--- jsr166/src/jsr166y/ForkJoinPool.java 2009/01/07 19:12:36 1.3
+++ jsr166/src/jsr166y/ForkJoinPool.java 2009/07/24 23:47:01 1.21
@@ -9,8 +9,6 @@ import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
-import sun.misc.Unsafe;
-import java.lang.reflect.*;
/**
* An {@link ExecutorService} for running {@link ForkJoinTask}s. A
@@ -27,7 +25,9 @@ import java.lang.reflect.*;
* (eventually blocking if none exist). This makes them efficient when
* most tasks spawn other subtasks (as do most ForkJoinTasks), as well
* as the mixed execution of some plain Runnable- or Callable- based
- * activities along with ForkJoinTasks. Otherwise, other
+ * activities along with ForkJoinTasks. When setting
+ * {@code setAsyncMode}, a ForkJoinPools may also be appropriate for
+ * use with fine-grained tasks that are never joined. Otherwise, other
* ExecutorService implementations are typically more appropriate
* choices.
*
@@ -36,24 +36,27 @@ import java.lang.reflect.*;
* adding, suspending, or resuming threads, even if some tasks are
* waiting to join others. However, no such adjustments are performed
* in the face of blocked IO or other unmanaged synchronization. The
- * nested ManagedBlocker
interface enables extension of
+ * nested {@code ManagedBlocker} interface enables extension of
* the kinds of synchronization accommodated. The target parallelism
- * level may also be changed dynamically (setParallelism
)
- * and dynamically thread construction can be limited using methods
- * setMaximumPoolSize
and/or
- * setMaintainsParallelism
.
+ * level may also be changed dynamically ({@code setParallelism})
+ * and thread construction can be limited using methods
+ * {@code setMaximumPoolSize} and/or
+ * {@code setMaintainsParallelism}.
*
*
In addition to execution and lifecycle control methods, this
* class provides status check methods (for example
- * getStealCount
) that are intended to aid in developing,
+ * {@code getStealCount}) that are intended to aid in developing,
* tuning, and monitoring fork/join applications. Also, method
- * toString
returns indications of pool state in a
+ * {@code toString} returns indications of pool state in a
* convenient form for informal monitoring.
*
*
Implementation notes: This implementation restricts the
* maximum number of running threads to 32767. Attempts to create
* pools with greater than the maximum result in
* IllegalArgumentExceptions.
+ *
+ * @since 1.7
+ * @author Doug Lea
*/
public class ForkJoinPool extends AbstractExecutorService {
@@ -79,13 +82,13 @@ public class ForkJoinPool extends Abstra
* Returns a new worker thread operating in the given pool.
*
* @param pool the pool this thread works in
- * @throws NullPointerException if pool is null;
+ * @throws NullPointerException if pool is null
*/
public ForkJoinWorkerThread newThread(ForkJoinPool pool);
}
/**
- * Default ForkJoinWorkerThreadFactory implementation, creates a
+ * Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
static class DefaultForkJoinWorkerThreadFactory
@@ -131,11 +134,11 @@ public class ForkJoinPool extends Abstra
new AtomicInteger();
/**
- * Array holding all worker threads in the pool. 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.
+ * 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;
@@ -151,7 +154,7 @@ public class ForkJoinPool extends Abstra
/**
* The uncaught exception handler used when any worker
- * abrupty terminates
+ * abruptly terminates
*/
private Thread.UncaughtExceptionHandler ueh;
@@ -179,9 +182,9 @@ public class ForkJoinPool extends Abstra
private final LinkedTransferQueue For example, here is a ManagedBlocker based on a
* ReentrantLock:
- * If the caller is not a ForkJoinTask, this method is behaviorally
* equivalent to
- * ("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool() {
this(Runtime.getRuntime().availableProcessors(),
@@ -328,15 +348,16 @@ public class ForkJoinPool extends Abstra
}
/**
- * Creates a ForkJoinPool with the indicated parellelism level
- * threads, and using the default ForkJoinWorkerThreadFactory,
+ * Creates a ForkJoinPool with the indicated parallelism level
+ * threads and using the default ForkJoinWorkerThreadFactory.
+ *
* @param parallelism the number of worker threads
* @throws IllegalArgumentException if parallelism less than or
* equal to zero
* @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}("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(int parallelism) {
this(parallelism, defaultForkJoinWorkerThreadFactory);
@@ -345,13 +366,14 @@ public class ForkJoinPool extends Abstra
/**
* Creates a ForkJoinPool with parallelism equal to the number of
* processors available on the system and using the given
- * ForkJoinWorkerThreadFactory,
+ * ForkJoinWorkerThreadFactory.
+ *
* @param factory the factory for creating new threads
* @throws NullPointerException if 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}("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(ForkJoinWorkerThreadFactory factory) {
this(Runtime.getRuntime().availableProcessors(), factory);
@@ -363,12 +385,12 @@ public class ForkJoinPool extends Abstra
* @param parallelism the targeted number of worker threads
* @param factory the factory for creating new threads
* @throws IllegalArgumentException if parallelism less than or
- * equal to zero, or greater than implementation limit.
+ * equal to zero, or greater than implementation limit
* @throws NullPointerException if 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}("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory) {
if (parallelism <= 0 || parallelism > MAX_THREADS)
@@ -385,11 +407,12 @@ public class ForkJoinPool extends Abstra
this.termination = workerLock.newCondition();
this.stealCount = new AtomicLong();
this.submissionQueue = new LinkedTransferQueue("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public Thread.UncaughtExceptionHandler
setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler h) {
@@ -677,10 +723,12 @@ public class ForkJoinPool extends Abstra
old = ueh;
ueh = h;
ForkJoinWorkerThread[] ws = workers;
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread w = ws[i];
- if (w != null)
- w.setUncaughtExceptionHandler(h);
+ if (ws != null) {
+ for (int i = 0; i < ws.length; ++i) {
+ ForkJoinWorkerThread w = ws[i];
+ if (w != null)
+ w.setUncaughtExceptionHandler(h);
+ }
}
} finally {
lock.unlock();
@@ -690,14 +738,15 @@ public class ForkJoinPool extends Abstra
/**
- * Sets the target paralleism level of this pool.
+ * 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.
+ * 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}("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public void setParallelism(int parallelism) {
checkPermission();
@@ -717,7 +766,7 @@ public class ForkJoinPool extends Abstra
} finally {
lock.unlock();
}
- signalIdleWorkers(false);
+ signalIdleWorkers();
}
/**
@@ -732,7 +781,7 @@ public class ForkJoinPool extends Abstra
/**
* Returns the number of worker threads that have started but not
* yet terminated. This result returned by this method may differ
- * from getParallelism
when threads are created to
+ * from {@code getParallelism} when threads are created to
* maintain parallelism when others are cooperatively blocked.
*
* @return the number of worker threads
@@ -744,6 +793,7 @@ public class ForkJoinPool extends Abstra
/**
* Returns the maximum number of threads allowed to exist in the
* pool, even if there are insufficient unblocked running threads.
+ *
* @return the maximum
*/
public int getMaximumPoolSize() {
@@ -755,8 +805,9 @@ public class ForkJoinPool extends Abstra
* pool, even if there are insufficient unblocked running threads.
* Setting this value has no effect on current pool size. It
* controls construction of new threads.
+ *
* @throws IllegalArgumentException if negative or greater then
- * internal implementation limit.
+ * internal implementation limit
*/
public void setMaximumPoolSize(int newMax) {
if (newMax < 0 || newMax > MAX_THREADS)
@@ -769,7 +820,8 @@ public class ForkJoinPool extends Abstra
* Returns 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;
+ * This setting is by default true.
+ *
* @return true if maintains parallelism
*/
public boolean getMaintainsParallelism() {
@@ -780,6 +832,7 @@ public class ForkJoinPool extends Abstra
* Sets whether this pool dynamically maintains its target
* parallelism level. If false, new threads are added only to
* avoid possible starvation.
+ *
* @param enable true to maintains parallelism
*/
public void setMaintainsParallelism(boolean enable) {
@@ -787,6 +840,42 @@ public class ForkJoinPool extends Abstra
}
/**
+ * 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 pool is quiescent, and
+ * typically only before any tasks are submitted. The effects of
+ * invocations at other times may be unpredictable.
+ *
+ * @param async if true, use locally FIFO scheduling
+ * @return the previous mode
+ */
+ 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 true if this pool uses local first-in-first-out
+ * scheduling mode for forked tasks that are never joined.
+ *
+ * @return true if this pool uses async mode
+ */
+ 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.
@@ -801,7 +890,8 @@ public class ForkJoinPool extends Abstra
* 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.
+ *
+ * @return the number of active threads
*/
public int getActiveThreadCount() {
return activeCountOf(runControl);
@@ -811,21 +901,23 @@ public class ForkJoinPool extends Abstra
* 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.
+ *
+ * @return the number of idle threads
*/
final int getIdleThreadCount() {
int c = runningCountOf(workerCounts) - activeCountOf(runControl);
- return (c <= 0)? 0 : c;
+ return (c <= 0) ? 0 : c;
}
/**
* Returns 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 true immediately upon idleness of all
+ * pending submissions to the pool. This method is conservative;
+ * it might not return true immediately upon idleness of all
* threads, but will eventually become true if threads remain
* inactive.
+ *
* @return true if all threads are currently idle
*/
public boolean isQuiescent() {
@@ -837,18 +929,19 @@ public class ForkJoinPool extends Abstra
* 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
+ * 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.
+ *
+ * @return the number of steals
*/
public long getStealCount() {
return stealCount.get();
}
/**
- * Accumulate steal count from a worker. Call only
- * when worker known to be idle.
+ * Accumulates steal count from a worker.
+ * Call only when worker known to be idle.
*/
private void updateStealCount(ForkJoinWorkerThread w) {
int sc = w.getAndClearStealCount();
@@ -863,15 +956,18 @@ public class ForkJoinPool extends Abstra
* 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.
+ *
+ * @return the number of queued tasks
*/
public long getQueuedTaskCount() {
long count = 0;
ForkJoinWorkerThread[] ws = workers;
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null)
- count += t.getQueueSize();
+ if (ws != null) {
+ for (int i = 0; i < ws.length; ++i) {
+ ForkJoinWorkerThread t = ws[i];
+ if (t != null)
+ count += t.getQueueSize();
+ }
}
return count;
}
@@ -880,7 +976,8 @@ public class ForkJoinPool extends Abstra
* Returns an estimate of the number 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.
+ *
+ * @return the number of queued submissions
*/
public int getQueuedSubmissionCount() {
return submissionQueue.size();
@@ -889,7 +986,8 @@ public class ForkJoinPool extends Abstra
/**
* Returns true if there are any tasks submitted to this pool
* that have not yet begun executing.
- * @return true
if there are any queued submissions.
+ *
+ * @return {@code true} if there are any queued submissions
*/
public boolean hasQueuedSubmissions() {
return !submissionQueue.isEmpty();
@@ -899,6 +997,7 @@ public class ForkJoinPool extends Abstra
* 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 null if none
*/
protected ForkJoinTask> pollSubmission() {
@@ -906,6 +1005,36 @@ public class ForkJoinPool extends Abstra
}
/**
+ * 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("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public void shutdown() {
checkPermission();
@@ -966,13 +1096,16 @@ public class ForkJoinPool extends Abstra
* waiting 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. Unlike some other executors,
- * this method cancels rather than collects non-executed tasks,
- * so always returns an empty list.
+ * this method cancels rather than collects non-executed tasks
+ * upon termination, so always returns an empty list. However, you
+ * can use method {@code drainTasksTo} before invoking this
+ * method to transfer unexecuted tasks to another collection.
+ *
* @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}("modifyThread")
,
+ * java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public Listtrue
if all tasks have completed following shut down.
+ * Returns {@code true} if all tasks have completed following shut down.
*
- * @return 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 true
if the process of termination has
+ * Returns {@code true} if the process of termination has
* commenced but possibly not yet completed.
*
- * @return true
if terminating
+ * @return {@code true} if terminating
*/
public boolean isTerminating() {
return runStateOf(runControl) >= TERMINATING;
}
/**
- * Returns true
if this pool has been shut down.
+ * Returns {@code true} if this pool has been shut down.
*
- * @return 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;
@@ -1015,8 +1148,8 @@ public class ForkJoinPool extends Abstra
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
- * @return true
if this executor terminated and
- * false
if the timeout elapsed before termination
+ * @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)
@@ -1040,9 +1173,10 @@ public class ForkJoinPool extends Abstra
// Shutdown and termination support
/**
- * Callback from terminating worker. Null out the corresponding
- * workers slot, and if terminating, try to terminate, else try to
- * shrink workers array.
+ * 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) {
@@ -1052,41 +1186,43 @@ public class ForkJoinPool extends Abstra
lock.lock();
try {
ForkJoinWorkerThread[] ws = workers;
- 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 (!isTerminating()) {
- tryShrinkWorkerArray();
- tryResumeSpare(true); // allow replacement
+ 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 (!isTerminating()) {
+ tryShrinkWorkerArray();
+ tryResumeSpare(true); // allow replacement
+ }
}
} finally {
lock.unlock();
}
- signalIdleWorkers(false);
+ signalIdleWorkers();
}
/**
- * Initiate termination.
+ * Initiates termination.
*/
private void terminate() {
if (transitionRunStateTo(TERMINATING)) {
stopAllWorkers();
resumeAllSpares();
- signalIdleWorkers(true);
+ signalIdleWorkers();
cancelQueuedSubmissions();
cancelQueuedWorkerTasks();
interruptUnterminatedWorkers();
- signalIdleWorkers(true); // resignal after interrupt
+ signalIdleWorkers(); // resignal after interrupt
}
}
/**
- * Possibly terminate when on shutdown state
+ * Possibly terminates when on shutdown state.
*/
private void terminateOnShutdown() {
if (!hasQueuedSubmissions() && canTerminateOnShutdown(runControl))
@@ -1094,7 +1230,7 @@ public class ForkJoinPool extends Abstra
}
/**
- * Clear out and cancel submissions
+ * Clears out and cancels submissions.
*/
private void cancelQueuedSubmissions() {
ForkJoinTask> task;
@@ -1103,17 +1239,19 @@ public class ForkJoinPool extends Abstra
}
/**
- * Clean out worker queues.
+ * Cleans out worker queues.
*/
private void cancelQueuedWorkerTasks() {
final ReentrantLock lock = this.workerLock;
lock.lock();
try {
ForkJoinWorkerThread[] ws = workers;
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null)
- t.cancelTasks();
+ if (ws != null) {
+ for (int i = 0; i < ws.length; ++i) {
+ ForkJoinWorkerThread t = ws[i];
+ if (t != null)
+ t.cancelTasks();
+ }
}
} finally {
lock.unlock();
@@ -1121,18 +1259,20 @@ public class ForkJoinPool extends Abstra
}
/**
- * Set each worker's status to terminating. Requires lock to avoid
- * conflicts with add/remove
+ * 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;
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null)
- t.shutdownNow();
+ if (ws != null) {
+ for (int i = 0; i < ws.length; ++i) {
+ ForkJoinWorkerThread t = ws[i];
+ if (t != null)
+ t.shutdownNow();
+ }
}
} finally {
lock.unlock();
@@ -1140,7 +1280,7 @@ public class ForkJoinPool extends Abstra
}
/**
- * Interrupt all unterminated workers. This is not required for
+ * Interrupts all unterminated workers. This is not required for
* sake of internal control, but may help unstick user code during
* shutdown.
*/
@@ -1149,12 +1289,14 @@ public class ForkJoinPool extends Abstra
lock.lock();
try {
ForkJoinWorkerThread[] ws = workers;
- for (int i = 0; i < ws.length; ++i) {
- ForkJoinWorkerThread t = ws[i];
- if (t != null && !t.isTerminated()) {
- try {
- t.interrupt();
- } catch (SecurityException ignore) {
+ 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) {
+ }
}
}
}
@@ -1165,75 +1307,93 @@ public class ForkJoinPool extends Abstra
/*
- * Nodes for event barrier to manage idle threads.
+ * 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 some,
- * they may wait for next count. Synchronization events occur only
- * in enough contexts to maintain overall liveness:
+ * 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
- * - Creation or termination of a worker
+ * - Resizes or other changes to the workers array
* - pool termination
* - A worker pushing a task on an empty queue
*
- * The last case (pushing a task) occurs often enough, and is
- * heavy enough compared to simple stack pushes to require some
- * special handling: Method signalNonEmptyWorkerQueue 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, a worker in sync
- * rescans for tasks after being enqueued if it was the first to
- * enqueue, and aborts the wait if finding one, also helping to
- * signal others. This works well because the worker has nothing
- * better to do anyway, and so might as well help alleviate the
- * overhead and contention on the threads actually doing work.
- *
- * Queue nodes are basic Treiber stack nodes, also used for spare
- * stack.
+ * 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(ForkJoinWorkerThread w, long c) {
+
+ WaitQueueNode(long c, ForkJoinWorkerThread w) {
count = c;
thread = w;
}
- final boolean signal() {
+
+ /**
+ * Wakes up waiter, returning false if known to already
+ */
+ boolean signal() {
ForkJoinWorkerThread t = thread;
+ if (t == null)
+ return false;
thread = null;
- if (t != null) {
- LockSupport.unpark(t);
- return true;
+ 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);
}
- return false;
}
}
/**
- * Release at least one thread waiting for event count to advance,
- * if one exists. If initial attempt fails, release all threads.
- * @param all if false, at first try to only release one thread
- * @return current event
+ * 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
*/
- private long releaseIdleWorkers(boolean all) {
- long c;
- for (;;) {
- WaitQueueNode q = barrierStack;
- c = eventCount;
- long qc;
- if (q == null || (qc = q.count) >= c)
- break;
- if (!all) {
- if (casBarrierStack(q, q.next) && q.signal())
- break;
- all = true;
- }
- else if (casBarrierStack(q, null)) {
+ final long ensureSync() {
+ long c = eventCount;
+ WaitQueueNode q;
+ while ((q = syncStack) != null && q.count < c) {
+ if (casBarrierStack(q, null)) {
do {
- q.signal();
+ q.signal();
} while ((q = q.next) != null);
break;
}
@@ -1242,94 +1402,111 @@ public class ForkJoinPool extends Abstra
}
/**
- * Returns current barrier event count
- * @return current barrier event count
- */
- final long getEventCount() {
- long ec = eventCount;
- releaseIdleWorkers(true); // release to ensure accurate result
- return ec;
- }
-
- /**
- * Increment event count and release at least one waiting thread,
- * if one exists (released threads will in turn wake up others).
- * @param all if true, try to wake up all
+ * Increments event count and releases waiting threads.
*/
- final void signalIdleWorkers(boolean all) {
+ private void signalIdleWorkers() {
long c;
- do;while (!casEventCount(c = eventCount, c+1));
- releaseIdleWorkers(all);
+ do {} while (!casEventCount(c = eventCount, c+1));
+ ensureSync();
}
/**
- * Wake up threads waiting to steal a task. Because method
- * sync rechecks availability, it is OK to only proceed if
- * queue appears to be non-empty.
+ * 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 signalNonEmptyWorkerQueue() {
- // If CAS fails another signaller must have succeeded
+ final void signalWork() {
long c;
- if (barrierStack != null && casEventCount(c = eventCount, c+1))
- releaseIdleWorkers(false);
+ 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 count, or some thread is
- * waiting on a previous count, or there is stealable work
- * available. Help wake up others on release.
+ * 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
- * @param prev previous value returned by sync (or 0)
- * @return current event count
*/
- final long sync(ForkJoinWorkerThread w, long prev) {
- updateStealCount(w);
+ final void sync(ForkJoinWorkerThread w) {
+ updateStealCount(w); // Transfer w's count while it is idle
- while (!w.isShutdown() && !isTerminating() &&
- (parallelism >= runningCountOf(workerCounts) ||
- !suspendIfSpare(w))) { // prefer suspend to waiting here
+ while (!w.isShutdown() && !isTerminating() && !suspendIfSpare(w)) {
+ long prev = w.lastEventCount;
WaitQueueNode node = null;
- boolean queued = false;
- for (;;) {
- if (!queued) {
- if (eventCount != prev)
- break;
- WaitQueueNode h = barrierStack;
- if (h != null && h.count != prev)
- break; // release below and maybe retry
- if (node == null)
- node = new WaitQueueNode(w, prev);
- queued = casBarrierStack(node.next = h, node);
- }
- else if (Thread.interrupted() ||
- node.thread == null ||
- (node.next == null && w.prescan()) ||
- eventCount != prev) {
- node.thread = null;
- if (eventCount == prev) // help trigger
- casEventCount(prev, prev+1);
+ 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;
}
- else
- LockSupport.park(this);
}
+ long ec = ensureSync();
+ if (ec != prev) {
+ w.lastEventCount = ec;
+ break;
+ }
+ }
+ }
+
+ /**
+ * Returns 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 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 (releaseIdleWorkers(false) != prev)
- return ec;
+ if (prev <= ec) // help signal
+ casEventCount(ec, ec+1);
}
- return prev; // return old count if aborted
+ return true;
+ }
+
+ /**
+ * Returns 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
/**
- * Decrement running count; if too low, add spare.
+ * 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
+ * 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
@@ -1343,7 +1520,7 @@ public class ForkJoinPool extends Abstra
* 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 enounter highly transient peaks when
+ * 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.
*
@@ -1352,12 +1529,14 @@ public class ForkJoinPool extends Abstra
* target counts, else create only to avoid starvation
* @return true if joinMe known to be done
*/
- final boolean preJoin(ForkJoinTask> joinMe, boolean maintainParallelism) {
+ 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))) { // CAS cheat
+ if (dec || (dec = casWorkerCounts(counts, --counts))) {
+ // CAS cheat
if (!needSpare(counts, maintainParallelism))
break;
if (joinMe.status < 0)
@@ -1372,7 +1551,8 @@ public class ForkJoinPool extends Abstra
/**
* Same idea as preJoin
*/
- final boolean preBlock(ManagedBlocker blocker, boolean maintainParallelism){
+ final boolean preBlock(ManagedBlocker blocker,
+ boolean maintainParallelism) {
maintainParallelism &= maintainsParallelism;
boolean dec = false;
while (spareStack == null || !tryResumeSpare(dec)) {
@@ -1396,6 +1576,7 @@ public class ForkJoinPool extends Abstra
* 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
*/
@@ -1408,31 +1589,14 @@ public class ForkJoinPool extends Abstra
return (tc < maxPoolSize &&
(rc == 0 || totalSurplus < 0 ||
(maintainParallelism &&
- runningDeficit > totalSurplus && mayHaveQueuedWork())));
- }
-
- /**
- * Returns true if at least one worker queue appears to be
- * nonempty. This is expensive but not often called. It is not
- * critical that this be accurate, but if not, more or fewer
- * running threads than desired might be maintained.
- */
- private boolean mayHaveQueuedWork() {
- ForkJoinWorkerThread[] ws = workers;
- int len = ws.length;
- ForkJoinWorkerThread v;
- for (int i = 0; i < len; ++i) {
- if ((v = ws[i]) != null && v.getRawQueueSize() > 0) {
- releaseIdleWorkers(false); // help wake up stragglers
- return true;
- }
- }
- return false;
+ runningDeficit > totalSurplus &&
+ ForkJoinWorkerThread.hasQueuedTasks(workers))));
}
/**
- * Add a spare worker if lock available and no more than the
- * expected numbers of threads exist
+ * 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) {
@@ -1465,7 +1629,7 @@ public class ForkJoinPool extends Abstra
}
/**
- * Add the kth spare worker. On entry, pool coounts are already
+ * Adds the kth spare worker. On entry, pool counts are already
* adjusted to reflect addition.
*/
private void createAndStartSpare(int k) {
@@ -1483,15 +1647,16 @@ public class ForkJoinPool extends Abstra
}
else
updateWorkerCount(-1); // adjust on failure
- signalIdleWorkers(false);
+ signalIdleWorkers();
}
/**
- * Suspend 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.
+ * 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) {
@@ -1499,17 +1664,12 @@ public class ForkJoinPool extends Abstra
int s;
while (parallelism < runningCountOf(s = workerCounts)) {
if (node == null)
- node = new WaitQueueNode(w, 0);
+ node = new WaitQueueNode(0, w);
if (casWorkerCounts(s, s-1)) { // representation-dependent
// push onto stack
- do;while (!casSpareStack(node.next = spareStack, node));
-
+ do {} while (!casSpareStack(node.next = spareStack, node));
// block until released by resumeSpare
- while (node.thread != null) {
- if (!Thread.interrupted())
- LockSupport.park(this);
- }
- w.activate(); // help warm up
+ node.awaitSpareRelease();
return true;
}
}
@@ -1517,7 +1677,8 @@ public class ForkJoinPool extends Abstra
}
/**
- * Try to pop and resume a spare thread.
+ * Tries to pop and resume a spare thread.
+ *
* @param updateCount if true, increment running count on success
* @return true if successful
*/
@@ -1535,8 +1696,8 @@ public class ForkJoinPool extends Abstra
}
/**
- * Pop and resume all spare threads. Same idea as
- * releaseIdleWorkers.
+ * Pops and resumes all spare threads. Same idea as ensureSync.
+ *
* @return true if any spares released
*/
private boolean resumeAllSpares() {
@@ -1554,7 +1715,7 @@ public class ForkJoinPool extends Abstra
}
/**
- * Pop and shutdown excessive spare threads. Call only while
+ * 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.
@@ -1577,48 +1738,39 @@ public class ForkJoinPool extends Abstra
}
/**
- * Returns approximate number of spares, just for diagnostics.
- */
- private int countSpares() {
- int sum = 0;
- for (WaitQueueNode q = spareStack; q != null; q = q.next)
- ++sum;
- return sum;
- }
-
- /**
* Interface for extending managed parallelism for tasks running
* in ForkJoinPools. A ManagedBlocker provides two methods.
- * Method isReleasable
must return true if blocking is not
- * necessary. Method block
blocks the current thread
- * if necessary (perhaps internally invoking isReleasable before
- * actually blocking.).
+ * Method {@code isReleasable} must return true if blocking is not
+ * necessary. Method {@code block} blocks the current thread if
+ * necessary (perhaps internally invoking {@code isReleasable}
+ * before actually blocking.).
+ *
*
- * 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());
- * }
+ *
*/
public static interface ManagedBlocker {
/**
* Possibly blocks the current thread, for example waiting for
* a lock or condition.
+ *
* @return true if no additional blocking is necessary (i.e.,
- * if isReleasable would return true).
+ * if isReleasable would return true)
* @throws InterruptedException if interrupted while waiting
- * (the method is not required to do so, but is allowe to).
+ * (the method is not required to do so, but is allowed to)
*/
boolean block() throws InterruptedException;
@@ -1633,20 +1785,20 @@ public class ForkJoinPool extends Abstra
* is a 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
+ * 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());
* }
- *
+ * }}maintainParallelism
is true and the pool supports
+ * {@code maintainParallelism} is true and the pool supports
* it ({@link #getMaintainsParallelism}), this method attempts to
- * maintain the pool's nominal parallelism. Otherwise if activates
+ * 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.
*
*
- * while (!blocker.isReleasable())
- * if (blocker.block())
- * return;
- *
+ * {@code
+ * while (!blocker.isReleasable())
+ * if (blocker.block())
+ * return;
+ * }
* If the caller is a ForkJoinTask, then the pool may first
* be expanded to ensure parallelism, and later adjusted.
*
@@ -1655,14 +1807,14 @@ public class ForkJoinPool extends Abstra
* 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.
+ * @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);
+ ForkJoinPool pool = ((t instanceof ForkJoinWorkerThread) ?
+ ((ForkJoinWorkerThread) t).pool : null);
if (!blocker.isReleasable()) {
try {
if (pool == null ||
@@ -1677,66 +1829,82 @@ public class ForkJoinPool extends Abstra
private static void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
- do;while (!blocker.isReleasable() && !blocker.block());
+ do {} while (!blocker.isReleasable() && !blocker.block());
}
// AbstractExecutorService overrides
protected