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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.6 by jsr166, Thu Mar 19 05:10:42 2009 UTC vs.
Revision 1.71 by dl, Wed Nov 14 17:20:38 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		/**
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
13	<	* execution. However, you can override initialization and termination
14	<	* cleanup methods surrounding the main task processing loop.  If you
15	<	* do create such a subclass, you will also need to supply a custom
16	<	* ForkJoinWorkerThreadFactory to use it in a 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
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 nonnull
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 probablistic 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 effciently 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.)
114	<	*
115	<	* 3. Selection control. We maintain policy of always choosing to
116	<	* run local tasks rather than stealing, and always trying to
117	<	* steal tasks before trying to run a new submission. All steals
118	<	* are currently performed in randomly-chosen deq-order. It may be
119	<	* worthwhile to bias these with locality / anti-locality
120	<	* information, but doing this well probably requires more
121	<	* lower-level information from JVMs than currently provided.
25	>	* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28		*/
29
30	<	/**
31	<	* 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;
30	>	final ForkJoinPool.WorkQueue workQueue; // Work-stealing mechanics
31	>	final ForkJoinPool pool;                // the pool this thread works in
32
33		/**
34	<	* Maximum work-stealing queue array size.  Must be less than or
35	<	* equal to 1 << 28 to ensure lack of index wraparound. (This
36	<	* is less than usual bounds, because we need leftshift by 3
135	<	* to be in int range).
34	>	* An initial name for a newly constructed worker, used until
35	>	* onStart can establish a useful name. This removes need to
36	>	* establish a name from worker startup path.
37		*/
38	<	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.
175	<	*/
176	<	private volatile int runState;
177	<
178	<	/**
179	<	* 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;
38	>	static final String provisionalName = "aForkJoinWorkerThread";
39
40		/**
41		* Creates a ForkJoinWorkerThread operating in the given pool.
42	+	*
43		* @param pool the pool this thread works in
44		* @throws NullPointerException if pool is null
45		*/
46		protected ForkJoinWorkerThread(ForkJoinPool pool) {
47	<	if (pool == null) throw new NullPointerException();
47	>	super(provisionalName); // bootstrap name
48	>	Thread.UncaughtExceptionHandler ueh = pool.ueh;
49	>	if (ueh != null)
50	>	setUncaughtExceptionHandler(ueh);
51	>	setDaemon(true);
52		this.pool = pool;
53	<	// Note: poolIndex is set by pool during construction
54	<	// Remaining initialization is deferred to onStart
53	>	pool.registerWorker(this.workQueue = new ForkJoinPool.WorkQueue
54	>	(pool, this, pool.localMode));
55		}
56
213	–	// Public access methods
214	–
57		/**
58	<	* Returns the pool hosting this thread
58	>	* Returns the pool hosting this thread.
59	>	*
60		* @return the pool
61		*/
62		public ForkJoinPool getPool() {
#	Line 226 | Line 69 | public class ForkJoinWorkerThread extend
69		* threads (minus one) that have ever been created in the pool.
70		* This method may be useful for applications that track status or
71		* collect results per-worker rather than per-task.
72	<	* @return the index number.
72	>	*
73	>	* @return the index number
74		*/
75		public int getPoolIndex() {
76	<	return poolIndex;
233	<	}
234	<
235	<
236	<	// Runstate management
237	<
238	<	// Runstate values. Order matters
239	<	private static final int RUNNING     = 0;
240	<	private static final int SHUTDOWN    = 1;
241	<	private static final int TERMINATING = 2;
242	<	private static final int TERMINATED  = 3;
243	<
244	<	final boolean isShutdown()    { return runState >= SHUTDOWN;  }
245	<	final boolean isTerminating() { return runState >= TERMINATING;  }
246	<	final boolean isTerminated()  { return runState == TERMINATED; }
247	<	final boolean shutdown()      { return transitionRunStateTo(SHUTDOWN); }
248	<	final boolean shutdownNow()   { return transitionRunStateTo(TERMINATING); }
249	<
250	<	/**
251	<	* Transition to at least the given state. Return true if not
252	<	* already at least given state.
253	<	*/
254	<	private boolean transitionRunStateTo(int state) {
255	<	for (;;) {
256	<	int s = runState;
257	<	if (s >= state)
258	<	return false;
259	<	if (_unsafe.compareAndSwapInt(this, runStateOffset, s, state))
260	<	return true;
261	<	}
262	<	}
263	<
264	<	/**
265	<	* Try to set status to active; fail on contention
266	<	*/
267	<	private boolean tryActivate() {
268	<	if (!active) {
269	<	if (!pool.tryIncrementActiveCount())
270	<	return false;
271	<	active = true;
272	<	}
273	<	return true;
274	<	}
275	<
276	<	/**
277	<	* Try to set status to active; fail on contention
278	<	*/
279	<	private boolean tryInactivate() {
280	<	if (active) {
281	<	if (!pool.tryDecrementActiveCount())
282	<	return false;
283	<	active = false;
284	<	}
285	<	return true;
286	<	}
287	<
288	<	/**
289	<	* Computes next value for random victim probe. Scans don't
290	<	* require a very high quality generator, but also not a crummy
291	<	* one. Marsaglia xor-shift is cheap and works well.
292	<	*/
293	<	private static int xorShift(int r) {
294	<	r ^= r << 1;
295	<	r ^= r >>> 3;
296	<	r ^= r << 10;
297	<	return r;
298	<	}
299	<
300	<	// Lifecycle methods
301	<
302	<	/**
303	<	* This method is required to be public, but should never be
304	<	* called explicitly. It performs the main run loop to execute
305	<	* ForkJoinTasks.
306	<	*/
307	<	public void run() {
308	<	Throwable exception = null;
309	<	try {
310	<	onStart();
311	<	pool.sync(this); // await first pool event
312	<	mainLoop();
313	<	} catch (Throwable ex) {
314	<	exception = ex;
315	<	} finally {
316	<	onTermination(exception);
317	<	}
318	<	}
319	<
320	<	/**
321	<	* Execute tasks until shut down.
322	<	*/
323	<	private void mainLoop() {
324	<	while (!isShutdown()) {
325	<	ForkJoinTask<?> t = pollTask();
326	<	if (t != null || (t = pollSubmission()) != null)
327	<	t.quietlyExec();
328	<	else if (tryInactivate())
329	<	pool.sync(this);
330	<	}
76	>	return workQueue.poolIndex;
77		}
78
79		/**
80		* Initializes internal state after construction but before
81		* processing any tasks. If you override this method, you must
82	<	* invoke super.onStart() at the beginning of the method.
82	>	* invoke {@code super.onStart()} at the beginning of the method.
83		* Initialization requires care: Most fields must have legal
84		* default values, to ensure that attempted accesses from other
85		* threads work correctly even before this thread starts
86		* processing tasks.
87		*/
88		protected void onStart() {
89	<	// Allocate while starting to improve chances of thread-local
90	<	// isolation
91	<	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
92	<	// Initial value of seed need not be especially random but
347	<	// should differ across workers and must be nonzero
348	<	int p = poolIndex + 1;
349	<	seed = p + (p << 8) + (p << 16) + (p << 24); // spread bits
89	>	String pref; // replace bootstrap name
90	>	if (provisionalName.equals(getName()) &&
91	>	(pref = pool.workerNamePrefix) != null)
92	>	setName(pref.concat(Long.toString(getId())));
93		}
94
95		/**
96	<	* Perform cleanup associated with termination of this worker
96	>	* Performs cleanup associated with termination of this worker
97		* thread.  If you override this method, you must invoke
98	<	* super.onTermination at the end of the overridden method.
98	>	* {@code super.onTermination} at the end of the overridden method.
99		*
100		* @param exception the exception causing this thread to abort due
101	<	* to an unrecoverable error, or null if completed normally.
101	>	* to an unrecoverable error, or {@code null} if completed normally
102		*/
103		protected void onTermination(Throwable exception) {
361	–	// Execute remaining local tasks unless aborting or terminating
362	–	while (exception == null &&  !pool.isTerminating() && base != sp) {
363	–	try {
364	–	ForkJoinTask<?> t = popTask();
365	–	if (t != null)
366	–	t.quietlyExec();
367	–	} catch(Throwable ex) {
368	–	exception = ex;
369	–	}
370	–	}
371	–	// Cancel other tasks, transition status, notify pool, and
372	–	// propagate exception to uncaught exception handler
373	–	try {
374	–	do;while (!tryInactivate()); // ensure inactive
375	–	cancelTasks();
376	–	runState = TERMINATED;
377	–	pool.workerTerminated(this);
378	–	} catch (Throwable ex) {        // Shouldn't ever happen
379	–	if (exception == null)      // but if so, at least rethrown
380	–	exception = ex;
381	–	} finally {
382	–	if (exception != null)
383	–	ForkJoinTask.rethrowException(exception);
384	–	}
385	–	}
386	–
387	–	// Intrinsics-based support for queue operations.
388	–
389	–	/**
390	–	* Add in store-order the given task at given slot of q to
391	–	* null. Caller must ensure q is nonnull and index is in range.
392	–	*/
393	–	private static void setSlot(ForkJoinTask<?>[] q, int i,
394	–	ForkJoinTask<?> t){
395	–	_unsafe.putOrderedObject(q, (i << qShift) + qBase, t);
396	–	}
397	–
398	–	/**
399	–	* CAS given slot of q to null. Caller must ensure q is nonnull
400	–	* and index is in range.
401	–	*/
402	–	private static boolean casSlotNull(ForkJoinTask<?>[] q, int i,
403	–	ForkJoinTask<?> t) {
404	–	return _unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
104		}
105
106		/**
107	<	* Sets sp in store-order.
108	<	*/
109	<	private void storeSp(int s) {
411	<	_unsafe.putOrderedInt(this, spOffset, s);
412	<	}
413	<
414	<	// Main queue methods
415	<
416	<	/**
417	<	* Pushes a task. Called only by current thread.
418	<	* @param t the task. Caller must ensure nonnull
419	<	*/
420	<	final void pushTask(ForkJoinTask<?> t) {
421	<	ForkJoinTask<?>[] q = queue;
422	<	int mask = q.length - 1;
423	<	int s = sp;
424	<	setSlot(q, s & mask, t);
425	<	storeSp(++s);
426	<	if ((s -= base) == 1)
427	<	pool.signalWork();
428	<	else if (s >= mask)
429	<	growQueue();
430	<	}
431	<
432	<	/**
433	<	* Tries to take a task from the base of the queue, failing if
434	<	* either empty or contended.
435	<	* @return a task, or null if none or contended.
436	<	*/
437	<	private ForkJoinTask<?> deqTask() {
438	<	ForkJoinTask<?> t;
439	<	ForkJoinTask<?>[] q;
440	<	int i;
441	<	int b;
442	<	if (sp != (b = base) &&
443	<	(q = queue) != null && // must read q after b
444	<	(t = q[i = (q.length - 1) & b]) != null &&
445	<	casSlotNull(q, i, t)) {
446	<	base = b + 1;
447	<	return t;
448	<	}
449	<	return null;
450	<	}
451	<
452	<	/**
453	<	* Returns a popped task, or null if empty. Ensures active status
454	<	* if nonnull. Called only by current thread.
455	<	*/
456	<	final ForkJoinTask<?> popTask() {
457	<	int s = sp;
458	<	while (s != base) {
459	<	if (tryActivate()) {
460	<	ForkJoinTask<?>[] q = queue;
461	<	int mask = q.length - 1;
462	<	int i = (s - 1) & mask;
463	<	ForkJoinTask<?> t = q[i];
464	<	if (t == null || !casSlotNull(q, i, t))
465	<	break;
466	<	storeSp(s - 1);
467	<	return t;
468	<	}
469	<	}
470	<	return null;
471	<	}
472	<
473	<	/**
474	<	* Specialized version of popTask to pop only if
475	<	* topmost element is the given task. Called only
476	<	* by current thread while active.
477	<	* @param t the task. Caller must ensure nonnull
478	<	*/
479	<	final boolean unpushTask(ForkJoinTask<?> t) {
480	<	ForkJoinTask<?>[] q = queue;
481	<	int mask = q.length - 1;
482	<	int s = sp - 1;
483	<	if (casSlotNull(q, s & mask, t)) {
484	<	storeSp(s);
485	<	return true;
486	<	}
487	<	return false;
488	<	}
489	<
490	<	/**
491	<	* Returns next task to pop.
492	<	*/
493	<	final ForkJoinTask<?> peekTask() {
494	<	ForkJoinTask<?>[] q = queue;
495	<	return q == null? null : q[(sp - 1) & (q.length - 1)];
496	<	}
497	<
498	<	/**
499	<	* Doubles queue array size. Transfers elements by emulating
500	<	* steals (deqs) from old array and placing, oldest first, into
501	<	* new array.
502	<	*/
503	<	private void growQueue() {
504	<	ForkJoinTask<?>[] oldQ = queue;
505	<	int oldSize = oldQ.length;
506	<	int newSize = oldSize << 1;
507	<	if (newSize > MAXIMUM_QUEUE_CAPACITY)
508	<	throw new RejectedExecutionException("Queue capacity exceeded");
509	<	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
510	<
511	<	int b = base;
512	<	int bf = b + oldSize;
513	<	int oldMask = oldSize - 1;
514	<	int newMask = newSize - 1;
515	<	do {
516	<	int oldIndex = b & oldMask;
517	<	ForkJoinTask<?> t = oldQ[oldIndex];
518	<	if (t != null && !casSlotNull(oldQ, oldIndex, t))
519	<	t = null;
520	<	setSlot(newQ, b & newMask, t);
521	<	} while (++b != bf);
522	<	pool.signalWork();
523	<	}
524	<
525	<	/**
526	<	* Tries to steal a task from another worker. Starts at a random
527	<	* index of workers array, and probes workers until finding one
528	<	* with non-empty queue or finding that all are empty.  It
529	<	* randomly selects the first n probes. If these are empty, it
530	<	* resorts to a full circular traversal, which is necessary to
531	<	* accurately set active status by caller. Also restarts if pool
532	<	* events occurred since last scan, which forces refresh of
533	<	* workers array, in case barrier was associated with resize.
534	<	*
535	<	* This method must be both fast and quiet -- usually avoiding
536	<	* memory accesses that could disrupt cache sharing etc other than
537	<	* those needed to check for and take tasks. This accounts for,
538	<	* among other things, updating random seed in place without
539	<	* storing it until exit.
540	<	*
541	<	* @return a task, or null if none found
542	<	*/
543	<	private ForkJoinTask<?> scan() {
544	<	ForkJoinTask<?> t = null;
545	<	int r = seed;                    // extract once to keep scan quiet
546	<	ForkJoinWorkerThread[] ws;       // refreshed on outer loop
547	<	int mask;                        // must be power 2 minus 1 and > 0
548	<	outer:do {
549	<	if ((ws = pool.workers) != null && (mask = ws.length - 1) > 0) {
550	<	int idx = r;
551	<	int probes = ~mask;      // use random index while negative
552	<	for (;;) {
553	<	r = xorShift(r);     // update random seed
554	<	ForkJoinWorkerThread v = ws[mask & idx];
555	<	if (v == null || v.sp == v.base) {
556	<	if (probes <= mask)
557	<	idx = (probes++ < 0)? r : (idx + 1);
558	<	else
559	<	break;
560	<	}
561	<	else if (!tryActivate() || (t = v.deqTask()) == null)
562	<	continue outer;  // restart on contention
563	<	else
564	<	break outer;
565	<	}
566	<	}
567	<	} while (pool.hasNewSyncEvent(this)); // retry on pool events
568	<	seed = r;
569	<	return t;
570	<	}
571	<
572	<	/**
573	<	* Pops or steals a task
574	<	* @return a task, if available
575	<	*/
576	<	final ForkJoinTask<?> pollTask() {
577	<	ForkJoinTask<?> t = popTask();
578	<	if (t == null && (t = scan()) != null)
579	<	++stealCount;
580	<	return t;
581	<	}
582	<
583	<	/**
584	<	* Returns a pool submission, if one exists, activating first.
585	<	* @return a submission, if available
586	<	*/
587	<	private ForkJoinTask<?> pollSubmission() {
588	<	ForkJoinPool p = pool;
589	<	while (p.hasQueuedSubmissions()) {
590	<	ForkJoinTask<?> t;
591	<	if (tryActivate() && (t = p.pollSubmission()) != null)
592	<	return t;
593	<	}
594	<	return null;
595	<	}
596	<
597	<	// Methods accessed only by Pool
598	<
599	<	/**
600	<	* Removes and cancels all tasks in queue.  Can be called from any
601	<	* thread.
602	<	*/
603	<	final void cancelTasks() {
604	<	ForkJoinTask<?> t;
605	<	while (base != sp && (t = deqTask()) != null)
606	<	t.cancelIgnoringExceptions();
607	<	}
608	<
609	<	/**
610	<	* Get and clear steal count for accumulation by pool.  Called
611	<	* only when known to be idle (in pool.sync and termination).
612	<	*/
613	<	final int getAndClearStealCount() {
614	<	int sc = stealCount;
615	<	stealCount = 0;
616	<	return sc;
617	<	}
618	<
619	<	/**
620	<	* Returns true if at least one worker in the given array appears
621	<	* to have at least one queued task.
622	<	* @param ws array of workers
623	<	*/
624	<	static boolean hasQueuedTasks(ForkJoinWorkerThread[] ws) {
625	<	if (ws != null) {
626	<	int len = ws.length;
627	<	for (int j = 0; j < 2; ++j) { // need two passes for clean sweep
628	<	for (int i = 0; i < len; ++i) {
629	<	ForkJoinWorkerThread w = ws[i];
630	<	if (w != null && w.sp != w.base)
631	<	return true;
632	<	}
633	<	}
634	<	}
635	<	return false;
636	<	}
637	<
638	<	// Support methods for ForkJoinTask
639	<
640	<	/**
641	<	* Returns an estimate of the number of tasks in the queue.
642	<	*/
643	<	final int getQueueSize() {
644	<	int n = sp - base;
645	<	return n < 0? 0 : n; // suppress momentarily negative values
646	<	}
647	<
648	<	/**
649	<	* Returns an estimate of the number of tasks, offset by a
650	<	* function of number of idle workers.
651	<	*/
652	<	final int getEstimatedSurplusTaskCount() {
653	<	// The halving approximates weighting idle vs non-idle workers
654	<	return (sp - base) - (pool.getIdleThreadCount() >>> 1);
655	<	}
656	<
657	<	/**
658	<	* Scan, returning early if joinMe done
659	<	*/
660	<	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
661	<	ForkJoinTask<?> t = pollTask();
662	<	if (t != null && joinMe.status < 0 && sp == base) {
663	<	pushTask(t); // unsteal if done and this task would be stealable
664	<	t = null;
665	<	}
666	<	return t;
667	<	}
668	<
669	<	/**
670	<	* Runs tasks until pool isQuiescent
107	>	* This method is required to be public, but should never be
108	>	* called explicitly. It performs the main run loop to execute
109	>	* {@link ForkJoinTask}s.
110		*/
111	<	final void helpQuiescePool() {
112	<	for (;;) {
674	<	ForkJoinTask<?> t = pollTask();
675	<	if (t != null)
676	<	t.quietlyExec();
677	<	else if (tryInactivate() && pool.isQuiescent())
678	<	break;
679	<	}
680	<	do;while (!tryActivate()); // re-activate on exit
681	<	}
682	<
683	<	// Temporary Unsafe mechanics for preliminary release
684	<	private static Unsafe getUnsafe() throws Throwable {
111	>	public void run() {
112	>	Throwable exception = null;
113		try {
114	<	return Unsafe.getUnsafe();
115	<	} catch (SecurityException se) {
114	>	onStart();
115	>	pool.runWorker(workQueue);
116	>	} catch (Throwable ex) {
117	>	exception = ex;
118	>	} finally {
119		try {
120	<	return java.security.AccessController.doPrivileged
121	<	(new java.security.PrivilegedExceptionAction<Unsafe>() {
122	<	public Unsafe run() throws Exception {
123	<	return getUnsafePrivileged();
124	<	}});
125	<	} catch (java.security.PrivilegedActionException e) {
695	<	throw e.getCause();
120	>	onTermination(exception);
121	>	} catch (Throwable ex) {
122	>	if (exception == null)
123	>	exception = ex;
124	>	} finally {
125	>	pool.deregisterWorker(this, exception);
126		}
127		}
128		}
699	–
700	–	private static Unsafe getUnsafePrivileged()
701	–	throws NoSuchFieldException, IllegalAccessException {
702	–	Field f = Unsafe.class.getDeclaredField("theUnsafe");
703	–	f.setAccessible(true);
704	–	return (Unsafe) f.get(null);
705	–	}
706	–
707	–	private static long fieldOffset(String fieldName)
708	–	throws NoSuchFieldException {
709	–	return _unsafe.objectFieldOffset
710	–	(ForkJoinWorkerThread.class.getDeclaredField(fieldName));
711	–	}
712	–
713	–	static final Unsafe _unsafe;
714	–	static final long baseOffset;
715	–	static final long spOffset;
716	–	static final long runStateOffset;
717	–	static final long qBase;
718	–	static final int qShift;
719	–	static {
720	–	try {
721	–	_unsafe = getUnsafe();
722	–	baseOffset = fieldOffset("base");
723	–	spOffset = fieldOffset("sp");
724	–	runStateOffset = fieldOffset("runState");
725	–	qBase = _unsafe.arrayBaseOffset(ForkJoinTask[].class);
726	–	int s = _unsafe.arrayIndexScale(ForkJoinTask[].class);
727	–	if ((s & (s-1)) != 0)
728	–	throw new Error("data type scale not a power of two");
729	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
730	–	} catch (Throwable e) {
731	–	throw new RuntimeException("Could not initialize intrinsics", e);
732	–	}
733	–	}
129		}

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