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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.10 by jsr166, Mon Jul 20 23:07:43 2009 UTC vs.
Revision 1.72 by dl, Mon Nov 19 18:12:42 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	<	import java.util.*;
9	<	import java.util.concurrent.*;
10	<	import java.util.concurrent.atomic.*;
11	<	import java.util.concurrent.locks.*;
12	<	import sun.misc.Unsafe;
13	<	import java.lang.reflect.*;
8	>
9	>	import java.util.concurrent.atomic.AtomicInteger;
10
11		/**
12	<	* A thread managed by a {@link ForkJoinPool}.  This class is
13	<	* subclassable solely for the sake of adding functionality -- there
14	<	* are no overridable methods dealing with scheduling or
15	<	* execution. However, you can override initialization and termination
16	<	* methods surrounding the main task processing loop.  If you do
17	<	* create such a subclass, you will also need to supply a custom
18	<	* ForkJoinWorkerThreadFactory to use it in a ForkJoinPool.
12	>	* A thread managed by a {@link ForkJoinPool}, which executes
13	>	* {@link ForkJoinTask}s.
14	>	* This class is subclassable solely for the sake of adding
15	>	* functionality -- there are no overridable methods dealing with
16	>	* scheduling or execution.  However, you can override initialization
17	>	* and termination methods surrounding the main task processing loop.
18	>	* If you do create such a subclass, you will also need to supply a
19	>	* custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to use it
20	>	* in a {@code ForkJoinPool}.
21		*
22	+	* @since 1.7
23	+	* @author Doug Lea
24		*/
25		public class ForkJoinWorkerThread extends Thread {
26		/*
27	<	* Algorithm overview:
28	<	*
29	<	* 1. Work-Stealing: Work-stealing queues are special forms of
30	<	* Deques that support only three of the four possible
31	<	* end-operations -- push, pop, and deq (aka steal), and only do
32	<	* so under the constraints that push and pop are called only from
33	<	* the owning thread, while deq may be called from other threads.
34	<	* (If you are unfamiliar with them, you probably want to read
35	<	* Herlihy and Shavit's book "The Art of Multiprocessor
36	<	* programming", chapter 16 describing these in more detail before
37	<	* proceeding.)  The main work-stealing queue design is roughly
38	<	* similar to "Dynamic Circular Work-Stealing Deque" by David
39	<	* Chase and Yossi Lev, SPAA 2005
40	<	* (http://research.sun.com/scalable/pubs/index.html).  The main
41	<	* difference ultimately stems from gc requirements that we null
42	<	* out taken slots as soon as we can, to maintain as small a
43	<	* footprint as possible even in programs generating huge numbers
44	<	* of tasks. To accomplish this, we shift the CAS arbitrating pop
45	<	* vs deq (steal) from being on the indices ("base" and "sp") to
46	<	* the slots themselves (mainly via method "casSlotNull()"). So,
47	<	* both a successful pop and deq mainly entail CAS'ing a non-null
48	<	* slot to null.  Because we rely on CASes of references, we do
49	<	* not need tag bits on base or sp.  They are simple ints as used
50	<	* in any circular array-based queue (see for example ArrayDeque).
51	<	* Updates to the indices must still be ordered in a way that
52	<	* guarantees that (sp - base) > 0 means the queue is empty, but
53	<	* otherwise may err on the side of possibly making the queue
54	<	* appear nonempty when a push, pop, or deq have not fully
55	<	* committed. Note that this means that the deq operation,
56	<	* considered individually, is not wait-free. One thief cannot
57	<	* successfully continue until another in-progress one (or, if
58	<	* previously empty, a push) completes.  However, in the
59	<	* aggregate, we ensure at least probabilistic non-blockingness. If
60	<	* an attempted steal fails, a thief always chooses a different
61	<	* random victim target to try next. So, in order for one thief to
62	<	* progress, it suffices for any in-progress deq or new push on
63	<	* any empty queue to complete. One reason this works well here is
64	<	* that apparently-nonempty often means soon-to-be-stealable,
65	<	* which gives threads a chance to activate if necessary before
66	<	* stealing (see below).
67	<	*
68	<	* Efficient implementation of this approach currently relies on
69	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
70	<	* correct orderings, reads and writes of variable base require
71	<	* volatile ordering.  Variable sp does not require volatile write
72	<	* but needs cheaper store-ordering on writes.  Because they are
73	<	* protected by volatile base reads, reads of the queue array and
74	<	* its slots do not need volatile load semantics, but writes (in
75	<	* push) require store order and CASes (in pop and deq) require
76	<	* (volatile) CAS semantics. Since these combinations aren't
77	<	* supported using ordinary volatiles, the only way to accomplish
78	<	* these efficiently is to use direct Unsafe calls. (Using external
79	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
80	<	* array is significantly slower because of memory locality and
81	<	* indirection effects.) Further, performance on most platforms is
82	<	* very sensitive to placement and sizing of the (resizable) queue
83	<	* array.  Even though these queues don't usually become all that
84	<	* big, the initial size must be large enough to counteract cache
85	<	* contention effects across multiple queues (especially in the
86	<	* presence of GC cardmarking). Also, to improve thread-locality,
87	<	* queues are currently initialized immediately after the thread
88	<	* gets the initial signal to start processing tasks.  However,
89	<	* all queue-related methods except pushTask are written in a way
90	<	* that allows them to instead be lazily allocated and/or disposed
91	<	* of when empty. All together, these low-level implementation
92	<	* choices produce as much as a factor of 4 performance
93	<	* improvement compared to naive implementations, and enable the
94	<	* processing of billions of tasks per second, sometimes at the
95	<	* expense of ugliness.
96	<	*
97	<	* 2. Run control: The primary run control is based on a global
98	<	* counter (activeCount) held by the pool. It uses an algorithm
99	<	* similar to that in Herlihy and Shavit section 17.6 to cause
100	<	* threads to eventually block when all threads declare they are
101	<	* inactive. (See variable "scans".)  For this to work, threads
102	<	* must be declared active when executing tasks, and before
103	<	* stealing a task. They must be inactive before blocking on the
104	<	* Pool Barrier (awaiting a new submission or other Pool
105	<	* event). In between, there is some free play which we take
106	<	* advantage of to avoid contention and rapid flickering of the
107	<	* global activeCount: If inactive, we activate only if a victim
108	<	* queue appears to be nonempty (see above).  Similarly, a thread
109	<	* tries to inactivate only after a full scan of other threads.
110	<	* The net effect is that contention on activeCount is rarely a
111	<	* measurable performance issue. (There are also a few other cases
112	<	* where we scan for work rather than retry/block upon
113	<	* contention.)
27	>	* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
28	>	* ForkJoinTasks. For explanation, see the internal documentation
29	>	* of class ForkJoinPool.
30		*
31	<	* 3. Selection control. We maintain policy of always choosing to
32	<	* run local tasks rather than stealing, and always trying to
33	<	* steal tasks before trying to run a new submission. All steals
34	<	* are currently performed in randomly-chosen deq-order. It may be
35	<	* worthwhile to bias these with locality / anti-locality
36	<	* information, but doing this well probably requires more
37	<	* lower-level information from JVMs than currently provided.
122	<	*/
123	<
124	<	/**
125	<	* Capacity of work-stealing queue array upon initialization.
126	<	* Must be a power of two. Initial size must be at least 2, but is
127	<	* padded to minimize cache effects.
128	<	*/
129	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
130	<
131	<	/**
132	<	* Maximum work-stealing queue array size.  Must be less than or
133	<	* equal to 1 << 28 to ensure lack of index wraparound. (This
134	<	* is less than usual bounds, because we need leftshift by 3
135	<	* to be in int range).
136	<	*/
137	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28;
138	<
139	<	/**
140	<	* The pool this thread works in. Accessed directly by ForkJoinTask
141	<	*/
142	<	final ForkJoinPool pool;
143	<
144	<	/**
145	<	* The work-stealing queue array. Size must be a power of two.
146	<	* Initialized when thread starts, to improve memory locality.
147	<	*/
148	<	private ForkJoinTask<?>[] queue;
149	<
150	<	/**
151	<	* Index (mod queue.length) of next queue slot to push to or pop
152	<	* from. It is written only by owner thread, via ordered store.
153	<	* Both sp and base are allowed to wrap around on overflow, but
154	<	* (sp - base) still estimates size.
155	<	*/
156	<	private volatile int sp;
157	<
158	<	/**
159	<	* Index (mod queue.length) of least valid queue slot, which is
160	<	* always the next position to steal from if nonempty.
161	<	*/
162	<	private volatile int base;
163	<
164	<	/**
165	<	* Activity status. When true, this worker is considered active.
166	<	* Must be false upon construction. It must be true when executing
167	<	* tasks, and BEFORE stealing a task. It must be false before
168	<	* calling pool.sync
169	<	*/
170	<	private boolean active;
171	<
172	<	/**
173	<	* Run state of this worker. Supports simple versions of the usual
174	<	* shutdown/shutdownNow control.
31	>	* This class just maintains links to its pool and WorkQueue.  The
32	>	* pool field is set upon construction, but the workQueue field is
33	>	* not set until the thread has started (unless forced early by a
34	>	* subclass constructor call to poolIndex()).  This provides
35	>	* better memory placement (because this thread allocates queue
36	>	* and bookkeeping fields) but because the field is non-final, we
37	>	* require that it never be accessed except by the owning thread.
38		*/
176	–	private volatile int runState;
39
40	<	/**
41	<	* Seed for random number generator for choosing steal victims.
180	<	* Uses Marsaglia xorshift. Must be nonzero upon initialization.
181	<	*/
182	<	private int seed;
183	<
184	<	/**
185	<	* Number of steals, transferred to pool when idle
186	<	*/
187	<	private int stealCount;
188	<
189	<	/**
190	<	* Index of this worker in pool array. Set once by pool before
191	<	* running, and accessed directly by pool during cleanup etc
192	<	*/
193	<	int poolIndex;
194	<
195	<	/**
196	<	* The last barrier event waited for. Accessed in pool callback
197	<	* methods, but only by current thread.
198	<	*/
199	<	long lastEventCount;
40	>	final ForkJoinPool pool;                // the pool this thread works in
41	>	ForkJoinPool.WorkQueue workQueue;       // Work-stealing mechanics
42
43		/**
44	<	* True if use local fifo, not default lifo, for local polling
44	>	* Sequence number for creating worker Names
45		*/
46	<	private boolean locallyFifo;
46	>	private static final AtomicInteger threadNumber = new AtomicInteger();
47
48		/**
49		* Creates a ForkJoinWorkerThread operating in the given pool.
50	+	*
51		* @param pool the pool this thread works in
52		* @throws NullPointerException if pool is null
53		*/
54		protected ForkJoinWorkerThread(ForkJoinPool pool) {
55	<	if (pool == null) throw new NullPointerException();
55	>	super(pool.workerNamePrefix.concat(Integer.toString(threadNumber.incrementAndGet())));
56	>	setDaemon(true);
57		this.pool = pool;
58	<	// Note: poolIndex is set by pool during construction
59	<	// Remaining initialization is deferred to onStart
58	>	Thread.UncaughtExceptionHandler ueh = pool.ueh;
59	>	if (ueh != null)
60	>	setUncaughtExceptionHandler(ueh);
61		}
62
218	–	// Public access methods
219	–
63		/**
64	<	* Returns the pool hosting this thread
64	>	* Returns the pool hosting this thread.
65	>	*
66		* @return the pool
67		*/
68		public ForkJoinPool getPool() {
#	Line 231 | Line 75 | public class ForkJoinWorkerThread extend
75		* threads (minus one) that have ever been created in the pool.
76		* This method may be useful for applications that track status or
77		* collect results per-worker rather than per-task.
78	<	* @return the index number.
78	>	*
79	>	* @return the index number
80		*/
81		public int getPoolIndex() {
82	<	return poolIndex;
83	<	}
84	<
85	<	/**
86	<	* Establishes local first-in-first-out scheduling mode for forked
242	<	* tasks that are never joined.
243	<	* @param async if true, use locally FIFO scheduling
244	<	*/
245	<	void setAsyncMode(boolean async) {
246	<	locallyFifo = async;
247	<	}
248	<
249	<	// Runstate management
250	<
251	<	// Runstate values. Order matters
252	<	private static final int RUNNING     = 0;
253	<	private static final int SHUTDOWN    = 1;
254	<	private static final int TERMINATING = 2;
255	<	private static final int TERMINATED  = 3;
256	<
257	<	final boolean isShutdown()    { return runState >= SHUTDOWN;  }
258	<	final boolean isTerminating() { return runState >= TERMINATING;  }
259	<	final boolean isTerminated()  { return runState == TERMINATED; }
260	<	final boolean shutdown()      { return transitionRunStateTo(SHUTDOWN); }
261	<	final boolean shutdownNow()   { return transitionRunStateTo(TERMINATING); }
262	<
263	<	/**
264	<	* Transition to at least the given state. Return true if not
265	<	* already at least given state.
266	<	*/
267	<	private boolean transitionRunStateTo(int state) {
268	<	for (;;) {
269	<	int s = runState;
270	<	if (s >= state)
271	<	return false;
272	<	if (_unsafe.compareAndSwapInt(this, runStateOffset, s, state))
273	<	return true;
274	<	}
275	<	}
276	<
277	<	/**
278	<	* Try to set status to active; fail on contention
279	<	*/
280	<	private boolean tryActivate() {
281	<	if (!active) {
282	<	if (!pool.tryIncrementActiveCount())
283	<	return false;
284	<	active = true;
285	<	}
286	<	return true;
287	<	}
288	<
289	<	/**
290	<	* Try to set status to active; fail on contention
291	<	*/
292	<	private boolean tryInactivate() {
293	<	if (active) {
294	<	if (!pool.tryDecrementActiveCount())
295	<	return false;
296	<	active = false;
297	<	}
298	<	return true;
299	<	}
300	<
301	<	/**
302	<	* Computes next value for random victim probe. Scans don't
303	<	* require a very high quality generator, but also not a crummy
304	<	* one. Marsaglia xor-shift is cheap and works well.
305	<	*/
306	<	private static int xorShift(int r) {
307	<	r ^= r << 1;
308	<	r ^= r >>> 3;
309	<	r ^= r << 10;
310	<	return r;
311	<	}
312	<
313	<	// Lifecycle methods
314	<
315	<	/**
316	<	* This method is required to be public, but should never be
317	<	* called explicitly. It performs the main run loop to execute
318	<	* ForkJoinTasks.
319	<	*/
320	<	public void run() {
321	<	Throwable exception = null;
322	<	try {
323	<	onStart();
324	<	pool.sync(this); // await first pool event
325	<	mainLoop();
326	<	} catch (Throwable ex) {
327	<	exception = ex;
328	<	} finally {
329	<	onTermination(exception);
330	<	}
331	<	}
332	<
333	<	/**
334	<	* Execute tasks until shut down.
335	<	*/
336	<	private void mainLoop() {
337	<	while (!isShutdown()) {
338	<	ForkJoinTask<?> t = pollTask();
339	<	if (t != null || (t = pollSubmission()) != null)
340	<	t.quietlyExec();
341	<	else if (tryInactivate())
342	<	pool.sync(this);
82	>	// force early registration if called before started
83	>	ForkJoinPool.WorkQueue q;
84	>	if ((q = workQueue) == null) {
85	>	pool.registerWorker(this);
86	>	q = workQueue;
87		}
88	+	return q.poolIndex;
89		}
90
91		/**
92		* Initializes internal state after construction but before
93		* processing any tasks. If you override this method, you must
94	<	* invoke super.onStart() at the beginning of the method.
94	>	* invoke {@code super.onStart()} at the beginning of the method.
95		* Initialization requires care: Most fields must have legal
96		* default values, to ensure that attempted accesses from other
97		* threads work correctly even before this thread starts
98		* processing tasks.
99		*/
100		protected void onStart() {
356	–	// Allocate while starting to improve chances of thread-local
357	–	// isolation
358	–	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
359	–	// Initial value of seed need not be especially random but
360	–	// should differ across workers and must be nonzero
361	–	int p = poolIndex + 1;
362	–	seed = p + (p << 8) + (p << 16) + (p << 24); // spread bits
101		}
102
103		/**
104	<	* Perform cleanup associated with termination of this worker
104	>	* Performs cleanup associated with termination of this worker
105		* thread.  If you override this method, you must invoke
106	<	* super.onTermination at the end of the overridden method.
106	>	* {@code super.onTermination} at the end of the overridden method.
107		*
108		* @param exception the exception causing this thread to abort due
109	<	* to an unrecoverable error, or null if completed normally.
109	>	* to an unrecoverable error, or {@code null} if completed normally
110		*/
111		protected void onTermination(Throwable exception) {
374	–	// Execute remaining local tasks unless aborting or terminating
375	–	while (exception == null &&  !pool.isTerminating() && base != sp) {
376	–	try {
377	–	ForkJoinTask<?> t = popTask();
378	–	if (t != null)
379	–	t.quietlyExec();
380	–	} catch(Throwable ex) {
381	–	exception = ex;
382	–	}
383	–	}
384	–	// Cancel other tasks, transition status, notify pool, and
385	–	// propagate exception to uncaught exception handler
386	–	try {
387	–	do;while (!tryInactivate()); // ensure inactive
388	–	cancelTasks();
389	–	runState = TERMINATED;
390	–	pool.workerTerminated(this);
391	–	} catch (Throwable ex) {        // Shouldn't ever happen
392	–	if (exception == null)      // but if so, at least rethrown
393	–	exception = ex;
394	–	} finally {
395	–	if (exception != null)
396	–	ForkJoinTask.rethrowException(exception);
397	–	}
398	–	}
399	–
400	–	// Intrinsics-based support for queue operations.
401	–
402	–	/**
403	–	* Adds in store-order the given task at given slot of q to null.
404	–	* Caller must ensure q is non-null and index is in range.
405	–	*/
406	–	private static void setSlot(ForkJoinTask<?>[] q, int i,
407	–	ForkJoinTask<?> t){
408	–	_unsafe.putOrderedObject(q, (i << qShift) + qBase, t);
409	–	}
410	–
411	–	/**
412	–	* CAS given slot of q to null. Caller must ensure q is non-null
413	–	* and index is in range.
414	–	*/
415	–	private static boolean casSlotNull(ForkJoinTask<?>[] q, int i,
416	–	ForkJoinTask<?> t) {
417	–	return _unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
418	–	}
419	–
420	–	/**
421	–	* Sets sp in store-order.
422	–	*/
423	–	private void storeSp(int s) {
424	–	_unsafe.putOrderedInt(this, spOffset, s);
425	–	}
426	–
427	–	// Main queue methods
428	–
429	–	/**
430	–	* Pushes a task. Called only by current thread.
431	–	* @param t the task. Caller must ensure non-null.
432	–	*/
433	–	final void pushTask(ForkJoinTask<?> t) {
434	–	ForkJoinTask<?>[] q = queue;
435	–	int mask = q.length - 1;
436	–	int s = sp;
437	–	setSlot(q, s & mask, t);
438	–	storeSp(++s);
439	–	if ((s -= base) == 1)
440	–	pool.signalWork();
441	–	else if (s >= mask)
442	–	growQueue();
443	–	}
444	–
445	–	/**
446	–	* Tries to take a task from the base of the queue, failing if
447	–	* either empty or contended.
448	–	* @return a task, or null if none or contended.
449	–	*/
450	–	final ForkJoinTask<?> deqTask() {
451	–	ForkJoinTask<?> t;
452	–	ForkJoinTask<?>[] q;
453	–	int i;
454	–	int b;
455	–	if (sp != (b = base) &&
456	–	(q = queue) != null && // must read q after b
457	–	(t = q[i = (q.length - 1) & b]) != null &&
458	–	casSlotNull(q, i, t)) {
459	–	base = b + 1;
460	–	return t;
461	–	}
462	–	return null;
463	–	}
464	–
465	–	/**
466	–	* Returns a popped task, or null if empty. Ensures active status
467	–	* if non-null. Called only by current thread.
468	–	*/
469	–	final ForkJoinTask<?> popTask() {
470	–	int s = sp;
471	–	while (s != base) {
472	–	if (tryActivate()) {
473	–	ForkJoinTask<?>[] q = queue;
474	–	int mask = q.length - 1;
475	–	int i = (s - 1) & mask;
476	–	ForkJoinTask<?> t = q[i];
477	–	if (t == null || !casSlotNull(q, i, t))
478	–	break;
479	–	storeSp(s - 1);
480	–	return t;
481	–	}
482	–	}
483	–	return null;
484	–	}
485	–
486	–	/**
487	–	* Specialized version of popTask to pop only if
488	–	* topmost element is the given task. Called only
489	–	* by current thread while active.
490	–	* @param t the task. Caller must ensure non-null
491	–	*/
492	–	final boolean unpushTask(ForkJoinTask<?> t) {
493	–	ForkJoinTask<?>[] q = queue;
494	–	int mask = q.length - 1;
495	–	int s = sp - 1;
496	–	if (casSlotNull(q, s & mask, t)) {
497	–	storeSp(s);
498	–	return true;
499	–	}
500	–	return false;
501	–	}
502	–
503	–	/**
504	–	* Returns next task.
505	–	*/
506	–	final ForkJoinTask<?> peekTask() {
507	–	ForkJoinTask<?>[] q = queue;
508	–	if (q == null)
509	–	return null;
510	–	int mask = q.length - 1;
511	–	int i = locallyFifo? base : (sp - 1);
512	–	return q[i & mask];
513	–	}
514	–
515	–	/**
516	–	* Doubles queue array size. Transfers elements by emulating
517	–	* steals (deqs) from old array and placing, oldest first, into
518	–	* new array.
519	–	*/
520	–	private void growQueue() {
521	–	ForkJoinTask<?>[] oldQ = queue;
522	–	int oldSize = oldQ.length;
523	–	int newSize = oldSize << 1;
524	–	if (newSize > MAXIMUM_QUEUE_CAPACITY)
525	–	throw new RejectedExecutionException("Queue capacity exceeded");
526	–	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
527	–
528	–	int b = base;
529	–	int bf = b + oldSize;
530	–	int oldMask = oldSize - 1;
531	–	int newMask = newSize - 1;
532	–	do {
533	–	int oldIndex = b & oldMask;
534	–	ForkJoinTask<?> t = oldQ[oldIndex];
535	–	if (t != null && !casSlotNull(oldQ, oldIndex, t))
536	–	t = null;
537	–	setSlot(newQ, b & newMask, t);
538	–	} while (++b != bf);
539	–	pool.signalWork();
540	–	}
541	–
542	–	/**
543	–	* Tries to steal a task from another worker. Starts at a random
544	–	* index of workers array, and probes workers until finding one
545	–	* with non-empty queue or finding that all are empty.  It
546	–	* randomly selects the first n probes. If these are empty, it
547	–	* resorts to a full circular traversal, which is necessary to
548	–	* accurately set active status by caller. Also restarts if pool
549	–	* events occurred since last scan, which forces refresh of
550	–	* workers array, in case barrier was associated with resize.
551	–	*
552	–	* This method must be both fast and quiet -- usually avoiding
553	–	* memory accesses that could disrupt cache sharing etc other than
554	–	* those needed to check for and take tasks. This accounts for,
555	–	* among other things, updating random seed in place without
556	–	* storing it until exit.
557	–	*
558	–	* @return a task, or null if none found
559	–	*/
560	–	private ForkJoinTask<?> scan() {
561	–	ForkJoinTask<?> t = null;
562	–	int r = seed;                    // extract once to keep scan quiet
563	–	ForkJoinWorkerThread[] ws;       // refreshed on outer loop
564	–	int mask;                        // must be power 2 minus 1 and > 0
565	–	outer:do {
566	–	if ((ws = pool.workers) != null && (mask = ws.length - 1) > 0) {
567	–	int idx = r;
568	–	int probes = ~mask;      // use random index while negative
569	–	for (;;) {
570	–	r = xorShift(r);     // update random seed
571	–	ForkJoinWorkerThread v = ws[mask & idx];
572	–	if (v == null || v.sp == v.base) {
573	–	if (probes <= mask)
574	–	idx = (probes++ < 0)? r : (idx + 1);
575	–	else
576	–	break;
577	–	}
578	–	else if (!tryActivate() || (t = v.deqTask()) == null)
579	–	continue outer;  // restart on contention
580	–	else
581	–	break outer;
582	–	}
583	–	}
584	–	} while (pool.hasNewSyncEvent(this)); // retry on pool events
585	–	seed = r;
586	–	return t;
587	–	}
588	–
589	–	/**
590	–	* gets and removes a local or stolen a task
591	–	* @return a task, if available
592	–	*/
593	–	final ForkJoinTask<?> pollTask() {
594	–	ForkJoinTask<?> t = locallyFifo? deqTask() : popTask();
595	–	if (t == null && (t = scan()) != null)
596	–	++stealCount;
597	–	return t;
598	–	}
599	–
600	–	/**
601	–	* gets a local task
602	–	* @return a task, if available
603	–	*/
604	–	final ForkJoinTask<?> pollLocalTask() {
605	–	return locallyFifo? deqTask() : popTask();
606	–	}
607	–
608	–	/**
609	–	* Returns a pool submission, if one exists, activating first.
610	–	* @return a submission, if available
611	–	*/
612	–	private ForkJoinTask<?> pollSubmission() {
613	–	ForkJoinPool p = pool;
614	–	while (p.hasQueuedSubmissions()) {
615	–	ForkJoinTask<?> t;
616	–	if (tryActivate() && (t = p.pollSubmission()) != null)
617	–	return t;
618	–	}
619	–	return null;
620	–	}
621	–
622	–	// Methods accessed only by Pool
623	–
624	–	/**
625	–	* Removes and cancels all tasks in queue.  Can be called from any
626	–	* thread.
627	–	*/
628	–	final void cancelTasks() {
629	–	ForkJoinTask<?> t;
630	–	while (base != sp && (t = deqTask()) != null)
631	–	t.cancelIgnoringExceptions();
632	–	}
633	–
634	–	/**
635	–	* Drains tasks to given collection c
636	–	* @return the number of tasks drained
637	–	*/
638	–	final int drainTasksTo(Collection<ForkJoinTask<?>> c) {
639	–	int n = 0;
640	–	ForkJoinTask<?> t;
641	–	while (base != sp && (t = deqTask()) != null) {
642	–	c.add(t);
643	–	++n;
644	–	}
645	–	return n;
646	–	}
647	–
648	–	/**
649	–	* Get and clear steal count for accumulation by pool.  Called
650	–	* only when known to be idle (in pool.sync and termination).
651	–	*/
652	–	final int getAndClearStealCount() {
653	–	int sc = stealCount;
654	–	stealCount = 0;
655	–	return sc;
656	–	}
657	–
658	–	/**
659	–	* Returns true if at least one worker in the given array appears
660	–	* to have at least one queued task.
661	–	* @param ws array of workers
662	–	*/
663	–	static boolean hasQueuedTasks(ForkJoinWorkerThread[] ws) {
664	–	if (ws != null) {
665	–	int len = ws.length;
666	–	for (int j = 0; j < 2; ++j) { // need two passes for clean sweep
667	–	for (int i = 0; i < len; ++i) {
668	–	ForkJoinWorkerThread w = ws[i];
669	–	if (w != null && w.sp != w.base)
670	–	return true;
671	–	}
672	–	}
673	–	}
674	–	return false;
675	–	}
676	–
677	–	// Support methods for ForkJoinTask
678	–
679	–	/**
680	–	* Returns an estimate of the number of tasks in the queue.
681	–	*/
682	–	final int getQueueSize() {
683	–	int n = sp - base;
684	–	return n < 0? 0 : n; // suppress momentarily negative values
685	–	}
686	–
687	–	/**
688	–	* Returns an estimate of the number of tasks, offset by a
689	–	* function of number of idle workers.
690	–	*/
691	–	final int getEstimatedSurplusTaskCount() {
692	–	// The halving approximates weighting idle vs non-idle workers
693	–	return (sp - base) - (pool.getIdleThreadCount() >>> 1);
694	–	}
695	–
696	–	/**
697	–	* Scan, returning early if joinMe done
698	–	*/
699	–	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
700	–	ForkJoinTask<?> t = pollTask();
701	–	if (t != null && joinMe.status < 0 && sp == base) {
702	–	pushTask(t); // unsteal if done and this task would be stealable
703	–	t = null;
704	–	}
705	–	return t;
112		}
113
114		/**
115	<	* Runs tasks until pool isQuiescent
115	>	* This method is required to be public, but should never be
116	>	* called explicitly. It performs the main run loop to execute
117	>	* {@link ForkJoinTask}s.
118		*/
119	<	final void helpQuiescePool() {
120	<	for (;;) {
713	<	ForkJoinTask<?> t = pollTask();
714	<	if (t != null)
715	<	t.quietlyExec();
716	<	else if (tryInactivate() && pool.isQuiescent())
717	<	break;
718	<	}
719	<	do;while (!tryActivate()); // re-activate on exit
720	<	}
721	<
722	<	// Temporary Unsafe mechanics for preliminary release
723	<	private static Unsafe getUnsafe() throws Throwable {
119	>	public void run() {
120	>	Throwable exception = null;
121		try {
122	<	return Unsafe.getUnsafe();
123	<	} catch (SecurityException se) {
122	>	pool.registerWorker(this);
123	>	onStart();
124	>	pool.runWorker(workQueue);
125	>	} catch (Throwable ex) {
126	>	exception = ex;
127	>	} finally {
128		try {
129	<	return java.security.AccessController.doPrivileged
130	<	(new java.security.PrivilegedExceptionAction<Unsafe>() {
131	<	public Unsafe run() throws Exception {
132	<	return getUnsafePrivileged();
133	<	}});
134	<	} catch (java.security.PrivilegedActionException e) {
734	<	throw e.getCause();
129	>	onTermination(exception);
130	>	} catch (Throwable ex) {
131	>	if (exception == null)
132	>	exception = ex;
133	>	} finally {
134	>	pool.deregisterWorker(this, exception);
135		}
136		}
137		}
738	–
739	–	private static Unsafe getUnsafePrivileged()
740	–	throws NoSuchFieldException, IllegalAccessException {
741	–	Field f = Unsafe.class.getDeclaredField("theUnsafe");
742	–	f.setAccessible(true);
743	–	return (Unsafe) f.get(null);
744	–	}
745	–
746	–	private static long fieldOffset(String fieldName)
747	–	throws NoSuchFieldException {
748	–	return _unsafe.objectFieldOffset
749	–	(ForkJoinWorkerThread.class.getDeclaredField(fieldName));
750	–	}
751	–
752	–	static final Unsafe _unsafe;
753	–	static final long baseOffset;
754	–	static final long spOffset;
755	–	static final long runStateOffset;
756	–	static final long qBase;
757	–	static final int qShift;
758	–	static {
759	–	try {
760	–	_unsafe = getUnsafe();
761	–	baseOffset = fieldOffset("base");
762	–	spOffset = fieldOffset("sp");
763	–	runStateOffset = fieldOffset("runState");
764	–	qBase = _unsafe.arrayBaseOffset(ForkJoinTask[].class);
765	–	int s = _unsafe.arrayIndexScale(ForkJoinTask[].class);
766	–	if ((s & (s-1)) != 0)
767	–	throw new Error("data type scale not a power of two");
768	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
769	–	} catch (Throwable e) {
770	–	throw new RuntimeException("Could not initialize intrinsics", e);
771	–	}
772	–	}
138		}

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