--- jsr166/src/jsr166y/ForkJoinPool.java 2009/12/05 11:39:03 1.52
+++ jsr166/src/jsr166y/ForkJoinPool.java 2010/08/11 18:45:12 1.61
@@ -13,16 +13,15 @@ import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
-import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.atomic.AtomicInteger;
-import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.CountDownLatch;
/**
* An {@link ExecutorService} for running {@link ForkJoinTask}s.
* A {@code ForkJoinPool} provides the entry point for submissions
- * from non-{@code ForkJoinTask}s, as well as management and
+ * from non-{@code ForkJoinTask} clients, as well as management and
* monitoring operations.
*
*
A {@code ForkJoinPool} differs from other kinds of {@link
@@ -31,29 +30,19 @@ import java.util.concurrent.atomic.Atomi
* execute subtasks created by other active tasks (eventually blocking
* waiting for work if none exist). This enables efficient processing
* when most tasks spawn other subtasks (as do most {@code
- * ForkJoinTask}s). A {@code ForkJoinPool} may also be used for mixed
- * execution of some plain {@code Runnable}- or {@code Callable}-
- * based activities along with {@code ForkJoinTask}s. When setting
- * {@linkplain #setAsyncMode async mode}, a {@code ForkJoinPool} may
- * also be appropriate for use with fine-grained tasks of any form
- * that are never joined. Otherwise, other {@code ExecutorService}
- * implementations are typically more appropriate choices.
+ * ForkJoinTask}s). When setting asyncMode to true in
+ * constructors, {@code ForkJoinPool}s may also be appropriate for use
+ * with event-style tasks that are never joined.
*
*
A {@code ForkJoinPool} is constructed with a given target
* parallelism level; by default, equal to the number of available
- * processors. Unless configured otherwise via {@link
- * #setMaintainsParallelism}, the pool attempts to maintain this
- * number of active (or available) threads by dynamically adding,
- * suspending, or resuming internal worker threads, even if some tasks
- * are stalled waiting to join others. However, no such adjustments
- * are performed in the face of blocked IO or other unmanaged
- * synchronization. The nested {@link ManagedBlocker} interface
- * enables extension of the kinds of synchronization accommodated.
- * The target parallelism level may also be changed dynamically
- * ({@link #setParallelism}). The total number of threads may be
- * limited using method {@link #setMaximumPoolSize}, in which case it
- * may become possible for the activities of a pool to stall due to
- * the lack of available threads to process new tasks.
+ * processors. The pool attempts to maintain enough active (or
+ * available) threads by dynamically adding, suspending, or resuming
+ * internal worker threads, even if some tasks are stalled waiting to
+ * join others. However, no such adjustments are guaranteed in the
+ * face of blocked IO or other unmanaged synchronization. The nested
+ * {@link ManagedBlocker} interface enables extension of the kinds of
+ * synchronization accommodated.
*
*
In addition to execution and lifecycle control methods, this
* class provides status check methods (for example
@@ -62,6 +51,40 @@ import java.util.concurrent.atomic.Atomi
* {@link #toString} returns indications of pool state in a
* convenient form for informal monitoring.
*
+ *
As is the case with other ExecutorServices, there are three
+ * main task execution methods summarized in the following
+ * table. These are designed to be used by clients not already engaged
+ * in fork/join computations in the current pool. The main forms of
+ * these methods accept instances of {@code ForkJoinTask}, but
+ * overloaded forms also allow mixed execution of plain {@code
+ * Runnable}- or {@code Callable}- based activities as well. However,
+ * tasks that are already executing in a pool should normally
+ * NOT use these pool execution methods, but instead use the
+ * within-computation forms listed in the table.
+ *
+ *
+ *
+ * |
+ * Call from non-fork/join clients |
+ * Call from within fork/join computations |
+ *
+ *
+ * Arange async execution |
+ * {@link #execute(ForkJoinTask)} |
+ * {@link ForkJoinTask#fork} |
+ *
+ *
+ * Await and obtain result |
+ * {@link #invoke(ForkJoinTask)} |
+ * {@link ForkJoinTask#invoke} |
+ *
+ *
+ * Arrange exec and obtain Future |
+ * {@link #submit(ForkJoinTask)} |
+ * {@link ForkJoinTask#fork} (ForkJoinTasks are Futures) |
+ *
+ *
+ *
* 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
@@ -86,7 +109,8 @@ import java.util.concurrent.atomic.Atomi
* {@code IllegalArgumentException}.
*
*
This implementation rejects submitted tasks (that is, by throwing
- * {@link RejectedExecutionException}) only when the pool is shut down.
+ * {@link RejectedExecutionException}) only when the pool is shut down
+ * or internal resources have been exhuasted.
*
* @since 1.7
* @author Doug Lea
@@ -94,16 +118,248 @@ import java.util.concurrent.atomic.Atomi
public class ForkJoinPool extends AbstractExecutorService {
/*
- * See the extended comments interspersed below for design,
- * rationale, and walkthroughs.
+ * Implementation Overview
+ *
+ * This class provides the central bookkeeping and control for a
+ * set of worker threads: Submissions from non-FJ threads enter
+ * into a submission queue. Workers take these tasks and typically
+ * split them into subtasks that may be stolen by other workers.
+ * The main work-stealing mechanics implemented in class
+ * ForkJoinWorkerThread give first priority to processing tasks
+ * from their own queues (LIFO or FIFO, depending on mode), then
+ * to randomized FIFO steals of tasks in other worker queues, and
+ * lastly to new submissions. These mechanics do not consider
+ * affinities, loads, cache localities, etc, so rarely provide the
+ * best possible performance on a given machine, but portably
+ * provide good throughput by averaging over these factors.
+ * (Further, even if we did try to use such information, we do not
+ * usually have a basis for exploiting it. For example, some sets
+ * of tasks profit from cache affinities, but others are harmed by
+ * cache pollution effects.)
+ *
+ * Beyond work-stealing support and essential bookkeeping, 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. 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:
+ *
+ * Helping: 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.
+ *
+ * Compensating: Unless there are already enough live threads,
+ * method helpMaintainParallelism() may create or or
+ * re-activate a spare thread to compensate for blocked
+ * joiners until they unblock.
+ *
+ * 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, we rely on multiple retries of
+ * each. Further, because it is impossible to keep exactly the
+ * target (parallelism) number of threads running at any given
+ * time, we allow compensation during joins to fail, and enlist
+ * all other threads to help out whenever they are not otherwise
+ * occupied (i.e., mainly in method preStep).
+ *
+ * The ManagedBlocker extension API can't use helping so relies
+ * only on compensation in method awaitBlocker.
+ *
+ * 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
+ * structure to support index-based random steals by workers.
+ * Updates to the array recording new workers and unrecording
+ * terminated ones are protected from each other by a lock
+ * (workerLock) but the array is otherwise concurrently readable,
+ * and accessed directly by workers. To simplify index-based
+ * operations, the array size is always a power of two, and all
+ * readers must tolerate null slots. Currently, all worker thread
+ * creation is on-demand, triggered by task submissions,
+ * replacement of terminated workers, and/or compensation for
+ * blocked workers. However, all other support code is set up to
+ * work with other policies.
+ *
+ * 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 unusual
+ * code constructions here). In essence, the workers array serves
+ * as a WeakReference mechanism. Thus for example the event queue
+ * stores worker indices, not worker references. Access to the
+ * workers in associated methods (for example releaseEventWaiters)
+ * 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 shutdown, in which case
+ * it is OK to give up. On termination, we just clobber these
+ * data structures without trying to use them.
+ *
+ * 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 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
+ * releaseEventWaiters, that removes and signals (unparks) workers
+ * not waiting on current count. To reduce stalls, 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. Method helpMaintainParallelism looks for
+ * suspended threads to resume before considering creating a new
+ * replacement. The spares themselves are encoded on another
+ * variant of a Treiber Stack, headed at field "spareWaiters".
+ * Note that the use of spares 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. To avoid
+ * long-term build-up of spares, the oldest spare (see
+ * ForkJoinWorkerThread.suspendAsSpare) occasionally wakes up if
+ * not signalled and calls tryTrimSpare, which uses two different
+ * thresholds: Always killing if the number of spares is greater
+ * that 25% of total, and killing others only at a slower rate
+ * (UNUSED_SPARE_TRIM_RATE_NANOS).
+ *
+ * 6. Deciding when to create new workers. The main dynamic
+ * control in this class is deciding when to create extra threads
+ * in method helpMaintainParallelism. We would like to keep
+ * exactly #parallelism threads running, which is an impossble
+ * task. We always need to create one when the number of running
+ * threads would become zero and all workers are busy. Beyond
+ * this, we must rely on heuristics that work well in the the
+ * presence of transients phenomena such as GC stalls, dynamic
+ * compilation, and wake-up lags. These transients are extremely
+ * common -- we are normally trying to fully saturate the CPUs on
+ * a machine, so almost any activity other than running tasks
+ * impedes accuracy. Our main defense is to allow some slack in
+ * creation thresholds, using rules that reflect the fact that the
+ * more threads we have running, the more likely that we are
+ * underestimating the number running threads. The rules also
+ * better cope 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 100
+ * 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.
+ *
+ * Beware that there is a lot of representation-level coupling
+ * among classes ForkJoinPool, ForkJoinWorkerThread, and
+ * ForkJoinTask. For example, direct access to "workers" array by
+ * workers, and direct access to ForkJoinTask.status by both
+ * ForkJoinPool and ForkJoinWorkerThread. There is little point
+ * 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 the required read orderings (which are sometimes
+ * critical). 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 coding oddities that
+ * help some methods perform reasonably even when interpreted (not
+ * compiled), at the expense of some messy constructions that
+ * reduce byte code counts.
+ *
+ * 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).
*/
- /** Mask for packing and unpacking shorts */
- private static final int shortMask = 0xffff;
-
- /** Max pool size -- must be a power of two minus 1 */
- private static final int MAX_THREADS = 0x7FFF;
-
/**
* Factory for creating new {@link ForkJoinWorkerThread}s.
* A {@code ForkJoinWorkerThreadFactory} must be defined and used
@@ -124,14 +380,10 @@ public class ForkJoinPool extends Abstra
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
- static class DefaultForkJoinWorkerThreadFactory
+ static class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
- try {
- return new ForkJoinWorkerThread(pool);
- } catch (OutOfMemoryError oom) {
- return null;
- }
+ return new ForkJoinWorkerThread(pool);
}
}
@@ -167,29 +419,37 @@ public class ForkJoinPool extends Abstra
new AtomicInteger();
/**
- * Array holding all worker threads in the pool. Initialized upon
- * first use. Array size must be a power of two. Updates and
- * replacements are protected by workerLock, but it is always kept
- * in a consistent enough state to be randomly accessed without
- * locking by workers performing work-stealing.
+ * Absolute bound for parallelism level. Twice this number plus
+ * one (i.e., 0xfff) must fit into a 16bit field to enable
+ * word-packing for some counts and indices.
+ */
+ private static final int MAX_WORKERS = 0x7fff;
+
+ /**
+ * Array holding all worker threads in the pool. Array size must
+ * be a power of two. Updates and replacements are protected by
+ * workerLock, but the array is always kept in a consistent enough
+ * state to be randomly accessed without locking by workers
+ * performing work-stealing, as well as other traversal-based
+ * methods in this class. All readers must tolerate that some
+ * array slots may be null.
*/
volatile ForkJoinWorkerThread[] workers;
/**
- * Lock protecting access to workers.
+ * Queue for external submissions.
*/
- private final ReentrantLock workerLock;
+ private final LinkedTransferQueue> submissionQueue;
/**
- * Condition for awaitTermination.
+ * Lock protecting updates to workers array.
*/
- private final Condition termination;
+ private final ReentrantLock workerLock;
/**
- * The uncaught exception handler used when any worker
- * abruptly terminates
+ * Latch released upon termination.
*/
- private Thread.UncaughtExceptionHandler ueh;
+ private final Phaser termination;
/**
* Creation factory for worker threads.
@@ -197,387 +457,803 @@ public class ForkJoinPool extends Abstra
private final ForkJoinWorkerThreadFactory factory;
/**
- * Head of stack of threads that were created to maintain
- * parallelism when other threads blocked, but have since
- * suspended when the parallelism level rose.
- */
- private volatile WaitQueueNode spareStack;
-
- /**
* Sum of per-thread steal counts, updated only when threads are
* idle or terminating.
*/
- private final AtomicLong stealCount;
+ private volatile long stealCount;
/**
- * Queue for external submissions.
+ * The last nanoTime that a spare thread was trimmed
*/
- private final LinkedTransferQueue> submissionQueue;
+ private volatile long trimTime;
/**
- * Head of Treiber stack for barrier sync. See below for explanation.
+ * The rate at which to trim unused spares
*/
- private volatile WaitQueueNode syncStack;
+ static final long UNUSED_SPARE_TRIM_RATE_NANOS =
+ 1000L * 1000L * 1000L; // 1 sec
/**
- * The count for event barrier
+ * Encoded record of top of treiber stack of threads waiting for
+ * events. The top 32 bits contain the count being waited for. The
+ * bottom 16 bits contains one plus the pool index of waiting
+ * worker thread. (Bits 16-31 are unused.)
*/
- private volatile long eventCount;
+ private volatile long eventWaiters;
+
+ private static final int EVENT_COUNT_SHIFT = 32;
+ private static final long WAITER_ID_MASK = (1L << 16) - 1L;
/**
- * Pool number, just for assigning useful names to worker threads
+ * 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
*/
- private final int poolNumber;
+ private volatile int eventCount;
+
+ /**
+ * Encoded record of top of treiber stack of spare threads waiting
+ * for resumption. The top 16 bits contain an arbitrary count to
+ * avoid ABA effects. The bottom 16bits contains one plus the pool
+ * index of waiting worker thread.
+ */
+ private volatile int spareWaiters;
+
+ private static final int SPARE_COUNT_SHIFT = 16;
+ private static final int SPARE_ID_MASK = (1 << 16) - 1;
+
+ /**
+ * 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
/**
- * The maximum allowed pool size
+ * 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.
*/
- private volatile int maxPoolSize;
+ 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;
/**
- * The desired parallelism level, updated only under workerLock.
+ * The target parallelism level.
+ * Accessed directly by ForkJoinWorkerThreads.
*/
- private volatile int parallelism;
+ final int parallelism;
/**
* True if use local fifo, not default lifo, for local polling
+ * Read by, and replicated by ForkJoinWorkerThreads
*/
- private volatile boolean locallyFifo;
+ final boolean locallyFifo;
/**
- * 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 into one int to ensure consistent snapshot when
- * making decisions about creating and suspending spare
- * threads. Updated only by CAS. Note: CASes in
- * updateRunningCount and preJoin assume that running active count
- * is in low word, so need to be modified if this changes.
+ * The uncaught exception handler used when any worker abruptly
+ * terminates.
*/
- private volatile int workerCounts;
+ private final Thread.UncaughtExceptionHandler ueh;
+
+ /**
+ * Pool number, just for assigning useful names to worker threads
+ */
+ private final int poolNumber;
+
- private static int totalCountOf(int s) { return s >>> 16; }
- private static int runningCountOf(int s) { return s & shortMask; }
- private static int workerCountsFor(int t, int r) { return (t << 16) + r; }
+ // Utilities for CASing fields. Note that several of these
+ // are manually inlined by callers
/**
- * Adds delta (which may be negative) to running count. This must
- * be called before (with negative arg) and after (with positive)
- * any managed synchronization (i.e., mainly, joins).
- *
- * @param delta the number to add
+ * Increments running count part of workerCounts
*/
- final void updateRunningCount(int delta) {
- int s;
- do {} while (!casWorkerCounts(s = workerCounts, s + delta));
+ final void incrementRunningCount() {
+ int c;
+ do {} while (!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ c = workerCounts,
+ c + ONE_RUNNING));
}
/**
- * Adds delta (which may be negative) to both total and running
- * count. This must be called upon creation and termination of
- * worker threads.
- *
- * @param delta the number to add
+ * Tries to decrement running count unless already zero
*/
- private void updateWorkerCount(int delta) {
- int d = delta + (delta << 16); // add to both lo and hi parts
- int s;
- do {} while (!casWorkerCounts(s = workerCounts, s + d));
+ final boolean tryDecrementRunningCount() {
+ int wc = workerCounts;
+ if ((wc & RUNNING_COUNT_MASK) == 0)
+ return false;
+ return UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ wc, wc - ONE_RUNNING);
}
/**
- * Lifecycle control. High word contains runState, 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. These two fields are bundled together to
- * support correct termination triggering. Note: activeCount
- * CAS'es cheat by assuming active count is in low word, so need
- * to be modified if this changes
+ * Forces decrement of encoded workerCounts, awaiting nonzero if
+ * (rarely) necessary when other count updates lag.
+ *
+ * @param dr -- either zero or ONE_RUNNING
+ * @param dt == either zero or ONE_TOTAL
*/
- private volatile int runControl;
-
- // RunState values. Order among values matters
- private static final int RUNNING = 0;
- private static final int SHUTDOWN = 1;
- private static final int TERMINATING = 2;
- private static final int TERMINATED = 3;
+ private void decrementWorkerCounts(int dr, int dt) {
+ for (;;) {
+ int wc = workerCounts;
+ if (wc == 0 && (runState & TERMINATED) != 0)
+ return; // lagging termination on a backout
+ if ((wc & RUNNING_COUNT_MASK) - dr < 0 ||
+ (wc >>> TOTAL_COUNT_SHIFT) - dt < 0)
+ Thread.yield();
+ if (UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ wc, wc - (dr + dt)))
+ return;
+ }
+ }
- private static int runStateOf(int c) { return c >>> 16; }
- private static int activeCountOf(int c) { return c & shortMask; }
- private static int runControlFor(int r, int a) { return (r << 16) + a; }
+ /**
+ * Increments event count
+ */
+ private void advanceEventCount() {
+ int c;
+ do {} while(!UNSAFE.compareAndSwapInt(this, eventCountOffset,
+ c = eventCount, c+1));
+ }
/**
* Tries incrementing active count; fails on contention.
- * Called by workers before/during executing tasks.
+ * Called by workers before executing tasks.
*
* @return true on success
*/
final boolean tryIncrementActiveCount() {
- int c = runControl;
- return casRunControl(c, c+1);
+ int c;
+ return UNSAFE.compareAndSwapInt(this, runStateOffset,
+ c = runState, c + ONE_ACTIVE);
}
/**
* Tries decrementing active count; fails on contention.
- * Possibly triggers termination on success.
- * Called by workers when they can't find tasks.
- *
- * @return true on success
+ * Called when workers cannot find tasks to run.
*/
final boolean tryDecrementActiveCount() {
- int c = runControl;
- int nextc = c - 1;
- if (!casRunControl(c, nextc))
- return false;
- if (canTerminateOnShutdown(nextc))
- terminateOnShutdown();
- return true;
+ int c;
+ return UNSAFE.compareAndSwapInt(this, runStateOffset,
+ c = runState, c - ONE_ACTIVE);
}
/**
- * Returns {@code true} if argument represents zero active count
- * and nonzero runstate, which is the triggering condition for
- * terminating on shutdown.
+ * Advances to at least the given level. Returns true if not
+ * already in at least the given level.
*/
- private static boolean canTerminateOnShutdown(int c) {
- // i.e. least bit is nonzero runState bit
- return ((c & -c) >>> 16) != 0;
+ private boolean advanceRunLevel(int level) {
+ for (;;) {
+ int s = runState;
+ if ((s & level) != 0)
+ return false;
+ if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, s | level))
+ return true;
+ }
}
+ // workers array maintenance
+
/**
- * Transition run state to at least the given state. Return true
- * if not already at least given state.
+ * Records and returns a workers array index for new worker.
*/
- private boolean transitionRunStateTo(int state) {
- for (;;) {
- int c = runControl;
- if (runStateOf(c) >= state)
- return false;
- if (casRunControl(c, runControlFor(state, activeCountOf(c))))
- return true;
+ 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 n = ws.length;
+ if (k < 0 || k >= n || ws[k] != null) {
+ for (k = 0; k < n && ws[k] != null; ++k)
+ ;
+ if (k == n)
+ ws = Arrays.copyOf(ws, n << 1);
+ }
+ ws[k] = w;
+ workers = ws; // volatile array write ensures slot visibility
+ } finally {
+ lock.unlock();
}
+ return k;
}
/**
- * Controls whether to add spares to maintain parallelism
+ * Nulls out record of worker in workers array
*/
- private volatile boolean maintainsParallelism;
+ 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();
+ }
+ }
- // Constructors
+ // adding and removing workers
/**
- * Creates a {@code ForkJoinPool} with parallelism equal to {@link
- * java.lang.Runtime#availableProcessors}, and using the {@linkplain
- * #defaultForkJoinWorkerThreadFactory default thread factory}.
- *
- * @throws SecurityException if a security manager exists and
- * the caller is not permitted to modify threads
- * because it does not hold {@link
- * java.lang.RuntimePermission}{@code ("modifyThread")}
+ * Tries to create and add new worker. Assumes that worker counts
+ * are already updated to accommodate the worker, so adjusts on
+ * failure.
*/
- public ForkJoinPool() {
- this(Runtime.getRuntime().availableProcessors(),
- defaultForkJoinWorkerThreadFactory);
+ private void addWorker() {
+ ForkJoinWorkerThread w = null;
+ try {
+ w = factory.newThread(this);
+ } finally { // Adjust on either null or exceptional factory return
+ if (w == null) {
+ decrementWorkerCounts(ONE_RUNNING, ONE_TOTAL);
+ tryTerminate(false); // in case of failure during shutdown
+ }
+ }
+ if (w != null)
+ w.start(recordWorker(w), ueh);
}
/**
- * Creates a {@code ForkJoinPool} with the indicated parallelism
- * level and using the {@linkplain
- * #defaultForkJoinWorkerThreadFactory default thread factory}.
+ * Final callback from terminating worker. Removes record of
+ * worker from array, and adjusts counts. If pool is shutting
+ * down, tries to complete terminatation.
*
- * @param parallelism the parallelism level
- * @throws IllegalArgumentException if parallelism less than or
- * equal to zero, or greater than implementation limit
- * @throws SecurityException if a security manager exists and
- * the caller is not permitted to modify threads
- * because it does not hold {@link
- * java.lang.RuntimePermission}{@code ("modifyThread")}
+ * @param w the worker
*/
- public ForkJoinPool(int parallelism) {
- this(parallelism, defaultForkJoinWorkerThreadFactory);
+ final void workerTerminated(ForkJoinWorkerThread w) {
+ forgetWorker(w);
+ decrementWorkerCounts(w.isTrimmed()? 0 : ONE_RUNNING, ONE_TOTAL);
+ while (w.stealCount != 0) // collect final count
+ tryAccumulateStealCount(w);
+ tryTerminate(false);
}
+ // Waiting for and signalling events
+
/**
- * Creates a {@code ForkJoinPool} with parallelism equal to {@link
- * java.lang.Runtime#availableProcessors}, and using the given
- * thread factory.
+ * Releases workers blocked on a count not equal to current count.
+ * Normally called after precheck that eventWaiters isn't zero to
+ * avoid wasted array checks.
*
- * @param factory the factory for creating new threads
- * @throws NullPointerException if the factory is null
- * @throws SecurityException if a security manager exists and
- * the caller is not permitted to modify threads
- * because it does not hold {@link
- * java.lang.RuntimePermission}{@code ("modifyThread")}
+ * @param signalling true if caller is a signalling worker so can
+ * exit upon (conservatively) detected contention by other threads
+ * who will continue to release
*/
- public ForkJoinPool(ForkJoinWorkerThreadFactory factory) {
- this(Runtime.getRuntime().availableProcessors(), factory);
+ private void releaseEventWaiters(boolean signalling) {
+ ForkJoinWorkerThread[] ws = workers;
+ int n = ws.length;
+ long h; // head of stack
+ ForkJoinWorkerThread w; int id, ec;
+ while ((id = ((int)((h = eventWaiters) & WAITER_ID_MASK)) - 1) >= 0 &&
+ (int)(h >>> EVENT_COUNT_SHIFT) != (ec = eventCount) &&
+ id < n && (w = ws[id]) != null) {
+ if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
+ h, h = w.nextWaiter))
+ LockSupport.unpark(w);
+ if (signalling && (eventCount != ec || eventWaiters != h))
+ break;
+ }
}
/**
- * Creates a {@code ForkJoinPool} with the given parallelism and
- * thread factory.
- *
- * @param parallelism the parallelism level
- * @param factory the factory for creating new threads
- * @throws IllegalArgumentException if parallelism less than or
- * equal to zero, or greater than implementation limit
- * @throws NullPointerException if the factory is null
- * @throws SecurityException if a security manager exists and
- * the caller is not permitted to modify threads
- * because it does not hold {@link
- * java.lang.RuntimePermission}{@code ("modifyThread")}
+ * Tries to advance eventCount and releases waiters. Called only
+ * from workers.
*/
- public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory) {
- if (parallelism <= 0 || parallelism > MAX_THREADS)
- throw new IllegalArgumentException();
- if (factory == null)
- throw new NullPointerException();
- checkPermission();
- this.factory = factory;
- this.parallelism = parallelism;
- this.maxPoolSize = MAX_THREADS;
- this.maintainsParallelism = true;
- this.poolNumber = poolNumberGenerator.incrementAndGet();
- this.workerLock = new ReentrantLock();
- this.termination = workerLock.newCondition();
- this.stealCount = new AtomicLong();
- this.submissionQueue = new LinkedTransferQueue>();
- // worker array and workers are lazily constructed
+ final void signalWork() {
+ int c; // try to increment event count -- CAS failure OK
+ UNSAFE.compareAndSwapInt(this, eventCountOffset, c = eventCount, c+1);
+ if (eventWaiters != 0L)
+ releaseEventWaiters(true);
}
/**
- * Creates a new worker thread using factory.
+ * Blocks worker until terminating or event count
+ * advances from last value held by worker
*
- * @param index the index to assign worker
- * @return new worker, or null if factory failed
+ * @param w the calling worker thread
*/
- private ForkJoinWorkerThread createWorker(int index) {
- Thread.UncaughtExceptionHandler h = ueh;
- ForkJoinWorkerThread w = factory.newThread(this);
- if (w != null) {
- w.poolIndex = index;
- w.setDaemon(true);
- w.setAsyncMode(locallyFifo);
- w.setName("ForkJoinPool-" + poolNumber + "-worker-" + index);
- if (h != null)
- w.setUncaughtExceptionHandler(h);
+ private void eventSync(ForkJoinWorkerThread w) {
+ int wec = w.lastEventCount;
+ long nh = (((long)wec) << EVENT_COUNT_SHIFT) | ((long)(w.poolIndex+1));
+ long h;
+ while ((runState < SHUTDOWN || !tryTerminate(false)) &&
+ ((h = eventWaiters) == 0L ||
+ (int)(h >>> EVENT_COUNT_SHIFT) == wec) &&
+ eventCount == wec) {
+ if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
+ w.nextWaiter = h, nh)) {
+ while (runState < TERMINATING && eventCount == wec) {
+ if (!tryAccumulateStealCount(w)) // transfer while idle
+ continue;
+ Thread.interrupted(); // clear/ignore interrupt
+ if (eventCount != wec)
+ break;
+ LockSupport.park(w);
+ }
+ break;
+ }
}
- return w;
+ w.lastEventCount = eventCount;
}
+ // Maintaining spares
+
/**
- * Returns a good size for worker array given pool size.
- * Currently requires size to be a power of two.
+ * Pushes worker onto the spare stack
*/
- private static int arraySizeFor(int poolSize) {
- if (poolSize <= 1)
- return 1;
- // See Hackers Delight, sec 3.2
- int c = poolSize >= MAX_THREADS ? MAX_THREADS : (poolSize - 1);
- c |= c >>> 1;
- c |= c >>> 2;
- c |= c >>> 4;
- c |= c >>> 8;
- c |= c >>> 16;
- return c + 1;
+ final void pushSpare(ForkJoinWorkerThread w) {
+ int ns = (++w.spareCount << SPARE_COUNT_SHIFT) | (w.poolIndex+1);
+ do {} while (!UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
+ w.nextSpare = spareWaiters,ns));
}
/**
- * Creates or resizes array if necessary to hold newLength.
- * Call only under exclusion.
- *
- * @return the array
+ * Tries (once) to resume a spare if running count is less than
+ * target parallelism. Fails on contention or stale workers.
*/
- private ForkJoinWorkerThread[] ensureWorkerArrayCapacity(int newLength) {
- ForkJoinWorkerThread[] ws = workers;
- if (ws == null)
- return workers = new ForkJoinWorkerThread[arraySizeFor(newLength)];
- else if (newLength > ws.length)
- return workers = Arrays.copyOf(ws, arraySizeFor(newLength));
- else
- return ws;
+ private void tryResumeSpare() {
+ int sw, id;
+ ForkJoinWorkerThread w;
+ ForkJoinWorkerThread[] ws;
+ if ((id = ((sw = spareWaiters) & SPARE_ID_MASK) - 1) >= 0 &&
+ id < (ws = workers).length && (w = ws[id]) != null &&
+ (workerCounts & RUNNING_COUNT_MASK) < parallelism &&
+ eventWaiters == 0L &&
+ spareWaiters == sw &&
+ UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
+ sw, w.nextSpare) &&
+ w.tryUnsuspend()) {
+ int c; // try increment; if contended, finish after unpark
+ boolean inc = UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ c = workerCounts,
+ c + ONE_RUNNING);
+ LockSupport.unpark(w);
+ if (!inc) {
+ do {} while(!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ c = workerCounts,
+ c + ONE_RUNNING));
+ }
+ }
}
/**
- * Tries to shrink workers into smaller array after one or more terminate.
+ * Callback from oldest spare occasionally waking up. Tries
+ * (once) to shutdown a spare if more than 25% spare overage, or
+ * if UNUSED_SPARE_TRIM_RATE_NANOS have elapsed and there are at
+ * least #parallelism running threads. Note that we don't need CAS
+ * or locks here because the method is called only from the oldest
+ * suspended spare occasionally waking (and even misfires are OK).
+ *
+ * @param now the wake up nanoTime of caller
*/
- private void tryShrinkWorkerArray() {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- int len = ws.length;
- int last = len - 1;
- while (last >= 0 && ws[last] == null)
- --last;
- int newLength = arraySizeFor(last+1);
- if (newLength < len)
- workers = Arrays.copyOf(ws, newLength);
+ final void tryTrimSpare(long now) {
+ long lastTrim = trimTime;
+ trimTime = now;
+ helpMaintainParallelism(); // first, help wake up any needed spares
+ int sw, id;
+ ForkJoinWorkerThread w;
+ ForkJoinWorkerThread[] ws;
+ int pc = parallelism;
+ int wc = workerCounts;
+ if ((wc & RUNNING_COUNT_MASK) >= pc &&
+ (((wc >>> TOTAL_COUNT_SHIFT) - pc) > (pc >>> 2) + 1 ||// approx 25%
+ now - lastTrim >= UNUSED_SPARE_TRIM_RATE_NANOS) &&
+ (id = ((sw = spareWaiters) & SPARE_ID_MASK) - 1) >= 0 &&
+ id < (ws = workers).length && (w = ws[id]) != null &&
+ UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
+ sw, w.nextSpare))
+ w.shutdown(false);
+ }
+
+ /**
+ * Does at most one of:
+ *
+ * 1. Help wake up existing workers waiting for work via
+ * releaseEventWaiters. (If any exist, then it probably doesn't
+ * matter right now if under target parallelism level.)
+ *
+ * 2. If below parallelism level and a spare exists, try (once)
+ * to resume it via tryResumeSpare.
+ *
+ * 3. If neither of the above, tries (once) to add a new
+ * worker if either there are not enough total, or if all
+ * existing workers are busy, there are either no running
+ * workers or the deficit is at least twice the surplus.
+ */
+ private void helpMaintainParallelism() {
+ // uglified to work better when not compiled
+ int pc, wc, rc, tc, rs; long h;
+ if ((h = eventWaiters) != 0L) {
+ if ((int)(h >>> EVENT_COUNT_SHIFT) != eventCount)
+ releaseEventWaiters(false); // avoid useless call
+ }
+ else if ((pc = parallelism) >
+ (rc = ((wc = workerCounts) & RUNNING_COUNT_MASK))) {
+ if (spareWaiters != 0)
+ tryResumeSpare();
+ else if ((rs = runState) < TERMINATING &&
+ ((tc = wc >>> TOTAL_COUNT_SHIFT) < pc ||
+ (tc == (rs & ACTIVE_COUNT_MASK) && // all busy
+ (rc == 0 || // must add
+ rc < pc - ((tc - pc) << 1)) && // within slack
+ tc < MAX_WORKERS && runState == rs)) && // recheck busy
+ workerCounts == wc &&
+ UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
+ wc + (ONE_RUNNING|ONE_TOTAL)))
+ addWorker();
+ }
+ }
+
+ /**
+ * Callback from workers invoked upon each top-level action (i.e.,
+ * stealing a task or taking a submission and running
+ * it). Performs one or more of the following:
+ *
+ * 1. If the worker cannot find work (misses > 0), updates its
+ * active status to inactive and updates activeCount unless
+ * this is the first miss and there is contention, in which
+ * case it may try again (either in this or a subsequent
+ * call).
+ *
+ * 2. If there are at least 2 misses, awaits the next task event
+ * via eventSync
+ *
+ * 3. If there are too many running threads, suspends this worker
+ * (first forcing inactivation if necessary). If it is not
+ * needed, it may be killed while suspended via
+ * tryTrimSpare. Otherwise, upon resume it rechecks to make
+ * sure that it is still needed.
+ *
+ * 4. Helps release and/or reactivate other workers via
+ * helpMaintainParallelism
+ *
+ * @param w the worker
+ * @param misses the number of scans by caller failing to find work
+ * (saturating at 2 just to avoid wraparound)
+ */
+ final void preStep(ForkJoinWorkerThread w, int misses) {
+ boolean active = w.active;
+ int pc = parallelism;
+ for (;;) {
+ int wc = workerCounts;
+ int rc = wc & RUNNING_COUNT_MASK;
+ if (active && (misses > 0 || rc > pc)) {
+ int rs; // try inactivate
+ if (UNSAFE.compareAndSwapInt(this, runStateOffset,
+ rs = runState, rs - ONE_ACTIVE))
+ active = w.active = false;
+ else if (misses > 1 || rc > pc ||
+ (rs & ACTIVE_COUNT_MASK) >= pc)
+ continue; // force inactivate
+ }
+ if (misses > 1) {
+ misses = 0; // don't re-sync
+ eventSync(w); // continue loop to recheck rc
+ }
+ else if (rc > pc) {
+ if (workerCounts == wc && // try to suspend as spare
+ UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ wc, wc - ONE_RUNNING) &&
+ !w.suspendAsSpare()) // false if killed
+ break;
+ }
+ else {
+ if (rc < pc || eventWaiters != 0L)
+ helpMaintainParallelism();
+ break;
+ }
}
}
/**
- * Initializes workers if necessary.
- */
- final void ensureWorkerInitialization() {
- ForkJoinWorkerThread[] ws = workers;
- if (ws == null) {
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- ws = workers;
- if (ws == null) {
- int ps = parallelism;
- updateWorkerCount(ps);
- ws = ensureWorkerArrayCapacity(ps);
- for (int i = 0; i < ps; ++i) {
- ForkJoinWorkerThread w = createWorker(i);
- if (w != null) {
- ws[i] = w;
- w.start();
- }
- else
- updateWorkerCount(-1);
- }
- }
- } finally {
- lock.unlock();
+ * Helps and/or blocks awaiting join of the given task.
+ * Alternates between helpJoinTask() and helpMaintainParallelism()
+ * as many times as there is a deficit in running count (or longer
+ * if running count would become zero), then blocks if task still
+ * not done.
+ *
+ * @param joinMe the task to join
+ */
+ final void awaitJoin(ForkJoinTask> joinMe, ForkJoinWorkerThread worker) {
+ int threshold = parallelism; // descend blocking thresholds
+ while (joinMe.status >= 0) {
+ boolean block; int wc;
+ worker.helpJoinTask(joinMe);
+ if (joinMe.status < 0)
+ break;
+ if (((wc = workerCounts) & RUNNING_COUNT_MASK) <= threshold) {
+ if (threshold > 0)
+ --threshold;
+ else
+ advanceEventCount(); // force release
+ block = false;
+ }
+ else
+ block = UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ wc, wc - ONE_RUNNING);
+ helpMaintainParallelism();
+ if (block) {
+ int c;
+ joinMe.internalAwaitDone();
+ do {} while (!UNSAFE.compareAndSwapInt
+ (this, workerCountsOffset,
+ c = workerCounts, c + ONE_RUNNING));
+ break;
}
}
}
/**
- * Worker creation and startup for threads added via setParallelism.
+ * Same idea as awaitJoin, but no helping
*/
- private void createAndStartAddedWorkers() {
- resumeAllSpares(); // Allow spares to convert to nonspare
- int ps = parallelism;
- ForkJoinWorkerThread[] ws = ensureWorkerArrayCapacity(ps);
- int len = ws.length;
- // Sweep through slots, to keep lowest indices most populated
- int k = 0;
- while (k < len) {
- if (ws[k] != null) {
- ++k;
- continue;
+ final void awaitBlocker(ManagedBlocker blocker)
+ throws InterruptedException {
+ int threshold = parallelism;
+ while (!blocker.isReleasable()) {
+ boolean block; int wc;
+ if (((wc = workerCounts) & RUNNING_COUNT_MASK) <= threshold) {
+ if (threshold > 0)
+ --threshold;
+ else
+ advanceEventCount();
+ block = false;
}
- int s = workerCounts;
- int tc = totalCountOf(s);
- int rc = runningCountOf(s);
- if (rc >= ps || tc >= ps)
+ else
+ block = UNSAFE.compareAndSwapInt(this, workerCountsOffset,
+ wc, wc - ONE_RUNNING);
+ helpMaintainParallelism();
+ if (block) {
+ try {
+ do {} while (!blocker.isReleasable() && !blocker.block());
+ } finally {
+ int c;
+ do {} while (!UNSAFE.compareAndSwapInt
+ (this, workerCountsOffset,
+ c = workerCounts, c + ONE_RUNNING));
+ }
break;
- if (casWorkerCounts (s, workerCountsFor(tc+1, rc+1))) {
- ForkJoinWorkerThread w = createWorker(k);
+ }
+ }
+ }
+
+ /**
+ * Possibly initiates and/or completes termination.
+ *
+ * @param now if true, unconditionally terminate, else only
+ * if shutdown and empty queue and no active workers
+ * @return true if now terminating or terminated
+ */
+ private boolean tryTerminate(boolean now) {
+ if (now)
+ advanceRunLevel(SHUTDOWN); // ensure at least SHUTDOWN
+ else if (runState < SHUTDOWN ||
+ !submissionQueue.isEmpty() ||
+ (runState & ACTIVE_COUNT_MASK) != 0)
+ return false;
+
+ if (advanceRunLevel(TERMINATING))
+ startTerminating();
+
+ // Finish now if all threads terminated; else in some subsequent call
+ if ((workerCounts >>> TOTAL_COUNT_SHIFT) == 0) {
+ advanceRunLevel(TERMINATED);
+ termination.arrive();
+ }
+ return true;
+ }
+
+ /**
+ * Actions on transition to TERMINATING
+ *
+ * Runs up to four passes through workers: (0) shutting down each
+ * quietly (without waking up if parked) to quickly spread
+ * notifications without unnecessary bouncing around event queues
+ * etc (1) wake up and help cancel tasks (2) interrupt (3) mop up
+ * races with interrupted workers
+ */
+ private void startTerminating() {
+ cancelSubmissions();
+ for (int passes = 0; passes < 4 && workerCounts != 0; ++passes) {
+ advanceEventCount();
+ eventWaiters = 0L; // clobber lists
+ spareWaiters = 0;
+ ForkJoinWorkerThread[] ws = workers;
+ int n = ws.length;
+ for (int i = 0; i < n; ++i) {
+ ForkJoinWorkerThread w = ws[i];
if (w != null) {
- ws[k++] = w;
- w.start();
- }
- else {
- updateWorkerCount(-1); // back out on failed creation
- break;
+ w.shutdown(true);
+ if (passes > 0 && !w.isTerminated()) {
+ w.cancelTasks();
+ LockSupport.unpark(w);
+ if (passes > 1) {
+ try {
+ w.interrupt();
+ } catch (SecurityException ignore) {
+ }
+ }
+ }
}
}
}
}
+ /**
+ * Clear out and cancel submissions, ignoring exceptions
+ */
+ private void cancelSubmissions() {
+ ForkJoinTask> task;
+ while ((task = submissionQueue.poll()) != null) {
+ try {
+ task.cancel(false);
+ } catch (Throwable ignore) {
+ }
+ }
+ }
+
+ // misc support for ForkJoinWorkerThread
+
+ /**
+ * Returns pool number
+ */
+ final int getPoolNumber() {
+ return poolNumber;
+ }
+
+ /**
+ * Tries to accumulates steal count from a worker, clearing
+ * the worker's value.
+ *
+ * @return true if worker steal count now zero
+ */
+ final boolean tryAccumulateStealCount(ForkJoinWorkerThread w) {
+ int sc = w.stealCount;
+ long c = stealCount;
+ // CAS even if zero, for fence effects
+ if (UNSAFE.compareAndSwapLong(this, stealCountOffset, c, c + sc)) {
+ if (sc != 0)
+ w.stealCount = 0;
+ return true;
+ }
+ return sc == 0;
+ }
+
+ /**
+ * Returns the approximate (non-atomic) number of idle threads per
+ * 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;
+ }
+
+ // Public and protected methods
+
+ // Constructors
+
+ /**
+ * Creates a {@code ForkJoinPool} with parallelism equal to {@link
+ * java.lang.Runtime#availableProcessors}, using the {@linkplain
+ * #defaultForkJoinWorkerThreadFactory default thread factory},
+ * no UncaughtExceptionHandler, and non-async LIFO processing mode.
+ *
+ * @throws SecurityException if a security manager exists and
+ * the caller is not permitted to modify threads
+ * because it does not hold {@link
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
+ */
+ public ForkJoinPool() {
+ this(Runtime.getRuntime().availableProcessors(),
+ defaultForkJoinWorkerThreadFactory, null, false);
+ }
+
+ /**
+ * Creates a {@code ForkJoinPool} with the indicated parallelism
+ * level, the {@linkplain
+ * #defaultForkJoinWorkerThreadFactory default thread factory},
+ * no UncaughtExceptionHandler, and non-async LIFO processing mode.
+ *
+ * @param parallelism the parallelism level
+ * @throws IllegalArgumentException if parallelism less than or
+ * equal to zero, or greater than implementation limit
+ * @throws SecurityException if a security manager exists and
+ * the caller is not permitted to modify threads
+ * because it does not hold {@link
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
+ */
+ public ForkJoinPool(int parallelism) {
+ this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
+ }
+
+ /**
+ * Creates a {@code ForkJoinPool} with the given parameters.
+ *
+ * @param parallelism the parallelism level. For default value,
+ * use {@link java.lang.Runtime#availableProcessors}.
+ * @param factory the factory for creating new threads. For default value,
+ * use {@link #defaultForkJoinWorkerThreadFactory}.
+ * @param handler the handler for internal worker threads that
+ * terminate due to unrecoverable errors encountered while executing
+ * tasks. For default value, use 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
.
+ * @throws IllegalArgumentException if parallelism less than or
+ * equal to zero, or greater than implementation limit
+ * @throws NullPointerException if the factory is null
+ * @throws SecurityException if a security manager exists and
+ * the caller is not permitted to modify threads
+ * because it does not hold {@link
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
+ */
+ public ForkJoinPool(int parallelism,
+ ForkJoinWorkerThreadFactory factory,
+ Thread.UncaughtExceptionHandler handler,
+ boolean asyncMode) {
+ checkPermission();
+ if (factory == null)
+ throw new NullPointerException();
+ if (parallelism <= 0 || parallelism > MAX_WORKERS)
+ 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();
+ this.trimTime = System.nanoTime();
+ }
+
+ /**
+ * 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_WORKERS < (1 >>> 16)
+ int size = pc < MAX_WORKERS ? pc + 1 : MAX_WORKERS;
+ size |= size >>> 1;
+ size |= size >>> 2;
+ size |= size >>> 4;
+ size |= size >>> 8;
+ return size + 1;
+ }
+
// Execution methods
/**
@@ -586,16 +1262,18 @@ public class ForkJoinPool extends Abstra
private void doSubmit(ForkJoinTask task) {
if (task == null)
throw new NullPointerException();
- if (isShutdown())
+ if (runState >= SHUTDOWN)
throw new RejectedExecutionException();
- if (workers == null)
- ensureWorkerInitialization();
submissionQueue.offer(task);
- signalIdleWorkers();
+ advanceEventCount();
+ helpMaintainParallelism(); // start or wake up workers
}
/**
* 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}.
*
* @param task the task
* @return the task's result
@@ -610,6 +1288,9 @@ public class ForkJoinPool extends Abstra
/**
* 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
@@ -637,6 +1318,23 @@ public class ForkJoinPool extends Abstra
}
/**
+ * 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
+ * @throws NullPointerException if the task is null
+ * @throws RejectedExecutionException if the task cannot be
+ * scheduled for execution
+ */
+ public ForkJoinTask submit(ForkJoinTask task) {
+ doSubmit(task);
+ return task;
+ }
+
+ /**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
@@ -674,21 +1372,6 @@ public class ForkJoinPool extends Abstra
}
/**
- * Submits a ForkJoinTask for execution.
- *
- * @param task the task to submit
- * @return the task
- * @throws NullPointerException if the task is null
- * @throws RejectedExecutionException if the task cannot be
- * scheduled for execution
- */
- public ForkJoinTask submit(ForkJoinTask task) {
- doSubmit(task);
- return task;
- }
-
-
- /**
* @throws NullPointerException {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
@@ -700,7 +1383,7 @@ public class ForkJoinPool extends Abstra
invoke(new InvokeAll(forkJoinTasks));
@SuppressWarnings({"unchecked", "rawtypes"})
- List> futures = (List>) (List) forkJoinTasks;
+ List> futures = (List>) (List) forkJoinTasks;
return futures;
}
@@ -714,8 +1397,6 @@ public class ForkJoinPool extends Abstra
private static final long serialVersionUID = -7914297376763021607L;
}
- // Configuration and status settings and queries
-
/**
* Returns the factory used for constructing new workers.
*
@@ -732,86 +1413,7 @@ public class ForkJoinPool extends Abstra
* @return the handler, or {@code null} if none
*/
public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
- Thread.UncaughtExceptionHandler h;
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- h = ueh;
- } finally {
- lock.unlock();
- }
- return h;
- }
-
- /**
- * Sets the handler for internal worker threads that terminate due
- * to unrecoverable errors encountered while executing tasks.
- * Unless set, the current default or ThreadGroup handler is used
- * as handler.
- *
- * @param h the new handler
- * @return the old handler, or {@code null} if none
- * @throws SecurityException if a security manager exists and
- * the caller is not permitted to modify threads
- * because it does not hold {@link
- * java.lang.RuntimePermission}{@code ("modifyThread")}
- */
- public Thread.UncaughtExceptionHandler
- setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler h) {
- checkPermission();
- Thread.UncaughtExceptionHandler old = null;
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- old = ueh;
- ueh = h;
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread w = ws[i];
- if (w != null)
- w.setUncaughtExceptionHandler(h);
- }
- }
- } finally {
- lock.unlock();
- }
- return old;
- }
-
-
- /**
- * Sets the target parallelism level of this pool.
- *
- * @param parallelism the target parallelism
- * @throws IllegalArgumentException if parallelism less than or
- * equal to zero or greater than maximum size bounds
- * @throws SecurityException if a security manager exists and
- * the caller is not permitted to modify threads
- * because it does not hold {@link
- * java.lang.RuntimePermission}{@code ("modifyThread")}
- */
- public void setParallelism(int parallelism) {
- checkPermission();
- if (parallelism <= 0 || parallelism > maxPoolSize)
- throw new IllegalArgumentException();
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- if (isProcessingTasks()) {
- int p = this.parallelism;
- this.parallelism = parallelism;
- if (workers != null) {
- if (parallelism > p)
- createAndStartAddedWorkers();
- else
- trimSpares();
- }
- }
- } finally {
- lock.unlock();
- }
- signalIdleWorkers();
+ return ueh;
}
/**
@@ -832,85 +1434,7 @@ public class ForkJoinPool extends Abstra
* @return the number of worker threads
*/
public int getPoolSize() {
- return totalCountOf(workerCounts);
- }
-
- /**
- * Returns the maximum number of threads allowed to exist in the
- * pool. Unless set using {@link #setMaximumPoolSize}, the
- * maximum is an implementation-defined value designed only to
- * prevent runaway growth.
- *
- * @return the maximum
- */
- public int getMaximumPoolSize() {
- return maxPoolSize;
- }
-
- /**
- * Sets the maximum number of threads allowed to exist in the
- * pool. The given value should normally be greater than or equal
- * to the {@link #getParallelism parallelism} level. Setting this
- * value has no effect on current pool size. It controls
- * construction of new threads.
- *
- * @throws IllegalArgumentException if negative or greater than
- * internal implementation limit
- */
- public void setMaximumPoolSize(int newMax) {
- if (newMax < 0 || newMax > MAX_THREADS)
- throw new IllegalArgumentException();
- maxPoolSize = newMax;
- }
-
-
- /**
- * Returns {@code true} if this pool dynamically maintains its
- * target parallelism level. If false, new threads are added only
- * to avoid possible starvation. This setting is by default true.
- *
- * @return {@code true} if maintains parallelism
- */
- public boolean getMaintainsParallelism() {
- return maintainsParallelism;
- }
-
- /**
- * Sets whether this pool dynamically maintains its target
- * parallelism level. If false, new threads are added only to
- * avoid possible starvation.
- *
- * @param enable {@code true} to maintain parallelism
- */
- public void setMaintainsParallelism(boolean enable) {
- maintainsParallelism = enable;
- }
-
- /**
- * Establishes local first-in-first-out scheduling mode for forked
- * tasks that are never joined. This mode may be more appropriate
- * than default locally stack-based mode in applications in which
- * worker threads only process asynchronous tasks. This method is
- * designed to be invoked only when the pool is quiescent, and
- * typically only before any tasks are submitted. The effects of
- * invocations at other times may be unpredictable.
- *
- * @param async if {@code true}, use locally FIFO scheduling
- * @return the previous mode
- * @see #getAsyncMode
- */
- public boolean setAsyncMode(boolean async) {
- boolean oldMode = locallyFifo;
- locallyFifo = async;
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null)
- t.setAsyncMode(async);
- }
- }
- return oldMode;
+ return workerCounts >>> TOTAL_COUNT_SHIFT;
}
/**
@@ -918,7 +1442,6 @@ public class ForkJoinPool extends Abstra
* scheduling mode for forked tasks that are never joined.
*
* @return {@code true} if this pool uses async mode
- * @see #setAsyncMode
*/
public boolean getAsyncMode() {
return locallyFifo;
@@ -927,12 +1450,13 @@ public class ForkJoinPool extends Abstra
/**
* Returns an estimate of the number of worker threads that are
* not blocked waiting to join tasks or for other managed
- * synchronization.
+ * synchronization. This method may overestimate the
+ * number of running threads.
*
* @return the number of worker threads
*/
public int getRunningThreadCount() {
- return runningCountOf(workerCounts);
+ return workerCounts & RUNNING_COUNT_MASK;
}
/**
@@ -943,19 +1467,7 @@ public class ForkJoinPool extends Abstra
* @return the number of active threads
*/
public int getActiveThreadCount() {
- return activeCountOf(runControl);
- }
-
- /**
- * Returns an estimate of the number of threads that are currently
- * idle waiting for tasks. This method may underestimate the
- * number of idle threads.
- *
- * @return the number of idle threads
- */
- final int getIdleThreadCount() {
- int c = runningCountOf(workerCounts) - activeCountOf(runControl);
- return (c <= 0) ? 0 : c;
+ return runState & ACTIVE_COUNT_MASK;
}
/**
@@ -970,7 +1482,7 @@ public class ForkJoinPool extends Abstra
* @return {@code true} if all threads are currently idle
*/
public boolean isQuiescent() {
- return activeCountOf(runControl) == 0;
+ return (runState & ACTIVE_COUNT_MASK) == 0;
}
/**
@@ -985,17 +1497,7 @@ public class ForkJoinPool extends Abstra
* @return the number of steals
*/
public long getStealCount() {
- return stealCount.get();
- }
-
- /**
- * Accumulates steal count from a worker.
- * Call only when worker known to be idle.
- */
- private void updateStealCount(ForkJoinWorkerThread w) {
- int sc = w.getAndClearStealCount();
- if (sc != 0)
- stealCount.addAndGet(sc);
+ return stealCount;
}
/**
@@ -1011,12 +1513,11 @@ public class ForkJoinPool extends Abstra
public long getQueuedTaskCount() {
long count = 0;
ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null)
- count += t.getQueueSize();
- }
+ int n = ws.length;
+ for (int i = 0; i < n; ++i) {
+ ForkJoinWorkerThread w = ws[i];
+ if (w != null)
+ count += w.getQueueSize();
}
return count;
}
@@ -1071,16 +1572,15 @@ public class ForkJoinPool extends Abstra
* @return the number of elements transferred
*/
protected int drainTasksTo(Collection super ForkJoinTask>> c) {
- int n = submissionQueue.drainTo(c);
+ int count = submissionQueue.drainTo(c);
ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread w = ws[i];
- if (w != null)
- n += w.drainTasksTo(c);
- }
+ int n = ws.length;
+ for (int i = 0; i < n; ++i) {
+ ForkJoinWorkerThread w = ws[i];
+ if (w != null)
+ count += w.drainTasksTo(c);
}
- return n;
+ return count;
}
/**
@@ -1091,36 +1591,34 @@ public class ForkJoinPool extends Abstra
* @return a string identifying this pool, as well as its state
*/
public String toString() {
- int ps = parallelism;
- int wc = workerCounts;
- int rc = runControl;
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;
return super.toString() +
- "[" + runStateToString(runStateOf(rc)) +
- ", parallelism = " + ps +
- ", size = " + totalCountOf(wc) +
- ", active = " + activeCountOf(rc) +
- ", running = " + runningCountOf(wc) +
+ "[" + runLevelToString(rs) +
+ ", parallelism = " + pc +
+ ", size = " + tc +
+ ", active = " + ac +
+ ", running = " + rc +
", steals = " + st +
", tasks = " + qt +
", submissions = " + qs +
"]";
}
- private static String runStateToString(int rs) {
- switch (rs) {
- case RUNNING: return "Running";
- case SHUTDOWN: return "Shutting down";
- case TERMINATING: return "Terminating";
- case TERMINATED: return "Terminated";
- default: throw new Error("Unknown run state");
- }
+ private static String runLevelToString(int s) {
+ return ((s & TERMINATED) != 0 ? "Terminated" :
+ ((s & TERMINATING) != 0 ? "Terminating" :
+ ((s & SHUTDOWN) != 0 ? "Shutting down" :
+ "Running")));
}
- // lifecycle control
-
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
@@ -1135,23 +1633,8 @@ public class ForkJoinPool extends Abstra
*/
public void shutdown() {
checkPermission();
- transitionRunStateTo(SHUTDOWN);
- if (canTerminateOnShutdown(runControl)) {
- if (workers == null) { // shutting down before workers created
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- if (workers == null) {
- terminate();
- transitionRunStateTo(TERMINATED);
- termination.signalAll();
- }
- } finally {
- lock.unlock();
- }
- }
- terminateOnShutdown();
- }
+ advanceRunLevel(SHUTDOWN);
+ tryTerminate(false);
}
/**
@@ -1172,7 +1655,7 @@ public class ForkJoinPool extends Abstra
*/
public List shutdownNow() {
checkPermission();
- terminate();
+ tryTerminate(true);
return Collections.emptyList();
}
@@ -1182,7 +1665,7 @@ public class ForkJoinPool extends Abstra
* @return {@code true} if all tasks have completed following shut down
*/
public boolean isTerminated() {
- return runStateOf(runControl) == TERMINATED;
+ return runState >= TERMINATED;
}
/**
@@ -1196,7 +1679,7 @@ public class ForkJoinPool extends Abstra
* @return {@code true} if terminating but not yet terminated
*/
public boolean isTerminating() {
- return runStateOf(runControl) == TERMINATING;
+ return (runState & (TERMINATING|TERMINATED)) == TERMINATING;
}
/**
@@ -1205,15 +1688,7 @@ public class ForkJoinPool extends Abstra
* @return {@code true} if this pool has been shut down
*/
public boolean isShutdown() {
- return runStateOf(runControl) >= SHUTDOWN;
- }
-
- /**
- * Returns true if pool is not terminating or terminated.
- * Used internally to suppress execution when terminating.
- */
- final boolean isProcessingTasks() {
- return runStateOf(runControl) < TERMINATING;
+ return runState >= SHUTDOWN;
}
/**
@@ -1229,554 +1704,10 @@ public class ForkJoinPool extends Abstra
*/
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
- long nanos = unit.toNanos(timeout);
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- for (;;) {
- if (isTerminated())
- return true;
- if (nanos <= 0)
- return false;
- nanos = termination.awaitNanos(nanos);
- }
- } finally {
- lock.unlock();
- }
- }
-
- // Shutdown and termination support
-
- /**
- * Callback from terminating worker. Nulls out the corresponding
- * workers slot, and if terminating, tries to terminate; else
- * tries to shrink workers array.
- *
- * @param w the worker
- */
- final void workerTerminated(ForkJoinWorkerThread w) {
- updateStealCount(w);
- updateWorkerCount(-1);
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- int idx = w.poolIndex;
- if (idx >= 0 && idx < ws.length && ws[idx] == w)
- ws[idx] = null;
- if (totalCountOf(workerCounts) == 0) {
- terminate(); // no-op if already terminating
- transitionRunStateTo(TERMINATED);
- termination.signalAll();
- }
- else if (isProcessingTasks()) {
- tryShrinkWorkerArray();
- tryResumeSpare(true); // allow replacement
- }
- }
- } finally {
- lock.unlock();
- }
- signalIdleWorkers();
- }
-
- /**
- * Initiates termination.
- */
- private void terminate() {
- if (transitionRunStateTo(TERMINATING)) {
- stopAllWorkers();
- resumeAllSpares();
- signalIdleWorkers();
- cancelQueuedSubmissions();
- cancelQueuedWorkerTasks();
- interruptUnterminatedWorkers();
- signalIdleWorkers(); // resignal after interrupt
- }
- }
-
- /**
- * Possibly terminates when on shutdown state.
- */
- private void terminateOnShutdown() {
- if (!hasQueuedSubmissions() && canTerminateOnShutdown(runControl))
- terminate();
- }
-
- /**
- * Clears out and cancels submissions.
- */
- private void cancelQueuedSubmissions() {
- ForkJoinTask> task;
- while ((task = pollSubmission()) != null)
- task.cancel(false);
- }
-
- /**
- * Cleans out worker queues.
- */
- private void cancelQueuedWorkerTasks() {
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null)
- t.cancelTasks();
- }
- }
- } finally {
- lock.unlock();
- }
- }
-
- /**
- * Sets each worker's status to terminating. Requires lock to avoid
- * conflicts with add/remove.
- */
- private void stopAllWorkers() {
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null)
- t.shutdownNow();
- }
- }
- } finally {
- lock.unlock();
- }
- }
-
- /**
- * Interrupts all unterminated workers. This is not required for
- * sake of internal control, but may help unstick user code during
- * shutdown.
- */
- private void interruptUnterminatedWorkers() {
- final ReentrantLock lock = this.workerLock;
- lock.lock();
- try {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null && !t.isTerminated()) {
- try {
- t.interrupt();
- } catch (SecurityException ignore) {
- }
- }
- }
- }
- } finally {
- lock.unlock();
- }
- }
-
- /*
- * Nodes for event barrier to manage idle threads. Queue nodes
- * are basic Treiber stack nodes, also used for spare stack.
- *
- * The event barrier has an event count and a wait queue (actually
- * a Treiber stack). Workers are enabled to look for work when
- * the eventCount is incremented. If they fail to find work, they
- * may wait for next count. Upon release, threads help others wake
- * up.
- *
- * Synchronization events occur only in enough contexts to
- * maintain overall liveness:
- *
- * - Submission of a new task to the pool
- * - Resizes or other changes to the workers array
- * - pool termination
- * - A worker pushing a task on an empty queue
- *
- * The case of pushing a task occurs often enough, and is heavy
- * enough compared to simple stack pushes, to require special
- * handling: Method signalWork returns without advancing count if
- * the queue appears to be empty. This would ordinarily result in
- * races causing some queued waiters not to be woken up. To avoid
- * this, the first worker enqueued in method sync rescans for
- * tasks after being enqueued, and helps signal if any are
- * found. This works well because the worker has nothing better to
- * do, and so might as well help alleviate the overhead and
- * contention on the threads actually doing work. Also, since
- * event counts increments on task availability exist to maintain
- * liveness (rather than to force refreshes etc), it is OK for
- * callers to exit early if contending with another signaller.
- */
- static final class WaitQueueNode {
- WaitQueueNode next; // only written before enqueued
- volatile ForkJoinWorkerThread thread; // nulled to cancel wait
- final long count; // unused for spare stack
-
- WaitQueueNode(long c, ForkJoinWorkerThread w) {
- count = c;
- thread = w;
- }
-
- /**
- * Wakes up waiter, also clearing thread field
- */
- void signal() {
- ForkJoinWorkerThread t = thread;
- if (t != null) {
- thread = null;
- LockSupport.unpark(t);
- }
- }
- }
-
- /**
- * Ensures that no thread is waiting for count to advance from the
- * current value of eventCount read on entry to this method, by
- * releasing waiting threads if necessary.
- */
- final void ensureSync() {
- long c = eventCount;
- WaitQueueNode q;
- while ((q = syncStack) != null && q.count < c) {
- if (casBarrierStack(q, null)) {
- do {
- q.signal();
- } while ((q = q.next) != null);
- break;
- }
- }
- }
-
- /**
- * Increments event count and releases waiting threads.
- */
- private void signalIdleWorkers() {
- long c;
- do {} while (!casEventCount(c = eventCount, c+1));
- ensureSync();
- }
-
- /**
- * Signals threads waiting to poll a task. Because method sync
- * rechecks availability, it is OK to only proceed if queue
- * appears to be non-empty, and OK if CAS to increment count
- * fails (since some other thread succeeded).
- */
- final void signalWork() {
- if (syncStack != null) {
- long c = eventCount;
- casEventCount(c, c+1);
- WaitQueueNode q = syncStack;
- if (q != null && q.count <= c) {
- if (casBarrierStack(q, q.next))
- q.signal();
- else
- ensureSync(); // awaken all on contention
- }
- }
- }
-
- /**
- * Possibly blocks until event count advances from last value held
- * by caller, or if excess threads, caller is resumed as spare, or
- * caller or pool is terminating. Updates caller's event on exit.
- *
- * @param w the calling worker thread
- */
- final void sync(ForkJoinWorkerThread w) {
- updateStealCount(w); // Transfer w's count while it is idle
-
- if (!w.isShutdown() && isProcessingTasks() && !suspendIfSpare(w)) {
- long prev = w.lastEventCount;
- WaitQueueNode node = null;
- WaitQueueNode h;
- long c;
- while ((c = eventCount) == prev &&
- ((h = syncStack) == null || h.count == prev)) {
- if (node == null)
- node = new WaitQueueNode(prev, w);
- if (casBarrierStack(node.next = h, node)) {
- if (!Thread.interrupted() &&
- node.thread != null &&
- eventCount == prev &&
- (h != null || // cover signalWork race
- (!ForkJoinWorkerThread.hasQueuedTasks(workers) &&
- eventCount == prev)))
- LockSupport.park(this);
- c = eventCount;
- if (node.thread != null) { // help signal if not unparked
- node.thread = null;
- if (c == prev)
- casEventCount(prev, prev + 1);
- }
- break;
- }
- }
- w.lastEventCount = c;
- ensureSync();
- }
- }
-
- /**
- * Returns {@code true} if a new sync event occurred since last
- * call to sync or this method, if so, updating caller's count.
- */
- final boolean hasNewSyncEvent(ForkJoinWorkerThread w) {
- long wc = w.lastEventCount;
- long c = eventCount;
- if (wc != c)
- w.lastEventCount = c;
- ensureSync();
- return wc != c || wc != eventCount;
- }
-
- // Parallelism maintenance
-
- /**
- * Decrements running count; if too low, adds spare.
- *
- * Conceptually, all we need to do here is add or resume a
- * spare thread when one is about to block (and remove or
- * suspend it later when unblocked -- see suspendIfSpare).
- * However, implementing this idea requires coping with
- * several problems: we have imperfect information about the
- * states of threads. Some count updates can and usually do
- * lag run state changes, despite arrangements to keep them
- * accurate (for example, when possible, updating counts
- * before signalling or resuming), especially when running on
- * dynamic JVMs that don't optimize the infrequent paths that
- * update counts. Generating too many threads can make these
- * problems become worse, because excess threads are more
- * likely to be context-switched with others, slowing them all
- * down, especially if there is no work available, so all are
- * busy scanning or idling. Also, excess spare threads can
- * only be suspended or removed when they are idle, not
- * immediately when they aren't needed. So adding threads will
- * raise parallelism level for longer than necessary. Also,
- * FJ applications often encounter highly transient peaks when
- * many threads are blocked joining, but for less time than it
- * takes to create or resume spares.
- *
- * @param joinMe if non-null, return early if done
- * @param maintainParallelism if true, try to stay within
- * target counts, else create only to avoid starvation
- * @return true if joinMe known to be done
- */
- final boolean preJoin(ForkJoinTask> joinMe,
- boolean maintainParallelism) {
- maintainParallelism &= maintainsParallelism; // overrride
- boolean dec = false; // true when running count decremented
- while (spareStack == null || !tryResumeSpare(dec)) {
- int counts = workerCounts;
- if (dec || (dec = casWorkerCounts(counts, --counts))) {
- if (!needSpare(counts, maintainParallelism))
- break;
- if (joinMe.status < 0)
- return true;
- if (tryAddSpare(counts))
- break;
- }
- }
- return false;
- }
-
- /**
- * Same idea as preJoin
- */
- final boolean preBlock(ManagedBlocker blocker,
- boolean maintainParallelism) {
- maintainParallelism &= maintainsParallelism;
- boolean dec = false;
- while (spareStack == null || !tryResumeSpare(dec)) {
- int counts = workerCounts;
- if (dec || (dec = casWorkerCounts(counts, --counts))) {
- if (!needSpare(counts, maintainParallelism))
- break;
- if (blocker.isReleasable())
- return true;
- if (tryAddSpare(counts))
- break;
- }
- }
- return false;
- }
-
- /**
- * Returns {@code true} if a spare thread appears to be needed.
- * If maintaining parallelism, returns true when the deficit in
- * running threads is more than the surplus of total threads, and
- * there is apparently some work to do. This self-limiting rule
- * means that the more threads that have already been added, the
- * less parallelism we will tolerate before adding another.
- *
- * @param counts current worker counts
- * @param maintainParallelism try to maintain parallelism
- */
- private boolean needSpare(int counts, boolean maintainParallelism) {
- int ps = parallelism;
- int rc = runningCountOf(counts);
- int tc = totalCountOf(counts);
- int runningDeficit = ps - rc;
- int totalSurplus = tc - ps;
- return (tc < maxPoolSize &&
- (rc == 0 || totalSurplus < 0 ||
- (maintainParallelism &&
- runningDeficit > totalSurplus &&
- ForkJoinWorkerThread.hasQueuedTasks(workers))));
- }
-
- /**
- * Adds a spare worker if lock available and no more than the
- * expected numbers of threads exist.
- *
- * @return true if successful
- */
- private boolean tryAddSpare(int expectedCounts) {
- final ReentrantLock lock = this.workerLock;
- int expectedRunning = runningCountOf(expectedCounts);
- int expectedTotal = totalCountOf(expectedCounts);
- boolean success = false;
- boolean locked = false;
- // confirm counts while locking; CAS after obtaining lock
try {
- for (;;) {
- int s = workerCounts;
- int tc = totalCountOf(s);
- int rc = runningCountOf(s);
- if (rc > expectedRunning || tc > expectedTotal)
- break;
- if (!locked && !(locked = lock.tryLock()))
- break;
- if (casWorkerCounts(s, workerCountsFor(tc+1, rc+1))) {
- createAndStartSpare(tc);
- success = true;
- break;
- }
- }
- } finally {
- if (locked)
- lock.unlock();
- }
- return success;
- }
-
- /**
- * Adds the kth spare worker. On entry, pool counts are already
- * adjusted to reflect addition.
- */
- private void createAndStartSpare(int k) {
- ForkJoinWorkerThread w = null;
- ForkJoinWorkerThread[] ws = ensureWorkerArrayCapacity(k + 1);
- int len = ws.length;
- // Probably, we can place at slot k. If not, find empty slot
- if (k < len && ws[k] != null) {
- for (k = 0; k < len && ws[k] != null; ++k)
- ;
- }
- if (k < len && isProcessingTasks() && (w = createWorker(k)) != null) {
- ws[k] = w;
- w.start();
- }
- else
- updateWorkerCount(-1); // adjust on failure
- signalIdleWorkers();
- }
-
- /**
- * Suspends calling thread w if there are excess threads. Called
- * only from sync. Spares are enqueued in a Treiber stack using
- * the same WaitQueueNodes as barriers. They are resumed mainly
- * in preJoin, but are also woken on pool events that require all
- * threads to check run state.
- *
- * @param w the caller
- */
- private boolean suspendIfSpare(ForkJoinWorkerThread w) {
- WaitQueueNode node = null;
- for (;;) {
- int s = workerCounts;
- int rc = runningCountOf(s);
- int tc = totalCountOf(s);
- int ps = parallelism;
- // use tc as bound if rc transiently out of sync
- if (tc <= ps || rc <= ps)
- return false; // not a spare
- if (node == null)
- node = new WaitQueueNode(0, w);
- if (casWorkerCounts(s, workerCountsFor(tc, rc - 1)))
- break;
- }
- // push onto stack
- do {} while (!casSpareStack(node.next = spareStack, node));
- // block until released by resumeSpare
- while (!Thread.interrupted() && node.thread != null)
- LockSupport.park(this);
- return true;
- }
-
- /**
- * Tries to pop and resume a spare thread.
- *
- * @param updateCount if true, increment running count on success
- * @return true if successful
- */
- private boolean tryResumeSpare(boolean updateCount) {
- WaitQueueNode q;
- while ((q = spareStack) != null) {
- if (casSpareStack(q, q.next)) {
- if (updateCount)
- updateRunningCount(1);
- q.signal();
- return true;
- }
- }
- return false;
- }
-
- /**
- * Pops and resumes all spare threads. Same idea as ensureSync.
- *
- * @return true if any spares released
- */
- private boolean resumeAllSpares() {
- WaitQueueNode q;
- while ( (q = spareStack) != null) {
- if (casSpareStack(q, null)) {
- do {
- updateRunningCount(1);
- q.signal();
- } while ((q = q.next) != null);
- return true;
- }
- }
- return false;
- }
-
- /**
- * Pops and shuts down excessive spare threads. Call only while
- * holding lock. This is not guaranteed to eliminate all excess
- * threads, only those suspended as spares, which are the ones
- * unlikely to be needed in the future.
- */
- private void trimSpares() {
- int surplus = totalCountOf(workerCounts) - parallelism;
- WaitQueueNode q;
- while (surplus > 0 && (q = spareStack) != null) {
- if (casSpareStack(q, null)) {
- do {
- updateRunningCount(1);
- ForkJoinWorkerThread w = q.thread;
- if (w != null && surplus > 0 &&
- runningCountOf(workerCounts) > 0 && w.shutdown())
- --surplus;
- q.signal();
- } while ((q = q.next) != null);
- }
+ return termination.awaitAdvanceInterruptibly(0, timeout, unit) > 0;
+ } catch(TimeoutException ex) {
+ return false;
}
}
@@ -1784,11 +1715,17 @@ 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). 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:
@@ -1806,6 +1743,26 @@ 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 {
/**
@@ -1829,14 +1786,7 @@ public class ForkJoinPool extends Abstra
* Blocks in accord with the given blocker. If the current thread
* is a {@link ForkJoinWorkerThread}, this method possibly
* arranges for a spare thread to be activated if necessary to
- * ensure parallelism while the current thread is blocked.
- *
- * If {@code maintainParallelism} is {@code true} and the pool
- * supports it ({@link #getMaintainsParallelism}), this method
- * attempts to maintain the pool's nominal parallelism. Otherwise
- * it activates a thread only if necessary to avoid complete
- * starvation. This option may be preferable when blockages use
- * timeouts, or are almost always brief.
+ * ensure sufficient parallelism while the current thread is blocked.
*
*
If the caller is not a {@link ForkJoinTask}, this method is
* behaviorally equivalent to
@@ -1850,33 +1800,18 @@ public class ForkJoinPool extends Abstra
* first be expanded to ensure parallelism, and later adjusted.
*
* @param blocker the blocker
- * @param maintainParallelism if {@code true} and supported by
- * this pool, attempt to maintain the pool's nominal parallelism;
- * otherwise activate a thread only if necessary to avoid
- * complete starvation.
* @throws InterruptedException if blocker.block did so
*/
- public static void managedBlock(ManagedBlocker blocker,
- boolean maintainParallelism)
+ public static void managedBlock(ManagedBlocker blocker)
throws InterruptedException {
Thread t = Thread.currentThread();
- ForkJoinPool pool = ((t instanceof ForkJoinWorkerThread) ?
- ((ForkJoinWorkerThread) t).pool : null);
- if (!blocker.isReleasable()) {
- try {
- if (pool == null ||
- !pool.preBlock(blocker, maintainParallelism))
- awaitBlocker(blocker);
- } finally {
- if (pool != null)
- pool.updateRunningCount(1);
- }
+ if (t instanceof ForkJoinWorkerThread) {
+ ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
+ w.pool.awaitBlocker(blocker);
+ }
+ else {
+ do {} while (!blocker.isReleasable() && !blocker.block());
}
- }
-
- private static void awaitBlocker(ManagedBlocker blocker)
- throws InterruptedException {
- do {} while (!blocker.isReleasable() && !blocker.block());
}
// AbstractExecutorService overrides. These rely on undocumented
@@ -1894,32 +1829,18 @@ public class ForkJoinPool extends Abstra
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = getUnsafe();
- private static final long eventCountOffset =
- objectFieldOffset("eventCount", ForkJoinPool.class);
private static final long workerCountsOffset =
objectFieldOffset("workerCounts", ForkJoinPool.class);
- private static final long runControlOffset =
- objectFieldOffset("runControl", ForkJoinPool.class);
- private static final long syncStackOffset =
- objectFieldOffset("syncStack",ForkJoinPool.class);
- private static final long spareStackOffset =
- objectFieldOffset("spareStack", ForkJoinPool.class);
-
- private boolean casEventCount(long cmp, long val) {
- return UNSAFE.compareAndSwapLong(this, eventCountOffset, cmp, val);
- }
- private boolean casWorkerCounts(int cmp, int val) {
- return UNSAFE.compareAndSwapInt(this, workerCountsOffset, cmp, val);
- }
- private boolean casRunControl(int cmp, int val) {
- return UNSAFE.compareAndSwapInt(this, runControlOffset, cmp, val);
- }
- private boolean casSpareStack(WaitQueueNode cmp, WaitQueueNode val) {
- return UNSAFE.compareAndSwapObject(this, spareStackOffset, cmp, val);
- }
- private boolean casBarrierStack(WaitQueueNode cmp, WaitQueueNode val) {
- return UNSAFE.compareAndSwapObject(this, syncStackOffset, cmp, val);
- }
+ 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 final long spareWaitersOffset =
+ objectFieldOffset("spareWaiters",ForkJoinPool.class);
private static long objectFieldOffset(String field, Class> klazz) {
try {