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android / toolchain / jdk / jdk9_jdk / 2283b9d1d8f03fea8fa192c248b58726a66a6eaf / . / src / share / classes / java / util / concurrent / ForkJoinPool.java
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/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package java.util.concurrent;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.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;
/**
* 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
* monitoring operations.
*
* <p>A {@code ForkJoinPool} differs from other kinds of {@link
* ExecutorService} mainly by virtue of employing
* <em>work-stealing</em>: all threads in the pool attempt to find and
* execute subtasks created by other active tasks (eventually blocking
* waiting for work if none exist). This enables efficient processing
* when most tasks spawn other subtasks (as do most {@code
* ForkJoinTask}s). A {@code ForkJoinPool} may also be used for mixed
* execution of some plain {@code Runnable}- or {@code Callable}-
* based activities along with {@code ForkJoinTask}s. When setting
* {@linkplain #setAsyncMode async mode}, a {@code ForkJoinPool} may
* also be appropriate for use with fine-grained tasks of any form
* that are never joined. Otherwise, other {@code ExecutorService}
* implementations are typically more appropriate choices.
*
* <p>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.
*
* <p>In addition to execution and lifecycle control methods, this
* class provides status check methods (for example
* {@link #getStealCount}) that are intended to aid in developing,
* tuning, and monitoring fork/join applications. Also, method
* {@link #toString} returns indications of pool state in a
* convenient form for informal monitoring.
*
* <p><b>Sample Usage.</b> Normally a single {@code ForkJoinPool} is
* used for all parallel task execution in a program or subsystem.
* Otherwise, use would not usually outweigh the construction and
* bookkeeping overhead of creating a large set of threads. For
* example, a common pool could be used for the {@code SortTasks}
* illustrated in {@link RecursiveAction}. Because {@code
* ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon
* daemon} mode, there is typically no need to explicitly {@link
* #shutdown} such a pool upon program exit.
*
* <pre>
* static final ForkJoinPool mainPool = new ForkJoinPool();
* ...
* public void sort(long[] array) {
* mainPool.invoke(new SortTask(array, 0, array.length));
* }
* </pre>
*
* <p><b>Implementation notes</b>: This implementation restricts the
* maximum number of running threads to 32767. Attempts to create
* pools with greater than the maximum number result in
* {@code IllegalArgumentException}.
*
* <p>This implementation rejects submitted tasks (that is, by throwing
* {@link RejectedExecutionException}) only when the pool is shut down.
*
* @since 1.7
* @author Doug Lea
*/
public class ForkJoinPool extends AbstractExecutorService {
/*
* See the extended comments interspersed below for design,
* rationale, and walkthroughs.
*/
/** 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
* for {@code ForkJoinWorkerThread} subclasses that extend base
* functionality or initialize threads with different contexts.
*/
public static interface ForkJoinWorkerThreadFactory {
/**
* Returns a new worker thread operating in the given pool.
*
* @param pool the pool this thread works in
* @throws NullPointerException if the pool is null
*/
public ForkJoinWorkerThread newThread(ForkJoinPool pool);
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
static class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
try {
return new ForkJoinWorkerThread(pool);
} catch (OutOfMemoryError oom) {
return null;
}
}
}
/**
* Creates a new ForkJoinWorkerThread. This factory is used unless
* overridden in ForkJoinPool constructors.
*/
public static final ForkJoinWorkerThreadFactory
defaultForkJoinWorkerThreadFactory =
new DefaultForkJoinWorkerThreadFactory();
/**
* Permission required for callers of methods that may start or
* kill threads.
*/
private static final RuntimePermission modifyThreadPermission =
new RuntimePermission("modifyThread");
/**
* If there is a security manager, makes sure caller has
* permission to modify threads.
*/
private static void checkPermission() {
SecurityManager security = System.getSecurityManager();
if (security != null)
security.checkPermission(modifyThreadPermission);
}
/**
* Generator for assigning sequence numbers as pool names.
*/
private static final AtomicInteger poolNumberGenerator =
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.
*/
volatile ForkJoinWorkerThread[] workers;
/**
* Lock protecting access to workers.
*/
private final ReentrantLock workerLock;
/**
* Condition for awaitTermination.
*/
private final Condition termination;
/**
* The uncaught exception handler used when any worker
* abruptly terminates
*/
private Thread.UncaughtExceptionHandler ueh;
/**
* Creation factory for worker threads.
*/
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;
/**
* Queue for external submissions.
*/
private final LinkedTransferQueue<ForkJoinTask<?>> submissionQueue;
/**
* Head of Treiber stack for barrier sync. See below for explanation.
*/
private volatile WaitQueueNode syncStack;
/**
* The count for event barrier
*/
private volatile long eventCount;
/**
* Pool number, just for assigning useful names to worker threads
*/
private final int poolNumber;
/**
* The maximum allowed pool size
*/
private volatile int maxPoolSize;
/**
* The desired parallelism level, updated only under workerLock.
*/
private volatile int parallelism;
/**
* True if use local fifo, not default lifo, for local polling
*/
private volatile 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.
*/
private volatile int workerCounts;
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; }
/**
* 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
*/
final void updateRunningCount(int delta) {
int s;
do {} while (!casWorkerCounts(s = workerCounts, s + delta));
}
/**
* 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
*/
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));
}
/**
* 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
*/
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 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; }
/**
* Tries incrementing active count; fails on contention.
* Called by workers before/during executing tasks.
*
* @return true on success
*/
final boolean tryIncrementActiveCount() {
int c = runControl;
return casRunControl(c, c+1);
}
/**
* 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
*/
final boolean tryDecrementActiveCount() {
int c = runControl;
int nextc = c - 1;
if (!casRunControl(c, nextc))
return false;
if (canTerminateOnShutdown(nextc))
terminateOnShutdown();
return true;
}
/**
* Returns {@code true} if argument represents zero active count
* and nonzero runstate, which is the triggering condition for
* terminating on shutdown.
*/
private static boolean canTerminateOnShutdown(int c) {
// i.e. least bit is nonzero runState bit
return ((c & -c) >>> 16) != 0;
}
/**
* Transition run state to at least the given state. Return true
* if not already at least given state.
*/
private boolean transitionRunStateTo(int state) {
for (;;) {
int c = runControl;
if (runStateOf(c) >= state)
return false;
if (casRunControl(c, runControlFor(state, activeCountOf(c))))
return true;
}
}
/**
* Controls whether to add spares to maintain parallelism
*/
private volatile boolean maintainsParallelism;
// Constructors
/**
* 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")}
*/
public ForkJoinPool() {
this(Runtime.getRuntime().availableProcessors(),
defaultForkJoinWorkerThreadFactory);
}
/**
* Creates a {@code ForkJoinPool} with the indicated parallelism
* level and using the {@linkplain
* #defaultForkJoinWorkerThreadFactory default thread factory}.
*
* @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);
}
/**
* Creates a {@code ForkJoinPool} with parallelism equal to {@link
* java.lang.Runtime#availableProcessors}, and using the given
* thread factory.
*
* @param factory the factory for creating new threads
* @throws NullPointerException if the factory is null
* @throws SecurityException if a security manager exists and
* the caller is not permitted to modify threads
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(ForkJoinWorkerThreadFactory factory) {
this(Runtime.getRuntime().availableProcessors(), factory);
}
/**
* 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")}
*/
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<ForkJoinTask<?>>();
// worker array and workers are lazily constructed
}
/**
* Creates a new worker thread using factory.
*
* @param index the index to assign worker
* @return new worker, or null if factory failed
*/
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);
}
return w;
}
/**
* Returns a good size for worker array given pool size.
* Currently requires size to be a power of two.
*/
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;
}
/**
* Creates or resizes array if necessary to hold newLength.
* Call only under exclusion.
*
* @return the array
*/
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;
}
/**
* Tries to shrink workers into smaller array after one or more terminate.
*/
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);
}
}
/**
* 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;
ws = ensureWorkerArrayCapacity(ps);
for (int i = 0; i < ps; ++i) {
ForkJoinWorkerThread w = createWorker(i);
if (w != null) {
ws[i] = w;
w.start();
updateWorkerCount(1);
}
}
}
} finally {
lock.unlock();
}
}
}
/**
* Worker creation and startup for threads added via setParallelism.
*/
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;
}
int s = workerCounts;
int tc = totalCountOf(s);
int rc = runningCountOf(s);
if (rc >= ps || tc >= ps)
break;
if (casWorkerCounts (s, workerCountsFor(tc+1, rc+1))) {
ForkJoinWorkerThread w = createWorker(k);
if (w != null) {
ws[k++] = w;
w.start();
}
else {
updateWorkerCount(-1); // back out on failed creation
break;
}
}
}
}
// Execution methods
/**
* Common code for execute, invoke and submit
*/
private <T> void doSubmit(ForkJoinTask<T> task) {
if (task == null)
throw new NullPointerException();
if (isShutdown())
throw new RejectedExecutionException();
if (workers == null)
ensureWorkerInitialization();
submissionQueue.offer(task);
signalIdleWorkers();
}
/**
* Performs the given task, returning its result upon completion.
*
* @param task the task
* @return the task's result
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> T invoke(ForkJoinTask<T> task) {
doSubmit(task);
return task.join();
}
/**
* Arranges for (asynchronous) execution of the given task.
*
* @param task the task
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public void execute(ForkJoinTask<?> task) {
doSubmit(task);
}
// AbstractExecutorService methods
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public void execute(Runnable task) {
ForkJoinTask<?> job;
if (task instanceof ForkJoinTask<?>) // avoid re-wrap
job = (ForkJoinTask<?>) task;
else
job = ForkJoinTask.adapt(task, null);
doSubmit(job);
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Callable<T> task) {
ForkJoinTask<T> job = ForkJoinTask.adapt(task);
doSubmit(job);
return job;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Runnable task, T result) {
ForkJoinTask<T> job = ForkJoinTask.adapt(task, result);
doSubmit(job);
return job;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public ForkJoinTask<?> submit(Runnable task) {
ForkJoinTask<?> job;
if (task instanceof ForkJoinTask<?>) // avoid re-wrap
job = (ForkJoinTask<?>) task;
else
job = ForkJoinTask.adapt(task, null);
doSubmit(job);
return job;
}
/**
* Submits a ForkJoinTask for execution.
*
* @param task the task to submit
* @return the task
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
doSubmit(task);
return task;
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
ArrayList<ForkJoinTask<T>> forkJoinTasks =
new ArrayList<ForkJoinTask<T>>(tasks.size());
for (Callable<T> task : tasks)
forkJoinTasks.add(ForkJoinTask.adapt(task));
invoke(new InvokeAll<T>(forkJoinTasks));
@SuppressWarnings({"unchecked", "rawtypes"})
List<Future<T>> futures = (List<Future<T>>) (List) forkJoinTasks;
return futures;
}
static final class InvokeAll<T> extends RecursiveAction {
final ArrayList<ForkJoinTask<T>> tasks;
InvokeAll(ArrayList<ForkJoinTask<T>> tasks) { this.tasks = tasks; }
public void compute() {
try { invokeAll(tasks); }
catch (Exception ignore) {}
}
private static final long serialVersionUID = -7914297376763021607L;
}
// Configuration and status settings and queries
/**
* Returns the factory used for constructing new workers.
*
* @return the factory used for constructing new workers
*/
public ForkJoinWorkerThreadFactory getFactory() {
return factory;
}
/**
* Returns the handler for internal worker threads that terminate
* due to unrecoverable errors encountered while executing tasks.
*
* @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 (parallelism > p)
createAndStartAddedWorkers();
else
trimSpares();
}
} finally {
lock.unlock();
}
signalIdleWorkers();
}
/**
* Returns the targeted parallelism level of this pool.
*
* @return the targeted parallelism level of this pool
*/
public int getParallelism() {
return parallelism;
}
/**
* Returns the number of worker threads that have started but not
* yet terminated. This result returned by this method may differ
* from {@link #getParallelism} when threads are created to
* maintain parallelism when others are cooperatively blocked.
*
* @return the number of worker threads
*/
public int getPoolSize() {
return 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;
}
/**
* Returns {@code true} if this pool uses local first-in-first-out
* 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;
}
/**
* Returns an estimate of the number of worker threads that are
* not blocked waiting to join tasks or for other managed
* synchronization.
*
* @return the number of worker threads
*/
public int getRunningThreadCount() {
return runningCountOf(workerCounts);
}
/**
* Returns an estimate of the number of threads that are currently
* stealing or executing tasks. This method may overestimate the
* number of active threads.
*
* @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;
}
/**
* Returns {@code true} if all worker threads are currently idle.
* An idle worker is one that cannot obtain a task to execute
* because none are available to steal from other threads, and
* there are no pending submissions to the pool. This method is
* conservative; it might not return {@code true} immediately upon
* idleness of all threads, but will eventually become true if
* threads remain inactive.
*
* @return {@code true} if all threads are currently idle
*/
public boolean isQuiescent() {
return activeCountOf(runControl) == 0;
}
/**
* Returns an estimate of the total number of tasks stolen from
* one thread's work queue by another. The reported value
* underestimates the actual total number of steals when the pool
* is not quiescent. This value may be useful for monitoring and
* tuning fork/join programs: in general, steal counts should be
* high enough to keep threads busy, but low enough to avoid
* overhead and contention across threads.
*
* @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);
}
/**
* Returns an estimate of the total number of tasks currently held
* in queues by worker threads (but not including tasks submitted
* to the pool that have not begun executing). This value is only
* an approximation, obtained by iterating across all threads in
* the pool. This method may be useful for tuning task
* granularities.
*
* @return the number of queued tasks
*/
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();
}
}
return count;
}
/**
* Returns an estimate of the number of tasks submitted to this
* pool that have not yet begun executing. This method takes time
* proportional to the number of submissions.
*
* @return the number of queued submissions
*/
public int getQueuedSubmissionCount() {
return submissionQueue.size();
}
/**
* Returns {@code true} if there are any tasks submitted to this
* pool that have not yet begun executing.
*
* @return {@code true} if there are any queued submissions
*/
public boolean hasQueuedSubmissions() {
return !submissionQueue.isEmpty();
}
/**
* Removes and returns the next unexecuted submission if one is
* available. This method may be useful in extensions to this
* class that re-assign work in systems with multiple pools.
*
* @return the next submission, or {@code null} if none
*/
protected ForkJoinTask<?> pollSubmission() {
return submissionQueue.poll();
}
/**
* Removes all available unexecuted submitted and forked tasks
* from scheduling queues and adds them to the given collection,
* without altering their execution status. These may include
* artificially generated or wrapped tasks. This method is
* designed to be invoked only when the pool is known to be
* quiescent. Invocations at other times may not remove all
* tasks. A failure encountered while attempting to add elements
* to collection {@code c} may result in elements being in
* neither, either or both collections when the associated
* exception is thrown. The behavior of this operation is
* undefined if the specified collection is modified while the
* operation is in progress.
*
* @param c the collection to transfer elements into
* @return the number of elements transferred
*/
protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
int n = 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);
}
}
return n;
}
/**
* Returns a string identifying this pool, as well as its state,
* including indications of run state, parallelism level, and
* worker and task counts.
*
* @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();
return super.toString() +
"[" + runStateToString(runStateOf(rc)) +
", parallelism = " + ps +
", size = " + totalCountOf(wc) +
", active = " + activeCountOf(rc) +
", running = " + runningCountOf(wc) +
", 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");
}
}
// lifecycle control
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
* Tasks that are in the process of being submitted concurrently
* during the course of this method may or may not be rejected.
*
* @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 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();
}
}
/**
* Attempts to cancel and/or stop all tasks, and reject all
* subsequently submitted tasks. Tasks that are in the process of
* being submitted or executed concurrently during the course of
* this method may or may not be rejected. This method cancels
* both existing and unexecuted tasks, in order to permit
* termination in the presence of task dependencies. So the method
* always returns an empty list (unlike the case for some other
* Executors).
*
* @return an empty list
* @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 List<Runnable> shutdownNow() {
checkPermission();
terminate();
return Collections.emptyList();
}
/**
* Returns {@code true} if all tasks have completed following shut down.
*
* @return {@code true} if all tasks have completed following shut down
*/
public boolean isTerminated() {
return runStateOf(runControl) == TERMINATED;
}
/**
* Returns {@code true} if the process of termination has
* commenced but not yet completed. This method may be useful for
* debugging. A return of {@code true} reported a sufficient
* period after shutdown may indicate that submitted tasks have
* ignored or suppressed interruption, causing this executor not
* to properly terminate.
*
* @return {@code true} if terminating but not yet terminated
*/
public boolean isTerminating() {
return runStateOf(runControl) == TERMINATING;
}
/**
* Returns {@code true} if this pool has been shut down.
*
* @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;
}
/**
* Blocks until all tasks have completed execution after a shutdown
* request, or the timeout occurs, or the current thread is
* interrupted, whichever happens first.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if this executor terminated and
* {@code false} if the timeout elapsed before termination
* @throws InterruptedException if interrupted while waiting
*/
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 (see
* syncIsReleasable) 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, returning false if known to already
*/
boolean signal() {
ForkJoinWorkerThread t = thread;
if (t == null)
return false;
thread = null;
LockSupport.unpark(t);
return true;
}
/**
* Awaits release on sync.
*/
void awaitSyncRelease(ForkJoinPool p) {
while (thread != null && !p.syncIsReleasable(this))
LockSupport.park(this);
}
/**
* Awaits resumption as spare.
*/
void awaitSpareRelease() {
while (thread != null) {
if (!Thread.interrupted())
LockSupport.park(this);
}
}
}
/**
* 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.
*
* @return the count
*/
final long 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;
}
}
return c;
}
/**
* 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 to skip under contention to
* increment count (since some other thread succeeded).
*/
final void signalWork() {
long c;
WaitQueueNode q;
if (syncStack != null &&
casEventCount(c = eventCount, c+1) &&
(((q = syncStack) != null && q.count <= c) &&
(!casBarrierStack(q, q.next) || !q.signal())))
ensureSync();
}
/**
* Waits 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
while (!w.isShutdown() && isProcessingTasks() && !suspendIfSpare(w)) {
long prev = w.lastEventCount;
WaitQueueNode node = null;
WaitQueueNode h;
while (eventCount == prev &&
((h = syncStack) == null || h.count == prev)) {
if (node == null)
node = new WaitQueueNode(prev, w);
if (casBarrierStack(node.next = h, node)) {
node.awaitSyncRelease(this);
break;
}
}
long ec = ensureSync();
if (ec != prev) {
w.lastEventCount = ec;
break;
}
}
}
/**
* Returns {@code true} if worker waiting on sync can proceed:
* - on signal (thread == null)
* - on event count advance (winning race to notify vs signaller)
* - on interrupt
* - if the first queued node, we find work available
* If node was not signalled and event count not advanced on exit,
* then we also help advance event count.
*
* @return {@code true} if node can be released
*/
final boolean syncIsReleasable(WaitQueueNode node) {
long prev = node.count;
if (!Thread.interrupted() && node.thread != null &&
(node.next != null ||
!ForkJoinWorkerThread.hasQueuedTasks(workers)) &&
eventCount == prev)
return false;
if (node.thread != null) {
node.thread = null;
long ec = eventCount;
if (prev <= ec) // help signal
casEventCount(ec, ec+1);
}
return true;
}
/**
* 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 lc = w.lastEventCount;
long ec = ensureSync();
if (ec == lc)
return false;
w.lastEventCount = ec;
return true;
}
// 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;
int s;
while (parallelism < runningCountOf(s = workerCounts)) {
if (node == null)
node = new WaitQueueNode(0, w);
if (casWorkerCounts(s, s-1)) { // representation-dependent
// push onto stack
do {} while (!casSpareStack(node.next = spareStack, node));
// block until released by resumeSpare
node.awaitSpareRelease();
return true;
}
}
return false;
}
/**
* 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);
}
}
}
/**
* Interface for extending managed parallelism for tasks running
* in {@link ForkJoinPool}s.
*
* <p>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).
*
* <p>For example, here is a ManagedBlocker based on a
* ReentrantLock:
* <pre> {@code
* class ManagedLocker implements ManagedBlocker {
* final ReentrantLock lock;
* boolean hasLock = false;
* ManagedLocker(ReentrantLock lock) { this.lock = lock; }
* public boolean block() {
* if (!hasLock)
* lock.lock();
* return true;
* }
* public boolean isReleasable() {
* return hasLock || (hasLock = lock.tryLock());
* }
* }}</pre>
*/
public static interface ManagedBlocker {
/**
* Possibly blocks the current thread, for example waiting for
* a lock or condition.
*
* @return {@code true} if no additional blocking is necessary
* (i.e., if isReleasable would return true)
* @throws InterruptedException if interrupted while waiting
* (the method is not required to do so, but is allowed to)
*/
boolean block() throws InterruptedException;
/**
* Returns {@code true} if blocking is unnecessary.
*/
boolean isReleasable();
}
/**
* 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.
*
* <p>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.
*
* <p>If the caller is not a {@link ForkJoinTask}, this method is
* behaviorally equivalent to
* <pre> {@code
* while (!blocker.isReleasable())
* if (blocker.block())
* return;
* }</pre>
*
* If the caller is a {@code ForkJoinTask}, then the pool may
* 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)
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);
}
}
}
private static void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
do {} while (!blocker.isReleasable() && !blocker.block());
}
// AbstractExecutorService overrides. These rely on undocumented
// fact that ForkJoinTask.adapt returns ForkJoinTasks that also
// implement RunnableFuture.
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return (RunnableFuture<T>) ForkJoinTask.adapt(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return (RunnableFuture<T>) ForkJoinTask.adapt(callable);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.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 long objectFieldOffset(String field, Class<?> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
}