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root/jsr166/jsr166/src/jsr166y/ForkJoinTask.java

Comparing jsr166/src/jsr166y/ForkJoinTask.java (file contents):
Revision 1.90 by dl, Sat Apr 21 11:45:20 2012 UTC vs.
Revision 1.93 by dl, Wed Nov 14 17:20:37 2012 UTC

#	Line 30 | Line 30 | import java.lang.reflect.Constructor;
30		* subtasks may be hosted by a small number of actual threads in a
31		* ForkJoinPool, at the price of some usage limitations.
32		*
33	<	* <p>A "main" {@code ForkJoinTask} begins execution when submitted
34	<	* to a {@link ForkJoinPool}.  Once started, it will usually in turn
35	<	* start other subtasks.  As indicated by the name of this class,
36	<	* many programs using {@code ForkJoinTask} employ only methods
37	<	* {@link #fork} and {@link #join}, or derivatives such as {@link
33	>	* <p>A "main" {@code ForkJoinTask} begins execution when it is
34	>	* explicitly submitted to a {@link ForkJoinPool}, or, if not already
35	>	* engaged in a ForkJoin computation, commenced in the {@link
36	>	* ForkJoinPool#commonPool} via {@link #fork}, {@link #invoke}, or
37	>	* related methods.  Once started, it will usually in turn start other
38	>	* subtasks.  As indicated by the name of this class, many programs
39	>	* using {@code ForkJoinTask} employ only methods {@link #fork} and
40	>	* {@link #join}, or derivatives such as {@link
41		* #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
42		* provides a number of other methods that can come into play in
43	<	* advanced usages, as well as extension mechanics that allow
44	<	* support of new forms of fork/join processing.
43	>	* advanced usages, as well as extension mechanics that allow support
44	>	* of new forms of fork/join processing.
45		*
46		* <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
47		* The efficiency of {@code ForkJoinTask}s stems from a set of
#	Line 123 | Line 126 | import java.lang.reflect.Constructor;
126		* other actions.  Normally, a concrete ForkJoinTask subclass declares
127		* fields comprising its parameters, established in a constructor, and
128		* then defines a {@code compute} method that somehow uses the control
129	<	* methods supplied by this base class. While these methods have
127	<	* {@code public} access (to allow instances of different task
128	<	* subclasses to call each other's methods), some of them may only be
129	<	* called from within other ForkJoinTasks (as may be determined using
130	<	* method {@link #inForkJoinPool}).  Attempts to invoke them in other
131	<	* contexts result in exceptions or errors, possibly including {@code
132	<	* ClassCastException}.
129	>	* methods supplied by this base class.
130		*
131		* <p>Method {@link #join} and its variants are appropriate for use
132		* only when completion dependencies are acyclic; that is, the
#	Line 287 | Line 284 | public abstract class ForkJoinTask<V> im
284		* @return status upon completion
285		*/
286		private int externalAwaitDone() {
290	–	boolean interrupted = false;
287		int s;
288	+	ForkJoinPool.externalHelpJoin(this);
289	+	boolean interrupted = false;
290		while ((s = status) >= 0) {
291		if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
292		synchronized (this) {
#	Line 316 | Line 314 | public abstract class ForkJoinTask<V> im
314		int s;
315		if (Thread.interrupted())
316		throw new InterruptedException();
317	+	ForkJoinPool.externalHelpJoin(this);
318		while ((s = status) >= 0) {
319		if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
320		synchronized (this) {
#	Line 329 | Line 328 | public abstract class ForkJoinTask<V> im
328		return s;
329		}
330
331	+
332		/**
333		* Implementation for join, get, quietlyJoin. Directly handles
334		* only cases of already-completed, external wait, and
#	Line 338 | Line 338 | public abstract class ForkJoinTask<V> im
338		*/
339		private int doJoin() {
340		int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
341	<	if ((s = status) >= 0) {
342	<	if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
343	<	if (!(w = (wt = (ForkJoinWorkerThread)t).workQueue).
344	<	tryUnpush(this) || (s = doExec()) >= 0)
345	<	s = wt.pool.awaitJoin(w, this);
346	<	}
347	<	else
348	<	s = externalAwaitDone();
349	<	}
350	<	return s;
341	>	return (s = status) < 0 ? s :
342	>	((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
343	>	(w = (wt = (ForkJoinWorkerThread)t).workQueue).
344	>	tryUnpush(this) && (s = doExec()) < 0 ? s :
345	>	wt.pool.awaitJoin(w, this) :
346	>	externalAwaitDone();
347		}
348
349		/**
#	Line 357 | Line 353 | public abstract class ForkJoinTask<V> im
353		*/
354		private int doInvoke() {
355		int s; Thread t; ForkJoinWorkerThread wt;
356	<	if ((s = doExec()) >= 0) {
357	<	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
358	<	s = (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue,
359	<	this);
364	<	else
365	<	s = externalAwaitDone();
366	<	}
367	<	return s;
356	>	return (s = doExec()) < 0 ? s :
357	>	((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
358	>	(wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
359	>	externalAwaitDone();
360		}
361
362		// Exception table support
#	Line 606 | Line 598 | public abstract class ForkJoinTask<V> im
598		}
599
600		/**
601	+	* A version of "sneaky throw" to relay exceptions
602	+	*/
603	+	static void rethrow(final Throwable ex) {
604	+	if (ex != null) {
605	+	if (ex instanceof Error)
606	+	throw (Error)ex;
607	+	if (ex instanceof RuntimeException)
608	+	throw (RuntimeException)ex;
609	+	throw uncheckedThrowable(ex, RuntimeException.class);
610	+	}
611	+	}
612	+
613	+	/**
614	+	* The sneaky part of sneaky throw, relying on generics
615	+	* limitations to evade compiler complaints about rethrowing
616	+	* unchecked exceptions
617	+	*/
618	+	@SuppressWarnings("unchecked") static <T extends Throwable>
619	+	T uncheckedThrowable(final Throwable t, final Class<T> c) {
620	+	return (T)t; // rely on vacuous cast
621	+	}
622	+
623	+	/**
624		* Throws exception, if any, associated with the given status.
625		*/
626		private void reportException(int s) {
627	<	Throwable ex = ((s == CANCELLED) ?  new CancellationException() :
628	<	(s == EXCEPTIONAL) ? getThrowableException() :
629	<	null);
630	<	if (ex != null)
616	<	U.throwException(ex);
627	>	if (s == CANCELLED)
628	>	throw new CancellationException();
629	>	if (s == EXCEPTIONAL)
630	>	rethrow(getThrowableException());
631		}
632
633		// public methods
634
635		/**
636	<	* Arranges to asynchronously execute this task.  While it is not
637	<	* necessarily enforced, it is a usage error to fork a task more
638	<	* than once unless it has completed and been reinitialized.
639	<	* Subsequent modifications to the state of this task or any data
640	<	* it operates on are not necessarily consistently observable by
641	<	* any thread other than the one executing it unless preceded by a
642	<	* call to {@link #join} or related methods, or a call to {@link
643	<	* #isDone} returning {@code true}.
644	<	*
645	<	* <p>This method may be invoked only from within {@code
646	<	* ForkJoinPool} computations (as may be determined using method
633	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
634	<	* result in exceptions or errors, possibly including {@code
635	<	* ClassCastException}.
636	>	* Arranges to asynchronously execute this task in the pool the
637	>	* current task is running in, if applicable, or using the {@link
638	>	* ForkJoinPool#commonPool} if not {@link #inForkJoinPool}.  While
639	>	* it is not necessarily enforced, it is a usage error to fork a
640	>	* task more than once unless it has completed and been
641	>	* reinitialized.  Subsequent modifications to the state of this
642	>	* task or any data it operates on are not necessarily
643	>	* consistently observable by any thread other than the one
644	>	* executing it unless preceded by a call to {@link #join} or
645	>	* related methods, or a call to {@link #isDone} returning {@code
646	>	* true}.
647		*
648		* @return {@code this}, to simplify usage
649		*/
650		public final ForkJoinTask<V> fork() {
651	<	((ForkJoinWorkerThread)Thread.currentThread()).workQueue.push(this);
651	>	Thread t;
652	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
653	>	((ForkJoinWorkerThread)t).workQueue.push(this);
654	>	else
655	>	ForkJoinPool.commonPool.externalPush(this);
656		return this;
657		}
658
#	Line 687 | Line 702 | public abstract class ForkJoinTask<V> im
702		* cancelled, completed normally or exceptionally, or left
703		* unprocessed.
704		*
690	–	* <p>This method may be invoked only from within {@code
691	–	* ForkJoinPool} computations (as may be determined using method
692	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
693	–	* result in exceptions or errors, possibly including {@code
694	–	* ClassCastException}.
695	–	*
705		* @param t1 the first task
706		* @param t2 the second task
707		* @throws NullPointerException if any task is null
#	Line 718 | Line 727 | public abstract class ForkJoinTask<V> im
727		* related methods to check if they have been cancelled, completed
728		* normally or exceptionally, or left unprocessed.
729		*
721	–	* <p>This method may be invoked only from within {@code
722	–	* ForkJoinPool} computations (as may be determined using method
723	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
724	–	* result in exceptions or errors, possibly including {@code
725	–	* ClassCastException}.
726	–	*
730		* @param tasks the tasks
731		* @throws NullPointerException if any task is null
732		*/
#	Line 751 | Line 754 | public abstract class ForkJoinTask<V> im
754		}
755		}
756		if (ex != null)
757	<	U.throwException(ex);
757	>	rethrow(ex);
758		}
759
760		/**
#	Line 767 | Line 770 | public abstract class ForkJoinTask<V> im
770		* cancelled, completed normally or exceptionally, or left
771		* unprocessed.
772		*
770	–	* <p>This method may be invoked only from within {@code
771	–	* ForkJoinPool} computations (as may be determined using method
772	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
773	–	* result in exceptions or errors, possibly including {@code
774	–	* ClassCastException}.
775	–	*
773		* @param tasks the collection of tasks
774		* @return the tasks argument, to simplify usage
775		* @throws NullPointerException if tasks or any element are null
#	Line 808 | Line 805 | public abstract class ForkJoinTask<V> im
805		}
806		}
807		if (ex != null)
808	<	U.throwException(ex);
808	>	rethrow(ex);
809		return tasks;
810		}
811
#	Line 991 | Line 988 | public abstract class ForkJoinTask<V> im
988		ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
989		p = wt.pool;
990		w = wt.workQueue;
991	<	s = p.helpJoinOnce(w, this); // no retries on failure
991	>	p.helpJoinOnce(w, this); // no retries on failure
992		}
993	+	else
994	+	ForkJoinPool.externalHelpJoin(this);
995		boolean canBlock = false;
996		boolean interrupted = false;
997		try {
998		while ((s = status) >= 0) {
999	<	if (w != null && w.runState < 0)
999	>	if (w != null && w.qlock < 0)
1000		cancelIgnoringExceptions(this);
1001		else if (!canBlock) {
1002	<	if (p == null || p.tryCompensate(this, null))
1002	>	if (p == null || p.tryCompensate())
1003		canBlock = true;
1004		}
1005		else {
#	Line 1068 | Line 1067 | public abstract class ForkJoinTask<V> im
1067		* be of use in designs in which many tasks are forked, but none
1068		* are explicitly joined, instead executing them until all are
1069		* processed.
1071	–	*
1072	–	* <p>This method may be invoked only from within {@code
1073	–	* ForkJoinPool} computations (as may be determined using method
1074	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1075	–	* result in exceptions or errors, possibly including {@code
1076	–	* ClassCastException}.
1070		*/
1071		public static void helpQuiesce() {
1072	<	ForkJoinWorkerThread wt =
1073	<	(ForkJoinWorkerThread)Thread.currentThread();
1074	<	wt.pool.helpQuiescePool(wt.workQueue);
1072	>	Thread t;
1073	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
1074	>	ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
1075	>	wt.pool.helpQuiescePool(wt.workQueue);
1076	>	}
1077	>	else
1078	>	ForkJoinPool.externalHelpQuiescePool();
1079		}
1080
1081		/**
#	Line 1131 | Line 1128 | public abstract class ForkJoinTask<V> im
1128
1129		/**
1130		* Tries to unschedule this task for execution. This method will
1131	<	* typically succeed if this task is the most recently forked task
1132	<	* by the current thread, and has not commenced executing in
1133	<	* another thread.  This method may be useful when arranging
1134	<	* alternative local processing of tasks that could have been, but
1135	<	* were not, stolen.
1139	<	*
1140	<	* <p>This method may be invoked only from within {@code
1141	<	* ForkJoinPool} computations (as may be determined using method
1142	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1143	<	* result in exceptions or errors, possibly including {@code
1144	<	* ClassCastException}.
1131	>	* typically (but is not guaranteed to) succeed if this task is
1132	>	* the most recently forked task by the current thread, and has
1133	>	* not commenced executing in another thread.  This method may be
1134	>	* useful when arranging alternative local processing of tasks
1135	>	* that could have been, but were not, stolen.
1136		*
1137		* @return {@code true} if unforked
1138		*/
1139		public boolean tryUnfork() {
1140	<	return ((ForkJoinWorkerThread)Thread.currentThread())
1141	<	.workQueue.tryUnpush(this);
1140	>	Thread t;
1141	>	return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1142	>	((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
1143	>	ForkJoinPool.tryExternalUnpush(this));
1144		}
1145
1146		/**
#	Line 1156 | Line 1149 | public abstract class ForkJoinTask<V> im
1149		* value may be useful for heuristic decisions about whether to
1150		* fork other tasks.
1151		*
1159	–	* <p>This method may be invoked only from within {@code
1160	–	* ForkJoinPool} computations (as may be determined using method
1161	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1162	–	* result in exceptions or errors, possibly including {@code
1163	–	* ClassCastException}.
1164	–	*
1152		* @return the number of tasks
1153		*/
1154		public static int getQueuedTaskCount() {
1155	<	return ((ForkJoinWorkerThread) Thread.currentThread())
1156	<	.workQueue.queueSize();
1155	>	Thread t; ForkJoinPool.WorkQueue q;
1156	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
1157	>	q = ((ForkJoinWorkerThread)t).workQueue;
1158	>	else
1159	>	q = ForkJoinPool.commonSubmitterQueue();
1160	>	return (q == null) ? 0 : q.queueSize();
1161		}
1162
1163		/**
1164		* Returns an estimate of how many more locally queued tasks are
1165		* held by the current worker thread than there are other worker
1166	<	* threads that might steal them.  This value may be useful for
1166	>	* threads that might steal them, or zero if this thread is not
1167	>	* operating in a ForkJoinPool. This value may be useful for
1168		* heuristic decisions about whether to fork other tasks. In many
1169		* usages of ForkJoinTasks, at steady state, each worker should
1170		* aim to maintain a small constant surplus (for example, 3) of
1171		* tasks, and to process computations locally if this threshold is
1172		* exceeded.
1173		*
1182	–	* <p>This method may be invoked only from within {@code
1183	–	* ForkJoinPool} computations (as may be determined using method
1184	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1185	–	* result in exceptions or errors, possibly including {@code
1186	–	* ClassCastException}.
1187	–	*
1174		* @return the surplus number of tasks, which may be negative
1175		*/
1176		public static int getSurplusQueuedTaskCount() {
1177	<	/*
1192	<	* The aim of this method is to return a cheap heuristic guide
1193	<	* for task partitioning when programmers, frameworks, tools,
1194	<	* or languages have little or no idea about task granularity.
1195	<	* In essence by offering this method, we ask users only about
1196	<	* tradeoffs in overhead vs expected throughput and its
1197	<	* variance, rather than how finely to partition tasks.
1198	<	*
1199	<	* In a steady state strict (tree-structured) computation,
1200	<	* each thread makes available for stealing enough tasks for
1201	<	* other threads to remain active. Inductively, if all threads
1202	<	* play by the same rules, each thread should make available
1203	<	* only a constant number of tasks.
1204	<	*
1205	<	* The minimum useful constant is just 1. But using a value of
1206	<	* 1 would require immediate replenishment upon each steal to
1207	<	* maintain enough tasks, which is infeasible.  Further,
1208	<	* partitionings/granularities of offered tasks should
1209	<	* minimize steal rates, which in general means that threads
1210	<	* nearer the top of computation tree should generate more
1211	<	* than those nearer the bottom. In perfect steady state, each
1212	<	* thread is at approximately the same level of computation
1213	<	* tree. However, producing extra tasks amortizes the
1214	<	* uncertainty of progress and diffusion assumptions.
1215	<	*
1216	<	* So, users will want to use values larger, but not much
1217	<	* larger than 1 to both smooth over transient shortages and
1218	<	* hedge against uneven progress; as traded off against the
1219	<	* cost of extra task overhead. We leave the user to pick a
1220	<	* threshold value to compare with the results of this call to
1221	<	* guide decisions, but recommend values such as 3.
1222	<	*
1223	<	* When all threads are active, it is on average OK to
1224	<	* estimate surplus strictly locally. In steady-state, if one
1225	<	* thread is maintaining say 2 surplus tasks, then so are
1226	<	* others. So we can just use estimated queue length.
1227	<	* However, this strategy alone leads to serious mis-estimates
1228	<	* in some non-steady-state conditions (ramp-up, ramp-down,
1229	<	* other stalls). We can detect many of these by further
1230	<	* considering the number of "idle" threads, that are known to
1231	<	* have zero queued tasks, so compensate by a factor of
1232	<	* (#idle/#active) threads.
1233	<	*/
1234	<	ForkJoinWorkerThread wt =
1235	<	(ForkJoinWorkerThread)Thread.currentThread();
1236	<	return wt.workQueue.queueSize() - wt.pool.idlePerActive();
1177	>	return ForkJoinPool.getSurplusQueuedTaskCount();
1178		}
1179
1180		// Extension methods
#	Line 1284 | Line 1225 | public abstract class ForkJoinTask<V> im
1225		* primarily to support extensions, and is unlikely to be useful
1226		* otherwise.
1227		*
1287	–	* <p>This method may be invoked only from within {@code
1288	–	* ForkJoinPool} computations (as may be determined using method
1289	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1290	–	* result in exceptions or errors, possibly including {@code
1291	–	* ClassCastException}.
1292	–	*
1228		* @return the next task, or {@code null} if none are available
1229		*/
1230		protected static ForkJoinTask<?> peekNextLocalTask() {
1231	<	return ((ForkJoinWorkerThread) Thread.currentThread()).workQueue.peek();
1231	>	Thread t; ForkJoinPool.WorkQueue q;
1232	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
1233	>	q = ((ForkJoinWorkerThread)t).workQueue;
1234	>	else
1235	>	q = ForkJoinPool.commonSubmitterQueue();
1236	>	return (q == null) ? null : q.peek();
1237		}
1238
1239		/**
1240		* Unschedules and returns, without executing, the next task
1241	<	* queued by the current thread but not yet executed.  This method
1242	<	* is designed primarily to support extensions, and is unlikely to
1243	<	* be useful otherwise.
1244	<	*
1305	<	* <p>This method may be invoked only from within {@code
1306	<	* ForkJoinPool} computations (as may be determined using method
1307	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1308	<	* result in exceptions or errors, possibly including {@code
1309	<	* ClassCastException}.
1241	>	* queued by the current thread but not yet executed, if the
1242	>	* current thread is operating in a ForkJoinPool.  This method is
1243	>	* designed primarily to support extensions, and is unlikely to be
1244	>	* useful otherwise.
1245		*
1246		* @return the next task, or {@code null} if none are available
1247		*/
1248		protected static ForkJoinTask<?> pollNextLocalTask() {
1249	<	return ((ForkJoinWorkerThread) Thread.currentThread())
1250	<	.workQueue.nextLocalTask();
1249	>	Thread t;
1250	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1251	>	((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
1252	>	null;
1253		}
1254
1255		/**
1256	<	* Unschedules and returns, without executing, the next task
1256	>	* If the current thread is operating in a ForkJoinPool,
1257	>	* unschedules and returns, without executing, the next task
1258		* queued by the current thread but not yet executed, if one is
1259		* available, or if not available, a task that was forked by some
1260		* other thread, if available. Availability may be transient, so a
1261	<	* {@code null} result does not necessarily imply quiescence
1262	<	* of the pool this task is operating in.  This method is designed
1261	>	* {@code null} result does not necessarily imply quiescence of
1262	>	* the pool this task is operating in.  This method is designed
1263		* primarily to support extensions, and is unlikely to be useful
1264		* otherwise.
1265		*
1328	–	* <p>This method may be invoked only from within {@code
1329	–	* ForkJoinPool} computations (as may be determined using method
1330	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1331	–	* result in exceptions or errors, possibly including {@code
1332	–	* ClassCastException}.
1333	–	*
1266		* @return a task, or {@code null} if none are available
1267		*/
1268		protected static ForkJoinTask<?> pollTask() {
1269	<	ForkJoinWorkerThread wt =
1270	<	(ForkJoinWorkerThread)Thread.currentThread();
1271	<	return wt.pool.nextTaskFor(wt.workQueue);
1269	>	Thread t; ForkJoinWorkerThread wt;
1270	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1271	>	(wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
1272	>	null;
1273		}
1274
1275		// tag operations
#	Line 1525 | Line 1458 | public abstract class ForkJoinTask<V> im
1458		// Unsafe mechanics
1459		private static final sun.misc.Unsafe U;
1460		private static final long STATUS;
1461	+
1462		static {
1463		exceptionTableLock = new ReentrantLock();
1464		exceptionTableRefQueue = new ReferenceQueue<Object>();
1465		exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
1466		try {
1467		U = getUnsafe();
1468	+	Class<?> k = ForkJoinTask.class;
1469		STATUS = U.objectFieldOffset
1470	<	(ForkJoinTask.class.getDeclaredField("status"));
1470	>	(k.getDeclaredField("status"));
1471		} catch (Exception e) {
1472		throw new Error(e);
1473		}

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