -parallelism && + (ws = workers) != null) { + for (ForkJoinWorkerThread w : ws) + if (w != null) + count += w.drainTasksTo(c); } return count; } @@ -1683,15 +1841,20 @@ public class ForkJoinPool extends Abstra 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; - // int pk = collectParkCount(); + long c = ctl; + int tc = pc + (short)(c >>> TC_SHIFT); + int rc = pc + (int)(c >> AC_SHIFT); + if (rc < 0) // ignore transient negative + rc = 0; + int ac = rc + blockedCount; + String level; + if ((c & STOP_BIT) != 0) + level = (tc == 0)? "Terminated" : "Terminating"; + else + level = shutdown? "Shutting down" : "Running"; return super.toString() + - "[" + runLevelToString(rs) + + "[" + level + ", parallelism = " + pc + ", size = " + tc + ", active = " + ac + @@ -1699,17 +1862,9 @@ public class ForkJoinPool extends Abstra ", steals = " + st + ", tasks = " + qt + ", submissions = " + qs + - // ", parks = " + pk + "]"; } - 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. @@ -1724,7 +1879,7 @@ public class ForkJoinPool extends Abstra */ public void shutdown() { checkPermission(); - advanceRunLevel(SHUTDOWN); + shutdown = true; tryTerminate(false); } @@ -1746,6 +1901,7 @@ public class ForkJoinPool extends Abstra */ public List shutdownNow() { checkPermission(); + shutdown = true; tryTerminate(true); return Collections.emptyList(); } @@ -1756,7 +1912,9 @@ 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) == -parallelism); } /** @@ -1764,13 +1922,25 @@ 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 IO, + * 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) != -parallelism); + } + + /** + * Returns true if terminating or terminated. Used by ForkJoinWorkerThread. + */ + final boolean isAtLeastTerminating() { + return (ctl & STOP_BIT) != 0L; } /** @@ -1779,7 +1949,7 @@ public class ForkJoinPool extends Abstra * @return {@code true} if this pool has been shut down */ public boolean isShutdown() { - return runState >= SHUTDOWN; + return shutdown; } /** @@ -1795,10 +1965,19 @@ public class ForkJoinPool extends Abstra */ public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException { + long nanos = unit.toNanos(timeout); + final ReentrantLock lock = this.submissionLock; + lock.lock(); try { - return termination.awaitAdvanceInterruptibly(0, timeout, unit) > 0; - } catch(TimeoutException ex) { - return false; + for (;;) { + if (isTerminated()) + return true; + if (nanos <= 0) + return false; + nanos = termination.awaitNanos(nanos); + } + } finally { + lock.unlock(); } } @@ -1806,11 +1985,19 @@ public class ForkJoinPool extends Abstra * 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: @@ -1828,6 +2015,26 @@ 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 { /** @@ -1870,8 +2077,10 @@ public class ForkJoinPool extends Abstra public static void managedBlock(ManagedBlocker blocker) throws InterruptedException { Thread t = Thread.currentThread(); - if (t instanceof ForkJoinWorkerThread) - ((ForkJoinWorkerThread) t).pool.awaitBlocker(blocker); + if (t instanceof ForkJoinWorkerThread) { + ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; + w.pool.awaitBlocker(blocker); + } else { do {} while (!blocker.isReleasable() && !blocker.block()); } @@ -1890,28 +2099,47 @@ public class ForkJoinPool extends Abstra } // Unsafe mechanics - - 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); - - private static long objectFieldOffset(String field, Class klazz) { + private static final sun.misc.Unsafe UNSAFE; + private static final long ctlOffset; + private static final long stealCountOffset; + private static final long blockedCountOffset; + private static final long quiescerCountOffset; + private static final long scanGuardOffset; + private static final long nextWorkerNumberOffset; + private static final long ABASE; + private static final int ASHIFT; + + static { + poolNumberGenerator = new AtomicInteger(); + workerSeedGenerator = new Random(); + modifyThreadPermission = new RuntimePermission("modifyThread"); + defaultForkJoinWorkerThreadFactory = + new DefaultForkJoinWorkerThreadFactory(); + int s; 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; - } + UNSAFE = getUnsafe(); + Class k = ForkJoinPool.class; + ctlOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("ctl")); + stealCountOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("stealCount")); + blockedCountOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("blockedCount")); + quiescerCountOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("quiescerCount")); + scanGuardOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("scanGuard")); + nextWorkerNumberOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("nextWorkerNumber")); + Class a = ForkJoinTask[].class; + ABASE = UNSAFE.arrayBaseOffset(a); + s = UNSAFE.arrayIndexScale(a); + } catch (Exception e) { + throw new Error(e); + } + if ((s & (s-1)) != 0) + throw new Error("data type scale not a power of two"); + ASHIFT = 31 - Integer.numberOfLeadingZeros(s); } /**