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root/jsr166/jsr166/src/jsr166y/ForkJoinWorkerThread.java

Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.16 by jsr166, Thu Jul 23 23:07:57 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	<	* methods surrounding the main task processing loop.  If you do
15	<	* 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
32	<	* Deques that support only three of the four possible
33	<	* end-operations -- push, pop, and deq (aka steal), and only do
34	<	* so under the constraints that push and pop are called only from
35	<	* the owning thread, while deq may be called from other threads.
36	<	* (If you are unfamiliar with them, you probably want to read
37	<	* Herlihy and Shavit's book "The Art of Multiprocessor
38	<	* programming", chapter 16 describing these in more detail before
39	<	* proceeding.)  The main work-stealing queue design is roughly
40	<	* similar to "Dynamic Circular Work-Stealing Deque" by David
41	<	* Chase and Yossi Lev, SPAA 2005
42	<	* (http://research.sun.com/scalable/pubs/index.html).  The main
43	<	* difference ultimately stems from gc requirements that we null
44	<	* out taken slots as soon as we can, to maintain as small a
45	<	* footprint as possible even in programs generating huge numbers
46	<	* of tasks. To accomplish this, we shift the CAS arbitrating pop
47	<	* vs deq (steal) from being on the indices ("base" and "sp") to
48	<	* the slots themselves (mainly via method "casSlotNull()"). So,
49	<	* both a successful pop and deq mainly entail CAS'ing a non-null
50	<	* slot to null.  Because we rely on CASes of references, we do
51	<	* not need tag bits on base or sp.  They are simple ints as used
52	<	* in any circular array-based queue (see for example ArrayDeque).
53	<	* Updates to the indices must still be ordered in a way that
54	<	* guarantees that (sp - base) > 0 means the queue is empty, but
55	<	* otherwise may err on the side of possibly making the queue
56	<	* appear nonempty when a push, pop, or deq have not fully
57	<	* committed. Note that this means that the deq operation,
58	<	* considered individually, is not wait-free. One thief cannot
59	<	* successfully continue until another in-progress one (or, if
60	<	* previously empty, a push) completes.  However, in the
61	<	* aggregate, we ensure at least probabilistic non-blockingness. If
62	<	* an attempted steal fails, a thief always chooses a different
63	<	* random victim target to try next. So, in order for one thief to
64	<	* progress, it suffices for any in-progress deq or new push on
65	<	* any empty queue to complete. One reason this works well here is
66	<	* that apparently-nonempty often means soon-to-be-stealable,
67	<	* which gives threads a chance to activate if necessary before
68	<	* stealing (see below).
69	<	*
70	<	* Efficient implementation of this approach currently relies on
71	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
72	<	* correct orderings, reads and writes of variable base require
73	<	* volatile ordering.  Variable sp does not require volatile write
74	<	* but needs cheaper store-ordering on writes.  Because they are
75	<	* protected by volatile base reads, reads of the queue array and
76	<	* its slots do not need volatile load semantics, but writes (in
77	<	* push) require store order and CASes (in pop and deq) require
78	<	* (volatile) CAS semantics. Since these combinations aren't
79	<	* supported using ordinary volatiles, the only way to accomplish
80	<	* these efficiently is to use direct Unsafe calls. (Using external
81	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
82	<	* array is significantly slower because of memory locality and
83	<	* indirection effects.) Further, performance on most platforms is
84	<	* very sensitive to placement and sizing of the (resizable) queue
85	<	* array.  Even though these queues don't usually become all that
86	<	* big, the initial size must be large enough to counteract cache
87	<	* contention effects across multiple queues (especially in the
88	<	* presence of GC cardmarking). Also, to improve thread-locality,
89	<	* queues are currently initialized immediately after the thread
90	<	* gets the initial signal to start processing tasks.  However,
91	<	* all queue-related methods except pushTask are written in a way
92	<	* that allows them to instead be lazily allocated and/or disposed
93	<	* of when empty. All together, these low-level implementation
94	<	* choices produce as much as a factor of 4 performance
95	<	* improvement compared to naive implementations, and enable the
96	<	* processing of billions of tasks per second, sometimes at the
97	<	* expense of ugliness.
98	<	*
99	<	* 2. Run control: The primary run control is based on a global
100	<	* counter (activeCount) held by the pool. It uses an algorithm
101	<	* similar to that in Herlihy and Shavit section 17.6 to cause
102	<	* threads to eventually block when all threads declare they are
103	<	* inactive. (See variable "scans".)  For this to work, threads
104	<	* must be declared active when executing tasks, and before
105	<	* stealing a task. They must be inactive before blocking on the
106	<	* Pool Barrier (awaiting a new submission or other Pool
107	<	* event). In between, there is some free play which we take
108	<	* advantage of to avoid contention and rapid flickering of the
109	<	* global activeCount: If inactive, we activate only if a victim
110	<	* queue appears to be nonempty (see above).  Similarly, a thread
111	<	* tries to inactivate only after a full scan of other threads.
112	<	* The net effect is that contention on activeCount is rarely a
113	<	* measurable performance issue. (There are also a few other cases
114	<	* where we scan for work rather than retry/block upon
115	<	* contention.)
116	<	*
117	<	* 3. Selection control. We maintain policy of always choosing to
118	<	* run local tasks rather than stealing, and always trying to
119	<	* steal tasks before trying to run a new submission. All steals
120	<	* are currently performed in randomly-chosen deq-order. It may be
121	<	* worthwhile to bias these with locality / anti-locality
122	<	* information, but doing this well probably requires more
123	<	* lower-level information from JVMs than currently provided.
124	<	*/
125	<
126	<	/**
127	<	* Capacity of work-stealing queue array upon initialization.
128	<	* Must be a power of two. Initial size must be at least 2, but is
129	<	* padded to minimize cache effects.
130	<	*/
131	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
132	<
133	<	/**
134	<	* Maximum work-stealing queue array size.  Must be less than or
135	<	* equal to 1 << 28 to ensure lack of index wraparound. (This
136	<	* is less than usual bounds, because we need leftshift by 3
137	<	* to be in int range).
138	<	*/
139	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28;
140	<
141	<	/**
142	<	* The pool this thread works in. Accessed directly by ForkJoinTask.
143	<	*/
144	<	final ForkJoinPool pool;
145	<
146	<	/**
147	<	* The work-stealing queue array. Size must be a power of two.
148	<	* Initialized when thread starts, to improve memory locality.
149	<	*/
150	<	private ForkJoinTask<?>[] queue;
151	<
152	<	/**
153	<	* Index (mod queue.length) of next queue slot to push to or pop
154	<	* from. It is written only by owner thread, via ordered store.
155	<	* Both sp and base are allowed to wrap around on overflow, but
156	<	* (sp - base) still estimates size.
157	<	*/
158	<	private volatile int sp;
159	<
160	<	/**
161	<	* Index (mod queue.length) of least valid queue slot, which is
162	<	* always the next position to steal from if nonempty.
25	>	* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28		*/
164	–	private volatile int base;
29
30	<	/**
31	<	* Activity status. When true, this worker is considered active.
168	<	* Must be false upon construction. It must be true when executing
169	<	* tasks, and BEFORE stealing a task. It must be false before
170	<	* calling pool.sync.
171	<	*/
172	<	private boolean active;
30	>	final ForkJoinPool.WorkQueue workQueue; // Work-stealing mechanics
31	>	final ForkJoinPool pool;                // the pool this thread works in
32
33		/**
34	<	* Run state of this worker. Supports simple versions of the usual
35	<	* shutdown/shutdownNow control.
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 volatile int runState;
179	<
180	<	/**
181	<	* Seed for random number generator for choosing steal victims.
182	<	* Uses Marsaglia xorshift. Must be nonzero upon initialization.
183	<	*/
184	<	private int seed;
185	<
186	<	/**
187	<	* Number of steals, transferred to pool when idle
188	<	*/
189	<	private int stealCount;
190	<
191	<	/**
192	<	* Index of this worker in pool array. Set once by pool before
193	<	* running, and accessed directly by pool during cleanup etc.
194	<	*/
195	<	int poolIndex;
196	<
197	<	/**
198	<	* The last barrier event waited for. Accessed in pool callback
199	<	* methods, but only by current thread.
200	<	*/
201	<	long lastEventCount;
202	<
203	<	/**
204	<	* True if use local fifo, not default lifo, for local polling
205	<	*/
206	<	private boolean locallyFifo;
38	>	static final String provisionalName = "aForkJoinWorkerThread";
39
40		/**
41		* Creates a ForkJoinWorkerThread operating in the given pool.
#	Line 212 | 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	>	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
221	–	// Public access methods
222	–
57		/**
58		* Returns the pool hosting this thread.
59		*
#	Line 239 | Line 73 | public class ForkJoinWorkerThread extend
73		* @return the index number
74		*/
75		public int getPoolIndex() {
76	<	return poolIndex;
243	<	}
244	<
245	<	/**
246	<	* Establishes local first-in-first-out scheduling mode for forked
247	<	* tasks that are never joined.
248	<	*
249	<	* @param async if true, use locally FIFO scheduling
250	<	*/
251	<	void setAsyncMode(boolean async) {
252	<	locallyFifo = async;
253	<	}
254	<
255	<	// Runstate management
256	<
257	<	// Runstate values. Order matters
258	<	private static final int RUNNING     = 0;
259	<	private static final int SHUTDOWN    = 1;
260	<	private static final int TERMINATING = 2;
261	<	private static final int TERMINATED  = 3;
262	<
263	<	final boolean isShutdown()    { return runState >= SHUTDOWN;  }
264	<	final boolean isTerminating() { return runState >= TERMINATING;  }
265	<	final boolean isTerminated()  { return runState == TERMINATED; }
266	<	final boolean shutdown()      { return transitionRunStateTo(SHUTDOWN); }
267	<	final boolean shutdownNow()   { return transitionRunStateTo(TERMINATING); }
268	<
269	<	/**
270	<	* Transitions to at least the given state.  Returns true if not
271	<	* already at least at given state.
272	<	*/
273	<	private boolean transitionRunStateTo(int state) {
274	<	for (;;) {
275	<	int s = runState;
276	<	if (s >= state)
277	<	return false;
278	<	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, state))
279	<	return true;
280	<	}
281	<	}
282	<
283	<	/**
284	<	* Tries to set status to active; fails on contention.
285	<	*/
286	<	private boolean tryActivate() {
287	<	if (!active) {
288	<	if (!pool.tryIncrementActiveCount())
289	<	return false;
290	<	active = true;
291	<	}
292	<	return true;
293	<	}
294	<
295	<	/**
296	<	* Tries to set status to inactive; fails on contention.
297	<	*/
298	<	private boolean tryInactivate() {
299	<	if (active) {
300	<	if (!pool.tryDecrementActiveCount())
301	<	return false;
302	<	active = false;
303	<	}
304	<	return true;
305	<	}
306	<
307	<	/**
308	<	* Computes next value for random victim probe.  Scans don't
309	<	* require a very high quality generator, but also not a crummy
310	<	* one.  Marsaglia xor-shift is cheap and works well.
311	<	*/
312	<	private static int xorShift(int r) {
313	<	r ^= r << 1;
314	<	r ^= r >>> 3;
315	<	r ^= r << 10;
316	<	return r;
317	<	}
318	<
319	<	// Lifecycle methods
320	<
321	<	/**
322	<	* This method is required to be public, but should never be
323	<	* called explicitly. It performs the main run loop to execute
324	<	* ForkJoinTasks.
325	<	*/
326	<	public void run() {
327	<	Throwable exception = null;
328	<	try {
329	<	onStart();
330	<	pool.sync(this); // await first pool event
331	<	mainLoop();
332	<	} catch (Throwable ex) {
333	<	exception = ex;
334	<	} finally {
335	<	onTermination(exception);
336	<	}
337	<	}
338	<
339	<	/**
340	<	* Executes tasks until shut down.
341	<	*/
342	<	private void mainLoop() {
343	<	while (!isShutdown()) {
344	<	ForkJoinTask<?> t = pollTask();
345	<	if (t != null || (t = pollSubmission()) != null)
346	<	t.quietlyExec();
347	<	else if (tryInactivate())
348	<	pool.sync(this);
349	<	}
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
366	<	// should differ across workers and must be nonzero
367	<	int p = poolIndex + 1;
368	<	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		/**
#	Line 374 | Line 98 | public class ForkJoinWorkerThread extend
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) {
380	–	// Execute remaining local tasks unless aborting or terminating
381	–	while (exception == null &&  !pool.isTerminating() && base != sp) {
382	–	try {
383	–	ForkJoinTask<?> t = popTask();
384	–	if (t != null)
385	–	t.quietlyExec();
386	–	} catch (Throwable ex) {
387	–	exception = ex;
388	–	}
389	–	}
390	–	// Cancel other tasks, transition status, notify pool, and
391	–	// propagate exception to uncaught exception handler
392	–	try {
393	–	do {} while (!tryInactivate()); // ensure inactive
394	–	cancelTasks();
395	–	runState = TERMINATED;
396	–	pool.workerTerminated(this);
397	–	} catch (Throwable ex) {        // Shouldn't ever happen
398	–	if (exception == null)      // but if so, at least rethrown
399	–	exception = ex;
400	–	} finally {
401	–	if (exception != null)
402	–	ForkJoinTask.rethrowException(exception);
403	–	}
104		}
105
406	–	// Intrinsics-based support for queue operations.
407	–
106		/**
107	<	* Adds in store-order the given task at given slot of q to null.
108	<	* Caller must ensure q is non-null and index is in range.
109	<	*/
412	<	private static void setSlot(ForkJoinTask<?>[] q, int i,
413	<	ForkJoinTask<?> t) {
414	<	UNSAFE.putOrderedObject(q, (i << qShift) + qBase, t);
415	<	}
416	<
417	<	/**
418	<	* CAS given slot of q to null. Caller must ensure q is non-null
419	<	* and index is in range.
420	<	*/
421	<	private static boolean casSlotNull(ForkJoinTask<?>[] q, int i,
422	<	ForkJoinTask<?> t) {
423	<	return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
424	<	}
425	<
426	<	/**
427	<	* Sets sp in store-order.
428	<	*/
429	<	private void storeSp(int s) {
430	<	UNSAFE.putOrderedInt(this, spOffset, s);
431	<	}
432	<
433	<	// Main queue methods
434	<
435	<	/**
436	<	* Pushes a task. Called only by current thread.
437	<	*
438	<	* @param t the task. Caller must ensure non-null.
439	<	*/
440	<	final void pushTask(ForkJoinTask<?> t) {
441	<	ForkJoinTask<?>[] q = queue;
442	<	int mask = q.length - 1;
443	<	int s = sp;
444	<	setSlot(q, s & mask, t);
445	<	storeSp(++s);
446	<	if ((s -= base) == 1)
447	<	pool.signalWork();
448	<	else if (s >= mask)
449	<	growQueue();
450	<	}
451	<
452	<	/**
453	<	* Tries to take a task from the base of the queue, failing if
454	<	* either empty or contended.
455	<	*
456	<	* @return a task, or null if none or contended
457	<	*/
458	<	final ForkJoinTask<?> deqTask() {
459	<	ForkJoinTask<?> t;
460	<	ForkJoinTask<?>[] q;
461	<	int i;
462	<	int b;
463	<	if (sp != (b = base) &&
464	<	(q = queue) != null && // must read q after b
465	<	(t = q[i = (q.length - 1) & b]) != null &&
466	<	casSlotNull(q, i, t)) {
467	<	base = b + 1;
468	<	return t;
469	<	}
470	<	return null;
471	<	}
472	<
473	<	/**
474	<	* Returns a popped task, or null if empty. Ensures active status
475	<	* if non-null. Called only by current thread.
476	<	*/
477	<	final ForkJoinTask<?> popTask() {
478	<	int s = sp;
479	<	while (s != base) {
480	<	if (tryActivate()) {
481	<	ForkJoinTask<?>[] q = queue;
482	<	int mask = q.length - 1;
483	<	int i = (s - 1) & mask;
484	<	ForkJoinTask<?> t = q[i];
485	<	if (t == null || !casSlotNull(q, i, t))
486	<	break;
487	<	storeSp(s - 1);
488	<	return t;
489	<	}
490	<	}
491	<	return null;
492	<	}
493	<
494	<	/**
495	<	* Specialized version of popTask to pop only if
496	<	* topmost element is the given task. Called only
497	<	* by current thread while active.
498	<	*
499	<	* @param t the task. Caller must ensure non-null.
500	<	*/
501	<	final boolean unpushTask(ForkJoinTask<?> t) {
502	<	ForkJoinTask<?>[] q = queue;
503	<	int mask = q.length - 1;
504	<	int s = sp - 1;
505	<	if (casSlotNull(q, s & mask, t)) {
506	<	storeSp(s);
507	<	return true;
508	<	}
509	<	return false;
510	<	}
511	<
512	<	/**
513	<	* Returns next task.
514	<	*/
515	<	final ForkJoinTask<?> peekTask() {
516	<	ForkJoinTask<?>[] q = queue;
517	<	if (q == null)
518	<	return null;
519	<	int mask = q.length - 1;
520	<	int i = locallyFifo ? base : (sp - 1);
521	<	return q[i & mask];
522	<	}
523	<
524	<	/**
525	<	* Doubles queue array size. Transfers elements by emulating
526	<	* steals (deqs) from old array and placing, oldest first, into
527	<	* new array.
528	<	*/
529	<	private void growQueue() {
530	<	ForkJoinTask<?>[] oldQ = queue;
531	<	int oldSize = oldQ.length;
532	<	int newSize = oldSize << 1;
533	<	if (newSize > MAXIMUM_QUEUE_CAPACITY)
534	<	throw new RejectedExecutionException("Queue capacity exceeded");
535	<	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
536	<
537	<	int b = base;
538	<	int bf = b + oldSize;
539	<	int oldMask = oldSize - 1;
540	<	int newMask = newSize - 1;
541	<	do {
542	<	int oldIndex = b & oldMask;
543	<	ForkJoinTask<?> t = oldQ[oldIndex];
544	<	if (t != null && !casSlotNull(oldQ, oldIndex, t))
545	<	t = null;
546	<	setSlot(newQ, b & newMask, t);
547	<	} while (++b != bf);
548	<	pool.signalWork();
549	<	}
550	<
551	<	/**
552	<	* Tries to steal a task from another worker. Starts at a random
553	<	* index of workers array, and probes workers until finding one
554	<	* with non-empty queue or finding that all are empty.  It
555	<	* randomly selects the first n probes. If these are empty, it
556	<	* resorts to a full circular traversal, which is necessary to
557	<	* accurately set active status by caller. Also restarts if pool
558	<	* events occurred since last scan, which forces refresh of
559	<	* workers array, in case barrier was associated with resize.
560	<	*
561	<	* This method must be both fast and quiet -- usually avoiding
562	<	* memory accesses that could disrupt cache sharing etc other than
563	<	* those needed to check for and take tasks. This accounts for,
564	<	* among other things, updating random seed in place without
565	<	* storing it until exit.
566	<	*
567	<	* @return a task, or null if none found
568	<	*/
569	<	private ForkJoinTask<?> scan() {
570	<	ForkJoinTask<?> t = null;
571	<	int r = seed;                    // extract once to keep scan quiet
572	<	ForkJoinWorkerThread[] ws;       // refreshed on outer loop
573	<	int mask;                        // must be power 2 minus 1 and > 0
574	<	outer:do {
575	<	if ((ws = pool.workers) != null && (mask = ws.length - 1) > 0) {
576	<	int idx = r;
577	<	int probes = ~mask;      // use random index while negative
578	<	for (;;) {
579	<	r = xorShift(r);     // update random seed
580	<	ForkJoinWorkerThread v = ws[mask & idx];
581	<	if (v == null || v.sp == v.base) {
582	<	if (probes <= mask)
583	<	idx = (probes++ < 0) ? r : (idx + 1);
584	<	else
585	<	break;
586	<	}
587	<	else if (!tryActivate() || (t = v.deqTask()) == null)
588	<	continue outer;  // restart on contention
589	<	else
590	<	break outer;
591	<	}
592	<	}
593	<	} while (pool.hasNewSyncEvent(this)); // retry on pool events
594	<	seed = r;
595	<	return t;
596	<	}
597	<
598	<	/**
599	<	* Gets and removes a local or stolen task.
600	<	*
601	<	* @return a task, if available
602	<	*/
603	<	final ForkJoinTask<?> pollTask() {
604	<	ForkJoinTask<?> t = locallyFifo ? deqTask() : popTask();
605	<	if (t == null && (t = scan()) != null)
606	<	++stealCount;
607	<	return t;
608	<	}
609	<
610	<	/**
611	<	* Gets a local task.
612	<	*
613	<	* @return a task, if available
614	<	*/
615	<	final ForkJoinTask<?> pollLocalTask() {
616	<	return locallyFifo ? deqTask() : popTask();
617	<	}
618	<
619	<	/**
620	<	* Returns a pool submission, if one exists, activating first.
621	<	*
622	<	* @return a submission, if available
623	<	*/
624	<	private ForkJoinTask<?> pollSubmission() {
625	<	ForkJoinPool p = pool;
626	<	while (p.hasQueuedSubmissions()) {
627	<	ForkJoinTask<?> t;
628	<	if (tryActivate() && (t = p.pollSubmission()) != null)
629	<	return t;
630	<	}
631	<	return null;
632	<	}
633	<
634	<	// Methods accessed only by Pool
635	<
636	<	/**
637	<	* Removes and cancels all tasks in queue.  Can be called from any
638	<	* thread.
639	<	*/
640	<	final void cancelTasks() {
641	<	ForkJoinTask<?> t;
642	<	while (base != sp && (t = deqTask()) != null)
643	<	t.cancelIgnoringExceptions();
644	<	}
645	<
646	<	/**
647	<	* Drains tasks to given collection c.
648	<	*
649	<	* @return the number of tasks drained
650	<	*/
651	<	final int drainTasksTo(Collection<ForkJoinTask<?>> c) {
652	<	int n = 0;
653	<	ForkJoinTask<?> t;
654	<	while (base != sp && (t = deqTask()) != null) {
655	<	c.add(t);
656	<	++n;
657	<	}
658	<	return n;
659	<	}
660	<
661	<	/**
662	<	* Gets and clears steal count for accumulation by pool.  Called
663	<	* only when known to be idle (in pool.sync and termination).
664	<	*/
665	<	final int getAndClearStealCount() {
666	<	int sc = stealCount;
667	<	stealCount = 0;
668	<	return sc;
669	<	}
670	<
671	<	/**
672	<	* Returns true if at least one worker in the given array appears
673	<	* to have at least one queued task.
674	<	*
675	<	* @param ws array of workers
676	<	*/
677	<	static boolean hasQueuedTasks(ForkJoinWorkerThread[] ws) {
678	<	if (ws != null) {
679	<	int len = ws.length;
680	<	for (int j = 0; j < 2; ++j) { // need two passes for clean sweep
681	<	for (int i = 0; i < len; ++i) {
682	<	ForkJoinWorkerThread w = ws[i];
683	<	if (w != null && w.sp != w.base)
684	<	return true;
685	<	}
686	<	}
687	<	}
688	<	return false;
689	<	}
690	<
691	<	// Support methods for ForkJoinTask
692	<
693	<	/**
694	<	* Returns an estimate of the number of tasks in the queue.
695	<	*/
696	<	final int getQueueSize() {
697	<	// suppress momentarily negative values
698	<	return Math.max(0, sp - base);
699	<	}
700	<
701	<	/**
702	<	* Returns an estimate of the number of tasks, offset by a
703	<	* function of number of idle workers.
704	<	*/
705	<	final int getEstimatedSurplusTaskCount() {
706	<	// The halving approximates weighting idle vs non-idle workers
707	<	return (sp - base) - (pool.getIdleThreadCount() >>> 1);
708	<	}
709	<
710	<	/**
711	<	* Scans, returning early if joinMe done.
712	<	*/
713	<	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
714	<	ForkJoinTask<?> t = pollTask();
715	<	if (t != null && joinMe.status < 0 && sp == base) {
716	<	pushTask(t); // unsteal if done and this task would be stealable
717	<	t = null;
718	<	}
719	<	return t;
720	<	}
721	<
722	<	/**
723	<	* Runs tasks until {@code 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 (;;) {
727	<	ForkJoinTask<?> t = pollTask();
728	<	if (t != null)
729	<	t.quietlyExec();
730	<	else if (tryInactivate() && pool.isQuiescent())
731	<	break;
732	<	}
733	<	do {} while (!tryActivate()); // re-activate on exit
734	<	}
735	<
736	<	// Temporary Unsafe mechanics for preliminary release
737	<	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) {
748	<	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		}
752	–
753	–	private static Unsafe getUnsafePrivileged()
754	–	throws NoSuchFieldException, IllegalAccessException {
755	–	Field f = Unsafe.class.getDeclaredField("theUnsafe");
756	–	f.setAccessible(true);
757	–	return (Unsafe) f.get(null);
758	–	}
759	–
760	–	private static long fieldOffset(String fieldName)
761	–	throws NoSuchFieldException {
762	–	return UNSAFE.objectFieldOffset
763	–	(ForkJoinWorkerThread.class.getDeclaredField(fieldName));
764	–	}
765	–
766	–	static final Unsafe UNSAFE;
767	–	static final long baseOffset;
768	–	static final long spOffset;
769	–	static final long runStateOffset;
770	–	static final long qBase;
771	–	static final int qShift;
772	–	static {
773	–	try {
774	–	UNSAFE = getUnsafe();
775	–	baseOffset = fieldOffset("base");
776	–	spOffset = fieldOffset("sp");
777	–	runStateOffset = fieldOffset("runState");
778	–	qBase = UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
779	–	int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
780	–	if ((s & (s-1)) != 0)
781	–	throw new Error("data type scale not a power of two");
782	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
783	–	} catch (Throwable e) {
784	–	throw new RuntimeException("Could not initialize intrinsics", e);
785	–	}
786	–	}
129		}

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