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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.67 by jsr166, Wed Jun 8 05:12:25 2011 UTC vs.
Revision 1.68 by dl, Thu Jan 26 00:08:13 2012 UTC

#	Line 6 | Line 6
6
7		package jsr166y;
8
9	–	import java.util.Collection;
10	–	import java.util.concurrent.RejectedExecutionException;
11	–
9		/**
10		* A thread managed by a {@link ForkJoinPool}, which executes
11		* {@link ForkJoinTask}s.
#	Line 25 | Line 22 | import java.util.concurrent.RejectedExec
22		*/
23		public class ForkJoinWorkerThread extends Thread {
24		/*
28	–	* Overview:
29	–	*
25		* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
26	<	* ForkJoinTasks. This class includes bookkeeping in support of
27	<	* worker activation, suspension, and lifecycle control described
33	<	* in more detail in the internal documentation of class
34	<	* ForkJoinPool. And as described further below, this class also
35	<	* includes special-cased support for some ForkJoinTask
36	<	* methods. But the main mechanics involve work-stealing:
37	<	*
38	<	* Work-stealing queues are special forms of Deques that support
39	<	* only three of the four possible end-operations -- push, pop,
40	<	* and deq (aka steal), under the further constraints that push
41	<	* and pop are called only from the owning thread, while deq may
42	<	* be called from other threads.  (If you are unfamiliar with
43	<	* them, you probably want to read Herlihy and Shavit's book "The
44	<	* Art of Multiprocessor programming", chapter 16 describing these
45	<	* in more detail before proceeding.)  The main work-stealing
46	<	* queue design is roughly similar to those in the papers "Dynamic
47	<	* Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
48	<	* (http://research.sun.com/scalable/pubs/index.html) and
49	<	* "Idempotent work stealing" by Michael, Saraswat, and Vechev,
50	<	* PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
51	<	* The main differences ultimately stem from gc requirements that
52	<	* we null out taken slots as soon as we can, to maintain as small
53	<	* a footprint as possible even in programs generating huge
54	<	* numbers of tasks. To accomplish this, we shift the CAS
55	<	* arbitrating pop vs deq (steal) from being on the indices
56	<	* ("queueBase" and "queueTop") to the slots themselves (mainly
57	<	* via method "casSlotNull()"). So, both a successful pop and deq
58	<	* mainly entail a CAS of a slot from non-null to null.  Because
59	<	* we rely on CASes of references, we do not need tag bits on
60	<	* queueBase or queueTop.  They are simple ints as used in any
61	<	* circular array-based queue (see for example ArrayDeque).
62	<	* Updates to the indices must still be ordered in a way that
63	<	* guarantees that queueTop == queueBase means the queue is empty,
64	<	* but otherwise may err on the side of possibly making the queue
65	<	* appear nonempty when a push, pop, or deq have not fully
66	<	* committed. Note that this means that the deq operation,
67	<	* considered individually, is not wait-free. One thief cannot
68	<	* successfully continue until another in-progress one (or, if
69	<	* previously empty, a push) completes.  However, in the
70	<	* aggregate, we ensure at least probabilistic non-blockingness.
71	<	* If an attempted steal fails, a thief always chooses a different
72	<	* random victim target to try next. So, in order for one thief to
73	<	* progress, it suffices for any in-progress deq or new push on
74	<	* any empty queue to complete.
75	<	*
76	<	* This approach also enables support for "async mode" where local
77	<	* task processing is in FIFO, not LIFO order; simply by using a
78	<	* version of deq rather than pop when locallyFifo is true (as set
79	<	* by the ForkJoinPool).  This allows use in message-passing
80	<	* frameworks in which tasks are never joined.  However neither
81	<	* mode considers affinities, loads, cache localities, etc, so
82	<	* rarely provide the best possible performance on a given
83	<	* machine, but portably provide good throughput by averaging over
84	<	* these factors.  (Further, even if we did try to use such
85	<	* information, we do not usually have a basis for exploiting
86	<	* it. For example, some sets of tasks profit from cache
87	<	* affinities, but others are harmed by cache pollution effects.)
88	<	*
89	<	* When a worker would otherwise be blocked waiting to join a
90	<	* task, it first tries a form of linear helping: Each worker
91	<	* records (in field currentSteal) the most recent task it stole
92	<	* from some other worker. Plus, it records (in field currentJoin)
93	<	* the task it is currently actively joining. Method joinTask uses
94	<	* these markers to try to find a worker to help (i.e., steal back
95	<	* a task from and execute it) that could hasten completion of the
96	<	* actively joined task. In essence, the joiner executes a task
97	<	* that would be on its own local deque had the to-be-joined task
98	<	* not been stolen. This may be seen as a conservative variant of
99	<	* the approach in Wagner & Calder "Leapfrogging: a portable
100	<	* technique for implementing efficient futures" SIGPLAN Notices,
101	<	* 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
102	<	* in that: (1) We only maintain dependency links across workers
103	<	* upon steals, rather than use per-task bookkeeping.  This may
104	<	* require a linear scan of workers array to locate stealers, but
105	<	* usually doesn't because stealers leave hints (that may become
106	<	* stale/wrong) of where to locate them. This isolates cost to
107	<	* when it is needed, rather than adding to per-task overhead.
108	<	* (2) It is "shallow", ignoring nesting and potentially cyclic
109	<	* mutual steals.  (3) It is intentionally racy: field currentJoin
110	<	* is updated only while actively joining, which means that we
111	<	* miss links in the chain during long-lived tasks, GC stalls etc
112	<	* (which is OK since blocking in such cases is usually a good
113	<	* idea).  (4) We bound the number of attempts to find work (see
114	<	* MAX_HELP) and fall back to suspending the worker and if
115	<	* necessary replacing it with another.
116	<	*
117	<	* Efficient implementation of these algorithms currently relies
118	<	* on an uncomfortable amount of "Unsafe" mechanics. To maintain
119	<	* correct orderings, reads and writes of variable queueBase
120	<	* require volatile ordering.  Variable queueTop need not be
121	<	* volatile because non-local reads always follow those of
122	<	* queueBase.  Similarly, because they are protected by volatile
123	<	* queueBase reads, reads of the queue array and its slots by
124	<	* other threads do not need volatile load semantics, but writes
125	<	* (in push) require store order and CASes (in pop and deq)
126	<	* require (volatile) CAS semantics.  (Michael, Saraswat, and
127	<	* Vechev's algorithm has similar properties, but without support
128	<	* for nulling slots.)  Since these combinations aren't supported
129	<	* using ordinary volatiles, the only way to accomplish these
130	<	* efficiently is to use direct Unsafe calls. (Using external
131	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
132	<	* array is significantly slower because of memory locality and
133	<	* indirection effects.)
134	<	*
135	<	* Further, performance on most platforms is very sensitive to
136	<	* placement and sizing of the (resizable) queue array.  Even
137	<	* though these queues don't usually become all that big, the
138	<	* initial size must be large enough to counteract cache
139	<	* contention effects across multiple queues (especially in the
140	<	* presence of GC cardmarking). Also, to improve thread-locality,
141	<	* queues are initialized after starting.
142	<	*/
143	<
144	<	/**
145	<	* Mask for pool indices encoded as shorts
146	<	*/
147	<	private static final int  SMASK  = 0xffff;
148	<
149	<	/**
150	<	* Capacity of work-stealing queue array upon initialization.
151	<	* Must be a power of two. Initial size must be at least 4, but is
152	<	* padded to minimize cache effects.
153	<	*/
154	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
155	<
156	<	/**
157	<	* Maximum size for queue array. Must be a power of two
158	<	* less than or equal to 1 << (31 - width of array entry) to
159	<	* ensure lack of index wraparound, but is capped at a lower
160	<	* value to help users trap runaway computations.
161	<	*/
162	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
163	<
164	<	/**
165	<	* The work-stealing queue array. Size must be a power of two.
166	<	* Initialized when started (as opposed to when constructed), to
167	<	* improve memory locality.
168	<	*/
169	<	ForkJoinTask<?>[] queue;
170	<
171	<	/**
172	<	* The pool this thread works in. Accessed directly by ForkJoinTask.
173	<	*/
174	<	final ForkJoinPool pool;
175	<
176	<	/**
177	<	* Index (mod queue.length) of next queue slot to push to or pop
178	<	* from. It is written only by owner thread, and accessed by other
179	<	* threads only after reading (volatile) queueBase.  Both queueTop
180	<	* and queueBase are allowed to wrap around on overflow, but
181	<	* (queueTop - queueBase) still estimates size.
182	<	*/
183	<	int queueTop;
184	<
185	<	/**
186	<	* Index (mod queue.length) of least valid queue slot, which is
187	<	* always the next position to steal from if nonempty.
188	<	*/
189	<	volatile int queueBase;
190	<
191	<	/**
192	<	* The index of most recent stealer, used as a hint to avoid
193	<	* traversal in method helpJoinTask. This is only a hint because a
194	<	* worker might have had multiple steals and this only holds one
195	<	* of them (usually the most current). Declared non-volatile,
196	<	* relying on other prevailing sync to keep reasonably current.
197	<	*/
198	<	int stealHint;
199	<
200	<	/**
201	<	* Index of this worker in pool array. Set once by pool before
202	<	* running, and accessed directly by pool to locate this worker in
203	<	* its workers array.
204	<	*/
205	<	final int poolIndex;
206	<
207	<	/**
208	<	* Encoded record for pool task waits. Usages are always
209	<	* surrounded by volatile reads/writes
210	<	*/
211	<	int nextWait;
212	<
213	<	/**
214	<	* Complement of poolIndex, offset by count of entries of task
215	<	* waits. Accessed by ForkJoinPool to manage event waiters.
216	<	*/
217	<	volatile int eventCount;
218	<
219	<	/**
220	<	* Seed for random number generator for choosing steal victims.
221	<	* Uses Marsaglia xorshift. Must be initialized as nonzero.
222	<	*/
223	<	int seed;
224	<
225	<	/**
226	<	* Number of steals. Directly accessed (and reset) by pool when
227	<	* idle.
228	<	*/
229	<	int stealCount;
230	<
231	<	/**
232	<	* True if this worker should or did terminate
233	<	*/
234	<	volatile boolean terminate;
235	<
236	<	/**
237	<	* Set to true before LockSupport.park; false on return
238	<	*/
239	<	volatile boolean parked;
240	<
241	<	/**
242	<	* True if use local fifo, not default lifo, for local polling.
243	<	* Shadows value from ForkJoinPool.
244	<	*/
245	<	final boolean locallyFifo;
246	<
247	<	/**
248	<	* The task most recently stolen from another worker (or
249	<	* submission queue).  All uses are surrounded by enough volatile
250	<	* reads/writes to maintain as non-volatile.
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28		*/
252	–	ForkJoinTask<?> currentSteal;
29
30	<	/**
31	<	* The task currently being joined, set only when actively trying
256	<	* to help other stealers in helpJoinTask. All uses are surrounded
257	<	* by enough volatile reads/writes to maintain as non-volatile.
258	<	*/
259	<	ForkJoinTask<?> currentJoin;
30	>	final ForkJoinPool.WorkQueue workQueue; // Work-stealing mechanics
31	>	final ForkJoinPool pool;                // the pool this thread works in
32
33		/**
34		* Creates a ForkJoinWorkerThread operating in the given pool.
#	Line 266 | Line 38 | public class ForkJoinWorkerThread extend
38		*/
39		protected ForkJoinWorkerThread(ForkJoinPool pool) {
40		super(pool.nextWorkerName());
41	<	this.pool = pool;
270	<	int k = pool.registerWorker(this);
271	<	poolIndex = k;
272	<	eventCount = ~k & SMASK; // clear wait count
273	<	locallyFifo = pool.locallyFifo;
41	>	setDaemon(true);
42		Thread.UncaughtExceptionHandler ueh = pool.ueh;
43		if (ueh != null)
44		setUncaughtExceptionHandler(ueh);
45	<	setDaemon(true);
45	>	this.pool = pool;
46	>	this.workQueue = new ForkJoinPool.WorkQueue(this, pool.localMode);
47	>	pool.registerWorker(this);
48		}
49
280	–	// Public methods
281	–
50		/**
51		* Returns the pool hosting this thread.
52		*
#	Line 298 | Line 66 | public class ForkJoinWorkerThread extend
66		* @return the index number
67		*/
68		public int getPoolIndex() {
69	<	return poolIndex;
69	>	return workQueue.poolIndex;
70		}
71
304	–	// Randomization
305	–
306	–	/**
307	–	* Computes next value for random victim probes and backoffs.
308	–	* Scans don't require a very high quality generator, but also not
309	–	* a crummy one.  Marsaglia xor-shift is cheap and works well
310	–	* enough.  Note: This is manually inlined in FJP.scan() to avoid
311	–	* writes inside busy loops.
312	–	*/
313	–	private int nextSeed() {
314	–	int r = seed;
315	–	r ^= r << 13;
316	–	r ^= r >>> 17;
317	–	r ^= r << 5;
318	–	return seed = r;
319	–	}
320	–
321	–	// Run State management
322	–
72		/**
73		* Initializes internal state after construction but before
74		* processing any tasks. If you override this method, you must
#	Line 330 | Line 79 | public class ForkJoinWorkerThread extend
79		* processing tasks.
80		*/
81		protected void onStart() {
333	–	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
334	–	int r = ForkJoinPool.workerSeedGenerator.nextInt();
335	–	seed = (r == 0) ? 1 : r; //  must be nonzero
82		}
83
84		/**
#	Line 344 | Line 90 | public class ForkJoinWorkerThread extend
90		* to an unrecoverable error, or {@code null} if completed normally
91		*/
92		protected void onTermination(Throwable exception) {
347	–	try {
348	–	terminate = true;
349	–	cancelTasks();
350	–	pool.deregisterWorker(this, exception);
351	–	} catch (Throwable ex) {        // Shouldn't ever happen
352	–	if (exception == null)      // but if so, at least rethrown
353	–	exception = ex;
354	–	} finally {
355	–	if (exception != null)
356	–	UNSAFE.throwException(exception);
357	–	}
93		}
94
95		/**
#	Line 366 | Line 101 | public class ForkJoinWorkerThread extend
101		Throwable exception = null;
102		try {
103		onStart();
104	<	pool.work(this);
104	>	pool.runWorker(this);
105		} catch (Throwable ex) {
106		exception = ex;
107		} finally {
373	–	onTermination(exception);
374	–	}
375	–	}
376	–
377	–	/*
378	–	* Intrinsics-based atomic writes for queue slots. These are
379	–	* basically the same as methods in AtomicReferenceArray, but
380	–	* specialized for (1) ForkJoinTask elements (2) requirement that
381	–	* nullness and bounds checks have already been performed by
382	–	* callers and (3) effective offsets are known not to overflow
383	–	* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
384	–	* need corresponding version for reads: plain array reads are OK
385	–	* because they are protected by other volatile reads and are
386	–	* confirmed by CASes.
387	–	*
388	–	* Most uses don't actually call these methods, but instead
389	–	* contain inlined forms that enable more predictable
390	–	* optimization.  We don't define the version of write used in
391	–	* pushTask at all, but instead inline there a store-fenced array
392	–	* slot write.
393	–	*
394	–	* Also in most methods, as a performance (not correctness) issue,
395	–	* we'd like to encourage compilers not to arbitrarily postpone
396	–	* setting queueTop after writing slot.  Currently there is no
397	–	* intrinsic for arranging this, but using Unsafe putOrderedInt
398	–	* may be a preferable strategy on some compilers even though its
399	–	* main effect is a pre-, not post- fence. To simplify possible
400	–	* changes, the option is left in comments next to the associated
401	–	* assignments.
402	–	*/
403	–
404	–	/**
405	–	* CASes slot i of array q from t to null. Caller must ensure q is
406	–	* non-null and index is in range.
407	–	*/
408	–	private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
409	–	ForkJoinTask<?> t) {
410	–	return UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null);
411	–	}
412	–
413	–	/**
414	–	* Performs a volatile write of the given task at given slot of
415	–	* array q.  Caller must ensure q is non-null and index is in
416	–	* range. This method is used only during resets and backouts.
417	–	*/
418	–	private static final void writeSlot(ForkJoinTask<?>[] q, int i,
419	–	ForkJoinTask<?> t) {
420	–	UNSAFE.putObjectVolatile(q, (i << ASHIFT) + ABASE, t);
421	–	}
422	–
423	–	// queue methods
424	–
425	–	/**
426	–	* Pushes a task. Call only from this thread.
427	–	*
428	–	* @param t the task. Caller must ensure non-null.
429	–	*/
430	–	final void pushTask(ForkJoinTask<?> t) {
431	–	ForkJoinTask<?>[] q; int s, m;
432	–	if ((q = queue) != null) {    // ignore if queue removed
433	–	long u = (((s = queueTop) & (m = q.length - 1)) << ASHIFT) + ABASE;
434	–	UNSAFE.putOrderedObject(q, u, t);
435	–	queueTop = s + 1;         // or use putOrderedInt
436	–	if ((s -= queueBase) <= 2)
437	–	pool.signalWork();
438	–	else if (s == m)
439	–	growQueue();
440	–	}
441	–	}
442	–
443	–	/**
444	–	* Creates or doubles queue array.  Transfers elements by
445	–	* emulating steals (deqs) from old array and placing, oldest
446	–	* first, into new array.
447	–	*/
448	–	private void growQueue() {
449	–	ForkJoinTask<?>[] oldQ = queue;
450	–	int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
451	–	if (size > MAXIMUM_QUEUE_CAPACITY)
452	–	throw new RejectedExecutionException("Queue capacity exceeded");
453	–	if (size < INITIAL_QUEUE_CAPACITY)
454	–	size = INITIAL_QUEUE_CAPACITY;
455	–	ForkJoinTask<?>[] q = queue = new ForkJoinTask<?>[size];
456	–	int mask = size - 1;
457	–	int top = queueTop;
458	–	int oldMask;
459	–	if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
460	–	for (int b = queueBase; b != top; ++b) {
461	–	long u = ((b & oldMask) << ASHIFT) + ABASE;
462	–	Object x = UNSAFE.getObjectVolatile(oldQ, u);
463	–	if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
464	–	UNSAFE.putObjectVolatile
465	–	(q, ((b & mask) << ASHIFT) + ABASE, x);
466	–	}
467	–	}
468	–	}
469	–
470	–	/**
471	–	* Tries to take a task from the base of the queue, failing if
472	–	* empty or contended. Note: Specializations of this code appear
473	–	* in locallyDeqTask and elsewhere.
474	–	*
475	–	* @return a task, or null if none or contended
476	–	*/
477	–	final ForkJoinTask<?> deqTask() {
478	–	ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
479	–	if (queueTop != (b = queueBase) &&
480	–	(q = queue) != null && // must read q after b
481	–	(i = (q.length - 1) & b) >= 0 &&
482	–	(t = q[i]) != null && queueBase == b &&
483	–	UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null)) {
484	–	queueBase = b + 1;
485	–	return t;
486	–	}
487	–	return null;
488	–	}
489	–
490	–	/**
491	–	* Tries to take a task from the base of own queue.  Called only
492	–	* by this thread.
493	–	*
494	–	* @return a task, or null if none
495	–	*/
496	–	final ForkJoinTask<?> locallyDeqTask() {
497	–	ForkJoinTask<?> t; int m, b, i;
498	–	ForkJoinTask<?>[] q = queue;
499	–	if (q != null && (m = q.length - 1) >= 0) {
500	–	while (queueTop != (b = queueBase)) {
501	–	if ((t = q[i = m & b]) != null &&
502	–	queueBase == b &&
503	–	UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE,
504	–	t, null)) {
505	–	queueBase = b + 1;
506	–	return t;
507	–	}
508	–	}
509	–	}
510	–	return null;
511	–	}
512	–
513	–	/**
514	–	* Returns a popped task, or null if empty.
515	–	* Called only by this thread.
516	–	*/
517	–	private ForkJoinTask<?> popTask() {
518	–	int m;
519	–	ForkJoinTask<?>[] q = queue;
520	–	if (q != null && (m = q.length - 1) >= 0) {
521	–	for (int s; (s = queueTop) != queueBase;) {
522	–	int i = m & --s;
523	–	long u = (i << ASHIFT) + ABASE; // raw offset
524	–	ForkJoinTask<?> t = q[i];
525	–	if (t == null)   // lost to stealer
526	–	break;
527	–	if (UNSAFE.compareAndSwapObject(q, u, t, null)) {
528	–	queueTop = s; // or putOrderedInt
529	–	return t;
530	–	}
531	–	}
532	–	}
533	–	return null;
534	–	}
535	–
536	–	/**
537	–	* Specialized version of popTask to pop only if topmost element
538	–	* is the given task. Called only by this thread.
539	–	*
540	–	* @param t the task. Caller must ensure non-null.
541	–	*/
542	–	final boolean unpushTask(ForkJoinTask<?> t) {
543	–	ForkJoinTask<?>[] q;
544	–	int s;
545	–	if ((q = queue) != null && (s = queueTop) != queueBase &&
546	–	UNSAFE.compareAndSwapObject
547	–	(q, (((q.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
548	–	queueTop = s; // or putOrderedInt
549	–	return true;
550	–	}
551	–	return false;
552	–	}
553	–
554	–	/**
555	–	* Returns next task, or null if empty or contended.
556	–	*/
557	–	final ForkJoinTask<?> peekTask() {
558	–	int m;
559	–	ForkJoinTask<?>[] q = queue;
560	–	if (q == null || (m = q.length - 1) < 0)
561	–	return null;
562	–	int i = locallyFifo ? queueBase : (queueTop - 1);
563	–	return q[i & m];
564	–	}
565	–
566	–	// Support methods for ForkJoinPool
567	–
568	–	/**
569	–	* Runs the given task, plus any local tasks until queue is empty
570	–	*/
571	–	final void execTask(ForkJoinTask<?> t) {
572	–	currentSteal = t;
573	–	for (;;) {
574	–	if (t != null)
575	–	t.doExec();
576	–	if (queueTop == queueBase)
577	–	break;
578	–	t = locallyFifo ? locallyDeqTask() : popTask();
579	–	}
580	–	++stealCount;
581	–	currentSteal = null;
582	–	}
583	–
584	–	/**
585	–	* Removes and cancels all tasks in queue.  Can be called from any
586	–	* thread.
587	–	*/
588	–	final void cancelTasks() {
589	–	ForkJoinTask<?> cj = currentJoin; // try to cancel ongoing tasks
590	–	if (cj != null && cj.status >= 0)
591	–	cj.cancelIgnoringExceptions();
592	–	ForkJoinTask<?> cs = currentSteal;
593	–	if (cs != null && cs.status >= 0)
594	–	cs.cancelIgnoringExceptions();
595	–	while (queueBase != queueTop) {
596	–	ForkJoinTask<?> t = deqTask();
597	–	if (t != null)
598	–	t.cancelIgnoringExceptions();
599	–	}
600	–	}
601	–
602	–	/**
603	–	* Drains tasks to given collection c.
604	–	*
605	–	* @return the number of tasks drained
606	–	*/
607	–	final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
608	–	int n = 0;
609	–	while (queueBase != queueTop) {
610	–	ForkJoinTask<?> t = deqTask();
611	–	if (t != null) {
612	–	c.add(t);
613	–	++n;
614	–	}
615	–	}
616	–	return n;
617	–	}
618	–
619	–	// Support methods for ForkJoinTask
620	–
621	–	/**
622	–	* Returns an estimate of the number of tasks in the queue.
623	–	*/
624	–	final int getQueueSize() {
625	–	return queueTop - queueBase;
626	–	}
627	–
628	–	/**
629	–	* Gets and removes a local task.
630	–	*
631	–	* @return a task, if available
632	–	*/
633	–	final ForkJoinTask<?> pollLocalTask() {
634	–	return locallyFifo ? locallyDeqTask() : popTask();
635	–	}
636	–
637	–	/**
638	–	* Gets and removes a local or stolen task.
639	–	*
640	–	* @return a task, if available
641	–	*/
642	–	final ForkJoinTask<?> pollTask() {
643	–	ForkJoinWorkerThread[] ws;
644	–	ForkJoinTask<?> t = pollLocalTask();
645	–	if (t != null || (ws = pool.workers) == null)
646	–	return t;
647	–	int n = ws.length; // cheap version of FJP.scan
648	–	int steps = n << 1;
649	–	int r = nextSeed();
650	–	int i = 0;
651	–	while (i < steps) {
652	–	ForkJoinWorkerThread w = ws[(i++ + r) & (n - 1)];
653	–	if (w != null && w.queueBase != w.queueTop && w.queue != null) {
654	–	if ((t = w.deqTask()) != null)
655	–	return t;
656	–	i = 0;
657	–	}
658	–	}
659	–	return null;
660	–	}
661	–
662	–	/**
663	–	* The maximum stolen->joining link depth allowed in helpJoinTask,
664	–	* as well as the maximum number of retries (allowing on average
665	–	* one staleness retry per level) per attempt to instead try
666	–	* compensation.  Depths for legitimate chains are unbounded, but
667	–	* we use a fixed constant to avoid (otherwise unchecked) cycles
668	–	* and bound staleness of traversal parameters at the expense of
669	–	* sometimes blocking when we could be helping.
670	–	*/
671	–	private static final int MAX_HELP = 16;
672	–
673	–	/**
674	–	* Possibly runs some tasks and/or blocks, until joinMe is done.
675	–	*
676	–	* @param joinMe the task to join
677	–	* @return completion status on exit
678	–	*/
679	–	final int joinTask(ForkJoinTask<?> joinMe) {
680	–	ForkJoinTask<?> prevJoin = currentJoin;
681	–	currentJoin = joinMe;
682	–	for (int s, retries = MAX_HELP;;) {
683	–	if ((s = joinMe.status) < 0) {
684	–	currentJoin = prevJoin;
685	–	return s;
686	–	}
687	–	if (retries > 0) {
688	–	if (queueTop != queueBase) {
689	–	if (!localHelpJoinTask(joinMe))
690	–	retries = 0;           // cannot help
691	–	}
692	–	else if (retries == MAX_HELP >>> 1) {
693	–	--retries;                 // check uncommon case
694	–	if (tryDeqAndExec(joinMe) >= 0)
695	–	Thread.yield();        // for politeness
696	–	}
697	–	else
698	–	retries = helpJoinTask(joinMe) ? MAX_HELP : retries - 1;
699	–	}
700	–	else {
701	–	retries = MAX_HELP;           // restart if not done
702	–	pool.tryAwaitJoin(joinMe);
703	–	}
704	–	}
705	–	}
706	–
707	–	/**
708	–	* If present, pops and executes the given task, or any other
709	–	* cancelled task
710	–	*
711	–	* @return false if any other non-cancelled task exists in local queue
712	–	*/
713	–	private boolean localHelpJoinTask(ForkJoinTask<?> joinMe) {
714	–	int s, i; ForkJoinTask<?>[] q; ForkJoinTask<?> t;
715	–	if ((s = queueTop) != queueBase && (q = queue) != null &&
716	–	(i = (q.length - 1) & --s) >= 0 &&
717	–	(t = q[i]) != null) {
718	–	if (t != joinMe && t.status >= 0)
719	–	return false;
720	–	if (UNSAFE.compareAndSwapObject
721	–	(q, (i << ASHIFT) + ABASE, t, null)) {
722	–	queueTop = s;           // or putOrderedInt
723	–	t.doExec();
724	–	}
725	–	}
726	–	return true;
727	–	}
728	–
729	–	/**
730	–	* Tries to locate and execute tasks for a stealer of the given
731	–	* task, or in turn one of its stealers, Traces
732	–	* currentSteal->currentJoin links looking for a thread working on
733	–	* a descendant of the given task and with a non-empty queue to
734	–	* steal back and execute tasks from.  The implementation is very
735	–	* branchy to cope with potential inconsistencies or loops
736	–	* encountering chains that are stale, unknown, or of length
737	–	* greater than MAX_HELP links.  All of these cases are dealt with
738	–	* by just retrying by caller.
739	–	*
740	–	* @param joinMe the task to join
741	–	* @param canSteal true if local queue is empty
742	–	* @return true if ran a task
743	–	*/
744	–	private boolean helpJoinTask(ForkJoinTask<?> joinMe) {
745	–	boolean helped = false;
746	–	int m = pool.scanGuard & SMASK;
747	–	ForkJoinWorkerThread[] ws = pool.workers;
748	–	if (ws != null && ws.length > m && joinMe.status >= 0) {
749	–	int levels = MAX_HELP;              // remaining chain length
750	–	ForkJoinTask<?> task = joinMe;      // base of chain
751	–	outer:for (ForkJoinWorkerThread thread = this;;) {
752	–	// Try to find v, the stealer of task, by first using hint
753	–	ForkJoinWorkerThread v = ws[thread.stealHint & m];
754	–	if (v == null || v.currentSteal != task) {
755	–	for (int j = 0; ;) {        // search array
756	–	if ((v = ws[j]) != null && v.currentSteal == task) {
757	–	thread.stealHint = j;
758	–	break;              // save hint for next time
759	–	}
760	–	if (++j > m)
761	–	break outer;        // can't find stealer
762	–	}
763	–	}
764	–	// Try to help v, using specialized form of deqTask
765	–	for (;;) {
766	–	ForkJoinTask<?>[] q; int b, i;
767	–	if (joinMe.status < 0)
768	–	break outer;
769	–	if ((b = v.queueBase) == v.queueTop ||
770	–	(q = v.queue) == null ||
771	–	(i = (q.length-1) & b) < 0)
772	–	break;                  // empty
773	–	long u = (i << ASHIFT) + ABASE;
774	–	ForkJoinTask<?> t = q[i];
775	–	if (task.status < 0)
776	–	break outer;            // stale
777	–	if (t != null && v.queueBase == b &&
778	–	UNSAFE.compareAndSwapObject(q, u, t, null)) {
779	–	v.queueBase = b + 1;
780	–	v.stealHint = poolIndex;
781	–	ForkJoinTask<?> ps = currentSteal;
782	–	currentSteal = t;
783	–	t.doExec();
784	–	currentSteal = ps;
785	–	helped = true;
786	–	}
787	–	}
788	–	// Try to descend to find v's stealer
789	–	ForkJoinTask<?> next = v.currentJoin;
790	–	if (--levels > 0 && task.status >= 0 &&
791	–	next != null && next != task) {
792	–	task = next;
793	–	thread = v;
794	–	}
795	–	else
796	–	break;  // max levels, stale, dead-end, or cyclic
797	–	}
798	–	}
799	–	return helped;
800	–	}
801	–
802	–	/**
803	–	* Performs an uncommon case for joinTask: If task t is at base of
804	–	* some workers queue, steals and executes it.
805	–	*
806	–	* @param t the task
807	–	* @return t's status
808	–	*/
809	–	private int tryDeqAndExec(ForkJoinTask<?> t) {
810	–	int m = pool.scanGuard & SMASK;
811	–	ForkJoinWorkerThread[] ws = pool.workers;
812	–	if (ws != null && ws.length > m && t.status >= 0) {
813	–	for (int j = 0; j <= m; ++j) {
814	–	ForkJoinTask<?>[] q; int b, i;
815	–	ForkJoinWorkerThread v = ws[j];
816	–	if (v != null &&
817	–	(b = v.queueBase) != v.queueTop &&
818	–	(q = v.queue) != null &&
819	–	(i = (q.length - 1) & b) >= 0 &&
820	–	q[i] ==  t) {
821	–	long u = (i << ASHIFT) + ABASE;
822	–	if (v.queueBase == b &&
823	–	UNSAFE.compareAndSwapObject(q, u, t, null)) {
824	–	v.queueBase = b + 1;
825	–	v.stealHint = poolIndex;
826	–	ForkJoinTask<?> ps = currentSteal;
827	–	currentSteal = t;
828	–	t.doExec();
829	–	currentSteal = ps;
830	–	}
831	–	break;
832	–	}
833	–	}
834	–	}
835	–	return t.status;
836	–	}
837	–
838	–	/**
839	–	* Implements ForkJoinTask.getSurplusQueuedTaskCount().  Returns
840	–	* an estimate of the number of tasks, offset by a function of
841	–	* number of idle workers.
842	–	*
843	–	* This method provides a cheap heuristic guide for task
844	–	* partitioning when programmers, frameworks, tools, or languages
845	–	* have little or no idea about task granularity.  In essence by
846	–	* offering this method, we ask users only about tradeoffs in
847	–	* overhead vs expected throughput and its variance, rather than
848	–	* how finely to partition tasks.
849	–	*
850	–	* In a steady state strict (tree-structured) computation, each
851	–	* thread makes available for stealing enough tasks for other
852	–	* threads to remain active. Inductively, if all threads play by
853	–	* the same rules, each thread should make available only a
854	–	* constant number of tasks.
855	–	*
856	–	* The minimum useful constant is just 1. But using a value of 1
857	–	* would require immediate replenishment upon each steal to
858	–	* maintain enough tasks, which is infeasible.  Further,
859	–	* partitionings/granularities of offered tasks should minimize
860	–	* steal rates, which in general means that threads nearer the top
861	–	* of computation tree should generate more than those nearer the
862	–	* bottom. In perfect steady state, each thread is at
863	–	* approximately the same level of computation tree. However,
864	–	* producing extra tasks amortizes the uncertainty of progress and
865	–	* diffusion assumptions.
866	–	*
867	–	* So, users will want to use values larger, but not much larger
868	–	* than 1 to both smooth over transient shortages and hedge
869	–	* against uneven progress; as traded off against the cost of
870	–	* extra task overhead. We leave the user to pick a threshold
871	–	* value to compare with the results of this call to guide
872	–	* decisions, but recommend values such as 3.
873	–	*
874	–	* When all threads are active, it is on average OK to estimate
875	–	* surplus strictly locally. In steady-state, if one thread is
876	–	* maintaining say 2 surplus tasks, then so are others. So we can
877	–	* just use estimated queue length (although note that (queueTop -
878	–	* queueBase) can be an overestimate because of stealers lagging
879	–	* increments of queueBase).  However, this strategy alone leads
880	–	* to serious mis-estimates in some non-steady-state conditions
881	–	* (ramp-up, ramp-down, other stalls). We can detect many of these
882	–	* by further considering the number of "idle" threads, that are
883	–	* known to have zero queued tasks, so compensate by a factor of
884	–	* (#idle/#active) threads.
885	–	*/
886	–	final int getEstimatedSurplusTaskCount() {
887	–	return queueTop - queueBase - pool.idlePerActive();
888	–	}
889	–
890	–	/**
891	–	* Runs tasks until {@code pool.isQuiescent()}. We piggyback on
892	–	* pool's active count ctl maintenance, but rather than blocking
893	–	* when tasks cannot be found, we rescan until all others cannot
894	–	* find tasks either. The bracketing by pool quiescerCounts
895	–	* updates suppresses pool auto-shutdown mechanics that could
896	–	* otherwise prematurely terminate the pool because all threads
897	–	* appear to be inactive.
898	–	*/
899	–	final void helpQuiescePool() {
900	–	boolean active = true;
901	–	ForkJoinTask<?> ps = currentSteal; // to restore below
902	–	ForkJoinPool p = pool;
903	–	p.addQuiescerCount(1);
904	–	for (;;) {
905	–	ForkJoinWorkerThread[] ws = p.workers;
906	–	ForkJoinWorkerThread v = null;
907	–	int n;
908	–	if (queueTop != queueBase)
909	–	v = this;
910	–	else if (ws != null && (n = ws.length) > 1) {
911	–	ForkJoinWorkerThread w;
912	–	int r = nextSeed(); // cheap version of FJP.scan
913	–	int steps = n << 1;
914	–	for (int i = 0; i < steps; ++i) {
915	–	if ((w = ws[(i + r) & (n - 1)]) != null &&
916	–	w.queueBase != w.queueTop) {
917	–	v = w;
918	–	break;
919	–	}
920	–	}
921	–	}
922	–	if (v != null) {
923	–	ForkJoinTask<?> t;
924	–	if (!active) {
925	–	active = true;
926	–	p.addActiveCount(1);
927	–	}
928	–	if ((t = (v != this) ? v.deqTask() :
929	–	locallyFifo ? locallyDeqTask() : popTask()) != null) {
930	–	currentSteal = t;
931	–	t.doExec();
932	–	currentSteal = ps;
933	–	}
934	–	}
935	–	else {
936	–	if (active) {
937	–	active = false;
938	–	p.addActiveCount(-1);
939	–	}
940	–	if (p.isQuiescent()) {
941	–	p.addActiveCount(1);
942	–	p.addQuiescerCount(-1);
943	–	break;
944	–	}
945	–	}
946	–	}
947	–	}
948	–
949	–	// Unsafe mechanics
950	–	private static final sun.misc.Unsafe UNSAFE;
951	–	private static final long ABASE;
952	–	private static final int ASHIFT;
953	–
954	–	static {
955	–	int s;
956	–	try {
957	–	UNSAFE = getUnsafe();
958	–	Class<?> a = ForkJoinTask[].class;
959	–	ABASE = UNSAFE.arrayBaseOffset(a);
960	–	s = UNSAFE.arrayIndexScale(a);
961	–	} catch (Exception e) {
962	–	throw new Error(e);
963	–	}
964	–	if ((s & (s-1)) != 0)
965	–	throw new Error("data type scale not a power of two");
966	–	ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
967	–	}
968	–
969	–	/**
970	–	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
971	–	* Replace with a simple call to Unsafe.getUnsafe when integrating
972	–	* into a jdk.
973	–	*
974	–	* @return a sun.misc.Unsafe
975	–	*/
976	–	private static sun.misc.Unsafe getUnsafe() {
977	–	try {
978	–	return sun.misc.Unsafe.getUnsafe();
979	–	} catch (SecurityException se) {
108		try {
109	<	return java.security.AccessController.doPrivileged
110	<	(new java.security
111	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
112	<	public sun.misc.Unsafe run() throws Exception {
113	<	java.lang.reflect.Field f = sun.misc
114	<	.Unsafe.class.getDeclaredField("theUnsafe");
987	<	f.setAccessible(true);
988	<	return (sun.misc.Unsafe) f.get(null);
989	<	}});
990	<	} catch (java.security.PrivilegedActionException e) {
991	<	throw new RuntimeException("Could not initialize intrinsics",
992	<	e.getCause());
109	>	onTermination(exception);
110	>	} catch (Throwable ex) {
111	>	if (exception == null)
112	>	exception = ex;
113	>	} finally {
114	>	pool.deregisterWorker(this, exception);
115		}
116		}
117		}
118		}
119	+

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