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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.26 by dl, Sun Aug 2 11:54:31 2009 UTC vs.
Revision 1.73 by dl, Wed Nov 21 19:54:39 2012 UTC

#	Line 1 | Line 1
1		/*
2		* Written by Doug Lea with assistance from members of JCP JSR-166
3		* Expert Group and released to the public domain, as explained at
4	<	* http://creativecommons.org/licenses/publicdomain
4	>	* http://creativecommons.org/publicdomain/zero/1.0/
5		*/
6
7		package jsr166y;
8
9	–	import java.util.concurrent.*;
10	–
11	–	import java.util.Collection;
12	–
9		/**
10	<	* A thread managed by a {@link ForkJoinPool}.  This class is
11	<	* subclassable solely for the sake of adding functionality -- there
12	<	* are no overridable methods dealing with scheduling or execution.
13	<	* However, you can override initialization and termination methods
14	<	* surrounding the main task processing loop.  If you do create such a
15	<	* subclass, you will also need to supply a custom {@link
16	<	* ForkJoinPool.ForkJoinWorkerThreadFactory} to use it in a {@code
17	<	* ForkJoinPool}.
10	>	* A thread managed by a {@link ForkJoinPool}, which executes
11	>	* {@link ForkJoinTask}s.
12	>	* This class is subclassable solely for the sake of adding
13	>	* functionality -- there are no overridable methods dealing with
14	>	* scheduling or execution.  However, you can override initialization
15	>	* and termination methods surrounding the main task processing loop.
16	>	* If you do create such a subclass, you will also need to supply a
17	>	* custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to use it
18	>	* in a {@code ForkJoinPool}.
19		*
20		* @since 1.7
21		* @author Doug Lea
22		*/
23		public class ForkJoinWorkerThread extends Thread {
24		/*
25	<	* Algorithm overview:
26	<	*
27	<	* 1. Work-Stealing: Work-stealing queues are special forms of
28	<	* Deques that support only three of the four possible
29	<	* end-operations -- push, pop, and deq (aka steal), and only do
30	<	* so under the constraints that push and pop are called only from
31	<	* the owning thread, while deq may be called from other threads.
32	<	* (If you are unfamiliar with them, you probably want to read
33	<	* Herlihy and Shavit's book "The Art of Multiprocessor
34	<	* programming", chapter 16 describing these in more detail before
38	<	* proceeding.)  The main work-stealing queue design is roughly
39	<	* similar to "Dynamic Circular Work-Stealing Deque" by David
40	<	* Chase and Yossi Lev, SPAA 2005
41	<	* (http://research.sun.com/scalable/pubs/index.html).  The main
42	<	* difference ultimately stems from gc requirements that we null
43	<	* out taken slots as soon as we can, to maintain as small a
44	<	* footprint as possible even in programs generating huge numbers
45	<	* of tasks. To accomplish this, we shift the CAS arbitrating pop
46	<	* vs deq (steal) from being on the indices ("base" and "sp") to
47	<	* the slots themselves (mainly via method "casSlotNull()"). So,
48	<	* both a successful pop and deq mainly entail CAS'ing a non-null
49	<	* slot to null.  Because we rely on CASes of references, we do
50	<	* not need tag bits on base or sp.  They are simple ints as used
51	<	* in any circular array-based queue (see for example ArrayDeque).
52	<	* Updates to the indices must still be ordered in a way that
53	<	* guarantees that (sp - base) > 0 means the queue is empty, but
54	<	* otherwise may err on the side of possibly making the queue
55	<	* appear nonempty when a push, pop, or deq have not fully
56	<	* committed. Note that this means that the deq operation,
57	<	* considered individually, is not wait-free. One thief cannot
58	<	* successfully continue until another in-progress one (or, if
59	<	* previously empty, a push) completes.  However, in the
60	<	* aggregate, we ensure at least probabilistic non-blockingness. If
61	<	* an attempted steal fails, a thief always chooses a different
62	<	* random victim target to try next. So, in order for one thief to
63	<	* progress, it suffices for any in-progress deq or new push on
64	<	* any empty queue to complete. One reason this works well here is
65	<	* that apparently-nonempty often means soon-to-be-stealable,
66	<	* which gives threads a chance to activate if necessary before
67	<	* stealing (see below).
68	<	*
69	<	* This approach also enables support for "async mode" where local
70	<	* task processing is in FIFO, not LIFO order; simply by using a
71	<	* version of deq rather than pop when locallyFifo is true (as set
72	<	* by the ForkJoinPool).  This allows use in message-passing
73	<	* frameworks in which tasks are never joined.
74	<	*
75	<	* Efficient implementation of this approach currently relies on
76	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
77	<	* correct orderings, reads and writes of variable base require
78	<	* volatile ordering.  Variable sp does not require volatile write
79	<	* but needs cheaper store-ordering on writes.  Because they are
80	<	* protected by volatile base reads, reads of the queue array and
81	<	* its slots do not need volatile load semantics, but writes (in
82	<	* push) require store order and CASes (in pop and deq) require
83	<	* (volatile) CAS semantics. Since these combinations aren't
84	<	* supported using ordinary volatiles, the only way to accomplish
85	<	* these efficiently is to use direct Unsafe calls. (Using external
86	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
87	<	* array is significantly slower because of memory locality and
88	<	* indirection effects.) Further, performance on most platforms is
89	<	* very sensitive to placement and sizing of the (resizable) queue
90	<	* array.  Even though these queues don't usually become all that
91	<	* big, the initial size must be large enough to counteract cache
92	<	* contention effects across multiple queues (especially in the
93	<	* presence of GC cardmarking). Also, to improve thread-locality,
94	<	* queues are currently initialized immediately after the thread
95	<	* gets the initial signal to start processing tasks.  However,
96	<	* all queue-related methods except pushTask are written in a way
97	<	* that allows them to instead be lazily allocated and/or disposed
98	<	* of when empty. All together, these low-level implementation
99	<	* choices produce as much as a factor of 4 performance
100	<	* improvement compared to naive implementations, and enable the
101	<	* processing of billions of tasks per second, sometimes at the
102	<	* expense of ugliness.
103	<	*
104	<	* 2. Run control: The primary run control is based on a global
105	<	* counter (activeCount) held by the pool. It uses an algorithm
106	<	* similar to that in Herlihy and Shavit section 17.6 to cause
107	<	* threads to eventually block when all threads declare they are
108	<	* inactive. (See variable "scans".)  For this to work, threads
109	<	* must be declared active when executing tasks, and before
110	<	* stealing a task. They must be inactive before blocking on the
111	<	* Pool Barrier (awaiting a new submission or other Pool
112	<	* event). In between, there is some free play which we take
113	<	* advantage of to avoid contention and rapid flickering of the
114	<	* global activeCount: If inactive, we activate only if a victim
115	<	* queue appears to be nonempty (see above).  Similarly, a thread
116	<	* tries to inactivate only after a full scan of other threads.
117	<	* The net effect is that contention on activeCount is rarely a
118	<	* measurable performance issue. (There are also a few other cases
119	<	* where we scan for work rather than retry/block upon
120	<	* contention.)
121	<	*
122	<	* 3. Selection control. We maintain policy of always choosing to
123	<	* run local tasks rather than stealing, and always trying to
124	<	* steal tasks before trying to run a new submission. All steals
125	<	* are currently performed in randomly-chosen deq-order. It may be
126	<	* worthwhile to bias these with locality / anti-locality
127	<	* information, but doing this well probably requires more
128	<	* lower-level information from JVMs than currently provided.
129	<	*/
130	<
131	<	/**
132	<	* Capacity of work-stealing queue array upon initialization.
133	<	* Must be a power of two. Initial size must be at least 2, but is
134	<	* padded to minimize cache effects.
135	<	*/
136	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
137	<
138	<	/**
139	<	* Maximum work-stealing queue array size.  Must be less than or
140	<	* equal to 1 << 28 to ensure lack of index wraparound. (This
141	<	* is less than usual bounds, because we need leftshift by 3
142	<	* to be in int range).
143	<	*/
144	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28;
145	<
146	<	/**
147	<	* The pool this thread works in. Accessed directly by ForkJoinTask.
148	<	*/
149	<	final ForkJoinPool pool;
150	<
151	<	/**
152	<	* The work-stealing queue array. Size must be a power of two.
153	<	* Initialized when thread starts, to improve memory locality.
25	>	* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28	>	*
29	>	* This class just maintains links to its pool and WorkQueue.  The
30	>	* pool field is set immediately upon construction, but the
31	>	* workQueue field is not set until a call to registerWorker
32	>	* completes. This leads to a visibility race, that is tolerated
33	>	* by requiring that the workQueue field is only accessed by the
34	>	* owning thread.
35		*/
155	–	private ForkJoinTask<?>[] queue;
36
37	<	/**
38	<	* Index (mod queue.length) of next queue slot to push to or pop
159	<	* from. It is written only by owner thread, via ordered store.
160	<	* Both sp and base are allowed to wrap around on overflow, but
161	<	* (sp - base) still estimates size.
162	<	*/
163	<	private volatile int sp;
164	<
165	<	/**
166	<	* Index (mod queue.length) of least valid queue slot, which is
167	<	* always the next position to steal from if nonempty.
168	<	*/
169	<	private volatile int base;
170	<
171	<	/**
172	<	* Activity status. When true, this worker is considered active.
173	<	* Must be false upon construction. It must be true when executing
174	<	* tasks, and BEFORE stealing a task. It must be false before
175	<	* calling pool.sync.
176	<	*/
177	<	private boolean active;
178	<
179	<	/**
180	<	* Run state of this worker. Supports simple versions of the usual
181	<	* shutdown/shutdownNow control.
182	<	*/
183	<	private volatile int runState;
184	<
185	<	/**
186	<	* Seed for random number generator for choosing steal victims.
187	<	* Uses Marsaglia xorshift. Must be nonzero upon initialization.
188	<	*/
189	<	private int seed;
190	<
191	<	/**
192	<	* Number of steals, transferred to pool when idle
193	<	*/
194	<	private int stealCount;
195	<
196	<	/**
197	<	* Index of this worker in pool array. Set once by pool before
198	<	* running, and accessed directly by pool during cleanup etc.
199	<	*/
200	<	int poolIndex;
201	<
202	<	/**
203	<	* The last barrier event waited for. Accessed in pool callback
204	<	* methods, but only by current thread.
205	<	*/
206	<	long lastEventCount;
207	<
208	<	/**
209	<	* True if use local fifo, not default lifo, for local polling
210	<	*/
211	<	private boolean locallyFifo;
37	>	final ForkJoinPool pool;                // the pool this thread works in
38	>	final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics
39
40		/**
41		* Creates a ForkJoinWorkerThread operating in the given pool.
#	Line 217 | Line 44 | public class ForkJoinWorkerThread extend
44		* @throws NullPointerException if pool is null
45		*/
46		protected ForkJoinWorkerThread(ForkJoinPool pool) {
47	<	if (pool == null) throw new NullPointerException();
47	>	// Use a placeholder until a useful name can be set in registerWorker
48	>	super("aForkJoinWorkerThread");
49		this.pool = pool;
50	<	// Note: poolIndex is set by pool during construction
223	<	// Remaining initialization is deferred to onStart
50	>	this.workQueue = pool.registerWorker(this);
51		}
52
226	–	// Public access methods
227	–
53		/**
54		* Returns the pool hosting this thread.
55		*
#	Line 244 | Line 69 | public class ForkJoinWorkerThread extend
69		* @return the index number
70		*/
71		public int getPoolIndex() {
72	<	return poolIndex;
248	<	}
249	<
250	<	/**
251	<	* Establishes local first-in-first-out scheduling mode for forked
252	<	* tasks that are never joined.
253	<	*
254	<	* @param async if true, use locally FIFO scheduling
255	<	*/
256	<	void setAsyncMode(boolean async) {
257	<	locallyFifo = async;
258	<	}
259	<
260	<	// Runstate management
261	<
262	<	// Runstate values. Order matters
263	<	private static final int RUNNING     = 0;
264	<	private static final int SHUTDOWN    = 1;
265	<	private static final int TERMINATING = 2;
266	<	private static final int TERMINATED  = 3;
267	<
268	<	final boolean isShutdown()    { return runState >= SHUTDOWN;  }
269	<	final boolean isTerminating() { return runState >= TERMINATING;  }
270	<	final boolean isTerminated()  { return runState == TERMINATED; }
271	<	final boolean shutdown()      { return transitionRunStateTo(SHUTDOWN); }
272	<	final boolean shutdownNow()   { return transitionRunStateTo(TERMINATING); }
273	<
274	<	/**
275	<	* Transitions to at least the given state.
276	<	*
277	<	* @return {@code true} if not already at least at given state
278	<	*/
279	<	private boolean transitionRunStateTo(int state) {
280	<	for (;;) {
281	<	int s = runState;
282	<	if (s >= state)
283	<	return false;
284	<	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, state))
285	<	return true;
286	<	}
287	<	}
288	<
289	<	/**
290	<	* Tries to set status to active; fails on contention.
291	<	*/
292	<	private boolean tryActivate() {
293	<	if (!active) {
294	<	if (!pool.tryIncrementActiveCount())
295	<	return false;
296	<	active = true;
297	<	}
298	<	return true;
299	<	}
300	<
301	<	/**
302	<	* Tries to set status to inactive; fails on contention.
303	<	*/
304	<	private boolean tryInactivate() {
305	<	if (active) {
306	<	if (!pool.tryDecrementActiveCount())
307	<	return false;
308	<	active = false;
309	<	}
310	<	return true;
311	<	}
312	<
313	<	/**
314	<	* Computes next value for random victim probe.  Scans don't
315	<	* require a very high quality generator, but also not a crummy
316	<	* one.  Marsaglia xor-shift is cheap and works well.
317	<	*/
318	<	private static int xorShift(int r) {
319	<	r ^= (r << 13);
320	<	r ^= (r >>> 17);
321	<	return r ^ (r << 5);
322	<	}
323	<
324	<	// Lifecycle methods
325	<
326	<	/**
327	<	* This method is required to be public, but should never be
328	<	* called explicitly. It performs the main run loop to execute
329	<	* ForkJoinTasks.
330	<	*/
331	<	public void run() {
332	<	Throwable exception = null;
333	<	try {
334	<	onStart();
335	<	pool.sync(this); // await first pool event
336	<	mainLoop();
337	<	} catch (Throwable ex) {
338	<	exception = ex;
339	<	} finally {
340	<	onTermination(exception);
341	<	}
342	<	}
343	<
344	<	/**
345	<	* Executes tasks until shut down.
346	<	*/
347	<	private void mainLoop() {
348	<	while (!isShutdown()) {
349	<	ForkJoinTask<?> t = pollTask();
350	<	if (t != null || (t = pollSubmission()) != null)
351	<	t.quietlyExec();
352	<	else if (tryInactivate())
353	<	pool.sync(this);
354	<	}
72	>	return workQueue.poolIndex;
73		}
74
75		/**
76		* Initializes internal state after construction but before
77		* processing any tasks. If you override this method, you must
78	<	* invoke super.onStart() at the beginning of the method.
78	>	* invoke {@code super.onStart()} at the beginning of the method.
79		* Initialization requires care: Most fields must have legal
80		* default values, to ensure that attempted accesses from other
81		* threads work correctly even before this thread starts
82		* processing tasks.
83		*/
84		protected void onStart() {
367	–	// Allocate while starting to improve chances of thread-local
368	–	// isolation
369	–	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
370	–	// Initial value of seed need not be especially random but
371	–	// should differ across workers and must be nonzero
372	–	int p = poolIndex + 1;
373	–	seed = p + (p << 8) + (p << 16) + (p << 24); // spread bits
85		}
86
87		/**
#	Line 382 | Line 93 | public class ForkJoinWorkerThread extend
93		* to an unrecoverable error, or {@code null} if completed normally
94		*/
95		protected void onTermination(Throwable exception) {
385	–	// Execute remaining local tasks unless aborting or terminating
386	–	while (exception == null &&  !pool.isTerminating() && base != sp) {
387	–	try {
388	–	ForkJoinTask<?> t = popTask();
389	–	if (t != null)
390	–	t.quietlyExec();
391	–	} catch (Throwable ex) {
392	–	exception = ex;
393	–	}
394	–	}
395	–	// Cancel other tasks, transition status, notify pool, and
396	–	// propagate exception to uncaught exception handler
397	–	try {
398	–	do {} while (!tryInactivate()); // ensure inactive
399	–	cancelTasks();
400	–	runState = TERMINATED;
401	–	pool.workerTerminated(this);
402	–	} catch (Throwable ex) {        // Shouldn't ever happen
403	–	if (exception == null)      // but if so, at least rethrown
404	–	exception = ex;
405	–	} finally {
406	–	if (exception != null)
407	–	ForkJoinTask.rethrowException(exception);
408	–	}
409	–	}
410	–
411	–	// Intrinsics-based support for queue operations.
412	–
413	–	/**
414	–	* Adds in store-order the given task at given slot of q to null.
415	–	* Caller must ensure q is non-null and index is in range.
416	–	*/
417	–	private static void setSlot(ForkJoinTask<?>[] q, int i,
418	–	ForkJoinTask<?> t) {
419	–	UNSAFE.putOrderedObject(q, (i << qShift) + qBase, t);
420	–	}
421	–
422	–	/**
423	–	* CAS given slot of q to null. Caller must ensure q is non-null
424	–	* and index is in range.
425	–	*/
426	–	private static boolean casSlotNull(ForkJoinTask<?>[] q, int i,
427	–	ForkJoinTask<?> t) {
428	–	return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
429	–	}
430	–
431	–	/**
432	–	* Sets sp in store-order.
433	–	*/
434	–	private void storeSp(int s) {
435	–	UNSAFE.putOrderedInt(this, spOffset, s);
436	–	}
437	–
438	–	// Main queue methods
439	–
440	–	/**
441	–	* Pushes a task. Called only by current thread.
442	–	*
443	–	* @param t the task. Caller must ensure non-null.
444	–	*/
445	–	final void pushTask(ForkJoinTask<?> t) {
446	–	ForkJoinTask<?>[] q = queue;
447	–	int mask = q.length - 1;
448	–	int s = sp;
449	–	setSlot(q, s & mask, t);
450	–	storeSp(++s);
451	–	if ((s -= base) == 1)
452	–	pool.signalWork();
453	–	else if (s >= mask)
454	–	growQueue();
455	–	}
456	–
457	–	/**
458	–	* Tries to take a task from the base of the queue, failing if
459	–	* either empty or contended.
460	–	*
461	–	* @return a task, or null if none or contended
462	–	*/
463	–	final ForkJoinTask<?> deqTask() {
464	–	ForkJoinTask<?> t;
465	–	ForkJoinTask<?>[] q;
466	–	int i;
467	–	int b;
468	–	if (sp != (b = base) &&
469	–	(q = queue) != null && // must read q after b
470	–	(t = q[i = (q.length - 1) & b]) != null &&
471	–	casSlotNull(q, i, t)) {
472	–	base = b + 1;
473	–	return t;
474	–	}
475	–	return null;
476	–	}
477	–
478	–	/**
479	–	* Tries to take a task from the base of own queue, activating if
480	–	* necessary, failing only if empty. Called only by current thread.
481	–	*
482	–	* @return a task, or null if none
483	–	*/
484	–	final ForkJoinTask<?> locallyDeqTask() {
485	–	int b;
486	–	while (sp != (b = base)) {
487	–	if (tryActivate()) {
488	–	ForkJoinTask<?>[] q = queue;
489	–	int i = (q.length - 1) & b;
490	–	ForkJoinTask<?> t = q[i];
491	–	if (t != null && casSlotNull(q, i, t)) {
492	–	base = b + 1;
493	–	return t;
494	–	}
495	–	}
496	–	}
497	–	return null;
498	–	}
499	–
500	–	/**
501	–	* Returns a popped task, or null if empty. Ensures active status
502	–	* if non-null. Called only by current thread.
503	–	*/
504	–	final ForkJoinTask<?> popTask() {
505	–	int s = sp;
506	–	while (s != base) {
507	–	if (tryActivate()) {
508	–	ForkJoinTask<?>[] q = queue;
509	–	int mask = q.length - 1;
510	–	int i = (s - 1) & mask;
511	–	ForkJoinTask<?> t = q[i];
512	–	if (t == null || !casSlotNull(q, i, t))
513	–	break;
514	–	storeSp(s - 1);
515	–	return t;
516	–	}
517	–	}
518	–	return null;
519	–	}
520	–
521	–	/**
522	–	* Specialized version of popTask to pop only if
523	–	* topmost element is the given task. Called only
524	–	* by current thread while active.
525	–	*
526	–	* @param t the task. Caller must ensure non-null.
527	–	*/
528	–	final boolean unpushTask(ForkJoinTask<?> t) {
529	–	ForkJoinTask<?>[] q = queue;
530	–	int mask = q.length - 1;
531	–	int s = sp - 1;
532	–	if (casSlotNull(q, s & mask, t)) {
533	–	storeSp(s);
534	–	return true;
535	–	}
536	–	return false;
537	–	}
538	–
539	–	/**
540	–	* Returns next task or null if empty or contended
541	–	*/
542	–	final ForkJoinTask<?> peekTask() {
543	–	ForkJoinTask<?>[] q = queue;
544	–	if (q == null)
545	–	return null;
546	–	int mask = q.length - 1;
547	–	int i = locallyFifo ? base : (sp - 1);
548	–	return q[i & mask];
549	–	}
550	–
551	–	/**
552	–	* Doubles queue array size. Transfers elements by emulating
553	–	* steals (deqs) from old array and placing, oldest first, into
554	–	* new array.
555	–	*/
556	–	private void growQueue() {
557	–	ForkJoinTask<?>[] oldQ = queue;
558	–	int oldSize = oldQ.length;
559	–	int newSize = oldSize << 1;
560	–	if (newSize > MAXIMUM_QUEUE_CAPACITY)
561	–	throw new RejectedExecutionException("Queue capacity exceeded");
562	–	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
563	–
564	–	int b = base;
565	–	int bf = b + oldSize;
566	–	int oldMask = oldSize - 1;
567	–	int newMask = newSize - 1;
568	–	do {
569	–	int oldIndex = b & oldMask;
570	–	ForkJoinTask<?> t = oldQ[oldIndex];
571	–	if (t != null && !casSlotNull(oldQ, oldIndex, t))
572	–	t = null;
573	–	setSlot(newQ, b & newMask, t);
574	–	} while (++b != bf);
575	–	pool.signalWork();
576	–	}
577	–
578	–	/**
579	–	* Tries to steal a task from another worker. Starts at a random
580	–	* index of workers array, and probes workers until finding one
581	–	* with non-empty queue or finding that all are empty.  It
582	–	* randomly selects the first n probes. If these are empty, it
583	–	* resorts to a full circular traversal, which is necessary to
584	–	* accurately set active status by caller. Also restarts if pool
585	–	* events occurred since last scan, which forces refresh of
586	–	* workers array, in case barrier was associated with resize.
587	–	*
588	–	* This method must be both fast and quiet -- usually avoiding
589	–	* memory accesses that could disrupt cache sharing etc other than
590	–	* those needed to check for and take tasks. This accounts for,
591	–	* among other things, updating random seed in place without
592	–	* storing it until exit.
593	–	*
594	–	* @return a task, or null if none found
595	–	*/
596	–	private ForkJoinTask<?> scan() {
597	–	ForkJoinTask<?> t = null;
598	–	int r = seed;                    // extract once to keep scan quiet
599	–	ForkJoinWorkerThread[] ws;       // refreshed on outer loop
600	–	int mask;                        // must be power 2 minus 1 and > 0
601	–	outer:do {
602	–	if ((ws = pool.workers) != null && (mask = ws.length - 1) > 0) {
603	–	int idx = r;
604	–	int probes = ~mask;      // use random index while negative
605	–	for (;;) {
606	–	r = xorShift(r);     // update random seed
607	–	ForkJoinWorkerThread v = ws[mask & idx];
608	–	if (v == null || v.sp == v.base) {
609	–	if (probes <= mask)
610	–	idx = (probes++ < 0) ? r : (idx + 1);
611	–	else
612	–	break;
613	–	}
614	–	else if (!tryActivate() || (t = v.deqTask()) == null)
615	–	continue outer;  // restart on contention
616	–	else
617	–	break outer;
618	–	}
619	–	}
620	–	} while (pool.hasNewSyncEvent(this)); // retry on pool events
621	–	seed = r;
622	–	return t;
623	–	}
624	–
625	–	/**
626	–	* Gets and removes a local or stolen task.
627	–	*
628	–	* @return a task, if available
629	–	*/
630	–	final ForkJoinTask<?> pollTask() {
631	–	ForkJoinTask<?> t = locallyFifo ? locallyDeqTask() : popTask();
632	–	if (t == null && (t = scan()) != null)
633	–	++stealCount;
634	–	return t;
635	–	}
636	–
637	–	/**
638	–	* Gets a local task.
639	–	*
640	–	* @return a task, if available
641	–	*/
642	–	final ForkJoinTask<?> pollLocalTask() {
643	–	return locallyFifo ? locallyDeqTask() : popTask();
644	–	}
645	–
646	–	/**
647	–	* Returns a pool submission, if one exists, activating first.
648	–	*
649	–	* @return a submission, if available
650	–	*/
651	–	private ForkJoinTask<?> pollSubmission() {
652	–	ForkJoinPool p = pool;
653	–	while (p.hasQueuedSubmissions()) {
654	–	ForkJoinTask<?> t;
655	–	if (tryActivate() && (t = p.pollSubmission()) != null)
656	–	return t;
657	–	}
658	–	return null;
659	–	}
660	–
661	–	// Methods accessed only by Pool
662	–
663	–	/**
664	–	* Removes and cancels all tasks in queue.  Can be called from any
665	–	* thread.
666	–	*/
667	–	final void cancelTasks() {
668	–	ForkJoinTask<?> t;
669	–	while (base != sp && (t = deqTask()) != null)
670	–	t.cancelIgnoringExceptions();
671	–	}
672	–
673	–	/**
674	–	* Drains tasks to given collection c.
675	–	*
676	–	* @return the number of tasks drained
677	–	*/
678	–	final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
679	–	int n = 0;
680	–	ForkJoinTask<?> t;
681	–	while (base != sp && (t = deqTask()) != null) {
682	–	c.add(t);
683	–	++n;
684	–	}
685	–	return n;
686	–	}
687	–
688	–	/**
689	–	* Gets and clears steal count for accumulation by pool.  Called
690	–	* only when known to be idle (in pool.sync and termination).
691	–	*/
692	–	final int getAndClearStealCount() {
693	–	int sc = stealCount;
694	–	stealCount = 0;
695	–	return sc;
96		}
97
98		/**
99	<	* Returns {@code true} if at least one worker in the given array
100	<	* appears to have at least one queued task.
101	<	*
702	<	* @param ws array of workers
703	<	*/
704	<	static boolean hasQueuedTasks(ForkJoinWorkerThread[] ws) {
705	<	if (ws != null) {
706	<	int len = ws.length;
707	<	for (int j = 0; j < 2; ++j) { // need two passes for clean sweep
708	<	for (int i = 0; i < len; ++i) {
709	<	ForkJoinWorkerThread w = ws[i];
710	<	if (w != null && w.sp != w.base)
711	<	return true;
712	<	}
713	<	}
714	<	}
715	<	return false;
716	<	}
717	<
718	<	// Support methods for ForkJoinTask
719	<
720	<	/**
721	<	* Returns an estimate of the number of tasks in the queue.
722	<	*/
723	<	final int getQueueSize() {
724	<	// suppress momentarily negative values
725	<	return Math.max(0, sp - base);
726	<	}
727	<
728	<	/**
729	<	* Returns an estimate of the number of tasks, offset by a
730	<	* function of number of idle workers.
731	<	*/
732	<	final int getEstimatedSurplusTaskCount() {
733	<	// The halving approximates weighting idle vs non-idle workers
734	<	return (sp - base) - (pool.getIdleThreadCount() >>> 1);
735	<	}
736	<
737	<	/**
738	<	* Scans, returning early if joinMe done.
739	<	*/
740	<	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
741	<	ForkJoinTask<?> t = pollTask();
742	<	if (t != null && joinMe.status < 0 && sp == base) {
743	<	pushTask(t); // unsteal if done and this task would be stealable
744	<	t = null;
745	<	}
746	<	return t;
747	<	}
748	<
749	<	/**
750	<	* Runs tasks until {@code pool.isQuiescent()}.
751	<	*/
752	<	final void helpQuiescePool() {
753	<	for (;;) {
754	<	ForkJoinTask<?> t = pollTask();
755	<	if (t != null)
756	<	t.quietlyExec();
757	<	else if (tryInactivate() && pool.isQuiescent())
758	<	break;
759	<	}
760	<	do {} while (!tryActivate()); // re-activate on exit
761	<	}
762	<
763	<	// Unsafe mechanics
764	<
765	<	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
766	<	private static final long spOffset =
767	<	objectFieldOffset("sp", ForkJoinWorkerThread.class);
768	<	private static final long runStateOffset =
769	<	objectFieldOffset("runState", ForkJoinWorkerThread.class);
770	<	private static final long qBase;
771	<	private static final int qShift;
772	<
773	<	static {
774	<	qBase = UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
775	<	int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
776	<	if ((s & (s-1)) != 0)
777	<	throw new Error("data type scale not a power of two");
778	<	qShift = 31 - Integer.numberOfLeadingZeros(s);
779	<	}
780	<
781	<	private static long objectFieldOffset(String field, Class<?> klazz) {
782	<	try {
783	<	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
784	<	} catch (NoSuchFieldException e) {
785	<	// Convert Exception to corresponding Error
786	<	NoSuchFieldError error = new NoSuchFieldError(field);
787	<	error.initCause(e);
788	<	throw error;
789	<	}
790	<	}
791	<
792	<	/**
793	<	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
794	<	* Replace with a simple call to Unsafe.getUnsafe when integrating
795	<	* into a jdk.
796	<	*
797	<	* @return a sun.misc.Unsafe
99	>	* This method is required to be public, but should never be
100	>	* called explicitly. It performs the main run loop to execute
101	>	* {@link ForkJoinTask}s.
102		*/
103	<	private static sun.misc.Unsafe getUnsafe() {
103	>	public void run() {
104	>	Throwable exception = null;
105		try {
106	<	return sun.misc.Unsafe.getUnsafe();
107	<	} catch (SecurityException se) {
106	>	onStart();
107	>	pool.runWorker(workQueue);
108	>	} catch (Throwable ex) {
109	>	exception = ex;
110	>	} finally {
111		try {
112	<	return java.security.AccessController.doPrivileged
113	<	(new java.security
114	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
115	<	public sun.misc.Unsafe run() throws Exception {
116	<	java.lang.reflect.Field f = sun.misc
117	<	.Unsafe.class.getDeclaredField("theUnsafe");
810	<	f.setAccessible(true);
811	<	return (sun.misc.Unsafe) f.get(null);
812	<	}});
813	<	} catch (java.security.PrivilegedActionException e) {
814	<	throw new RuntimeException("Could not initialize intrinsics",
815	<	e.getCause());
112	>	onTermination(exception);
113	>	} catch (Throwable ex) {
114	>	if (exception == null)
115	>	exception = ex;
116	>	} finally {
117	>	pool.deregisterWorker(this, exception);
118		}
119		}
120		}

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