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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.31 by dl, Mon Apr 5 15:52:26 2010 UTC vs.
Revision 1.69 by dl, Mon Feb 20 18:20:06 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.Random;
12	–	import java.util.Collection;
13	–	import java.util.concurrent.locks.LockSupport;
14	–
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		/*
30	–	* Overview:
31	–	*
25		* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
26	<	* ForkJoinTasks. This class includes bookkeeping in support of
27	<	* worker activation, suspension, and lifecycle control described
35	<	* in more detail in the internal documentation of class
36	<	* ForkJoinPool. And as described further below, this class also
37	<	* includes special-cased support for some ForkJoinTask
38	<	* methods. But the main mechanics involve work-stealing:
39	<	*
40	<	* Work-stealing queues are special forms of Deques that support
41	<	* only three of the four possible end-operations -- push, pop,
42	<	* and deq (aka steal), under the further constraints that push
43	<	* and pop are called only from the owning thread, while deq may
44	<	* be called from other threads.  (If you are unfamiliar with
45	<	* them, you probably want to read Herlihy and Shavit's book "The
46	<	* Art of Multiprocessor programming", chapter 16 describing these
47	<	* in more detail before proceeding.)  The main work-stealing
48	<	* queue design is roughly similar to those in the papers "Dynamic
49	<	* Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
50	<	* (http://research.sun.com/scalable/pubs/index.html) and
51	<	* "Idempotent work stealing" by Michael, Saraswat, and Vechev,
52	<	* PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
53	<	* The main differences ultimately stem from gc requirements that
54	<	* we null out taken slots as soon as we can, to maintain as small
55	<	* a footprint as possible even in programs generating huge
56	<	* numbers of tasks. To accomplish this, we shift the CAS
57	<	* arbitrating pop vs deq (steal) from being on the indices
58	<	* ("base" and "sp") to the slots themselves (mainly via method
59	<	* "casSlotNull()"). So, both a successful pop and deq mainly
60	<	* entail a CAS of a slot from non-null to null.  Because we rely
61	<	* on CASes of references, we do not need tag bits on base or sp.
62	<	* They are simple ints as used in any circular array-based queue
63	<	* (see for example ArrayDeque).  Updates to the indices must
64	<	* still be ordered in a way that guarantees that sp == base means
65	<	* the queue is empty, but otherwise may err on the side of
66	<	* possibly making the queue appear nonempty when a push, pop, or
67	<	* deq have not fully committed. Note that this means that the deq
68	<	* operation, considered individually, is not wait-free. One thief
69	<	* cannot successfully continue until another in-progress one (or,
70	<	* if previously empty, a push) completes.  However, in the
71	<	* aggregate, we ensure at least probabilistic non-blockingness.
72	<	* If an attempted steal fails, a thief always chooses a different
73	<	* random victim target to try next. So, in order for one thief to
74	<	* progress, it suffices for any in-progress deq or new push on
75	<	* any empty queue to complete. One reason this works well here is
76	<	* that apparently-nonempty often means soon-to-be-stealable,
77	<	* which gives threads a chance to set activation status if
78	<	* necessary before stealing.
79	<	*
80	<	* This approach also enables support for "async mode" where local
81	<	* task processing is in FIFO, not LIFO order; simply by using a
82	<	* version of deq rather than pop when locallyFifo is true (as set
83	<	* by the ForkJoinPool).  This allows use in message-passing
84	<	* frameworks in which tasks are never joined.
85	<	*
86	<	* Efficient implementation of this approach currently relies on
87	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
88	<	* correct orderings, reads and writes of variable base require
89	<	* volatile ordering.  Variable sp does not require volatile
90	<	* writes but still needs store-ordering, which we accomplish by
91	<	* pre-incrementing sp before filling the slot with an ordered
92	<	* store.  (Pre-incrementing also enables backouts used in
93	<	* scanWhileJoining.)  Because they are protected by volatile base
94	<	* reads, reads of the queue array and its slots by other threads
95	<	* do not need volatile load semantics, but writes (in push)
96	<	* require store order and CASes (in pop and deq) require
97	<	* (volatile) CAS semantics.  (Michael, Saraswat, and Vechev's
98	<	* algorithm has similar properties, but without support for
99	<	* nulling slots.)  Since these combinations aren't supported
100	<	* using ordinary volatiles, the only way to accomplish these
101	<	* efficiently is to use direct Unsafe calls. (Using external
102	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
103	<	* array is significantly slower because of memory locality and
104	<	* indirection effects.)
105	<	*
106	<	* Further, performance on most platforms is very sensitive to
107	<	* placement and sizing of the (resizable) queue array.  Even
108	<	* though these queues don't usually become all that big, the
109	<	* initial size must be large enough to counteract cache
110	<	* contention effects across multiple queues (especially in the
111	<	* presence of GC cardmarking). Also, to improve thread-locality,
112	<	* queues are initialized after starting.  All together, these
113	<	* low-level implementation choices produce as much as a factor of
114	<	* 4 performance improvement compared to naive implementations,
115	<	* and enable the processing of billions of tasks per second,
116	<	* sometimes at the expense of ugliness.
117	<	*/
118	<
119	<	/**
120	<	* Generator for initial random seeds for random victim
121	<	* selection. This is used only to create initial seeds. Random
122	<	* steals use a cheaper xorshift generator per steal attempt. We
123	<	* expect only rare contention on seedGenerator, so just use a
124	<	* plain Random.
125	<	*/
126	<	private static final Random seedGenerator = new Random();
127	<
128	<	/**
129	<	* The timeout value for suspending spares. Spare workers that
130	<	* remain unsignalled for more than this time may be trimmed
131	<	* (killed and removed from pool).  Since our goal is to avoid
132	<	* long-term thread buildup, the exact value of timeout does not
133	<	* matter too much so long as it avoids most false-alarm timeouts
134	<	* under GC stalls or momentarily high system load.
135	<	*/
136	<	private static final long SPARE_KEEPALIVE_NANOS =
137	<	5L * 1000L * 1000L * 1000L; // 5 secs
138	<
139	<	/**
140	<	* Capacity of work-stealing queue array upon initialization.
141	<	* Must be a power of two. Initial size must be at least 2, but is
142	<	* padded to minimize cache effects.
143	<	*/
144	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
145	<
146	<	/**
147	<	* Maximum work-stealing queue array size.  Must be less than or
148	<	* equal to 1 << 28 to ensure lack of index wraparound. (This
149	<	* is less than usual bounds, because we need leftshift by 3
150	<	* to be in int range).
151	<	*/
152	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28;
153	<
154	<	/**
155	<	* The pool this thread works in. Accessed directly by ForkJoinTask.
156	<	*/
157	<	final ForkJoinPool pool;
158	<
159	<	/**
160	<	* The work-stealing queue array. Size must be a power of two.
161	<	* Initialized in onStart, to improve memory locality.
162	<	*/
163	<	private ForkJoinTask<?>[] queue;
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	<	* Index (mod queue.length) of next queue slot to push to or pop
173	<	* from. It is written only by owner thread, and accessed by other
174	<	* threads only after reading (volatile) base.  Both sp and base
175	<	* are allowed to wrap around on overflow, but (sp - base) still
176	<	* estimates size.
177	<	*/
178	<	private int sp;
179	<
180	<	/**
181	<	* Run state of this worker. In addition to the usual run levels,
182	<	* tracks if this worker is suspended as a spare, and if it was
183	<	* killed (trimmed) while suspended. However, "active" status is
184	<	* maintained separately.
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28		*/
186	–	private volatile int runState;
29
30	<	private static final int TERMINATING = 0x01;
31	<	private static final int TERMINATED  = 0x02;
190	<	private static final int SUSPENDED   = 0x04; // inactive spare
191	<	private static final int TRIMMED     = 0x08; // killed while suspended
192	<
193	<	/**
194	<	* Number of LockSupport.park calls to block this thread for
195	<	* suspension or event waits. Used for internal instrumention;
196	<	* currently not exported but included because volatile write upon
197	<	* park also provides a workaround for a JVM bug.
198	<	*/
199	<	private volatile int parkCount;
200	<
201	<	/**
202	<	* Number of steals, transferred and reset in pool callbacks pool
203	<	* when idle Accessed directly by pool.
204	<	*/
205	<	int stealCount;
206	<
207	<	/**
208	<	* Seed for random number generator for choosing steal victims.
209	<	* Uses Marsaglia xorshift. Must be initialized as nonzero.
210	<	*/
211	<	private int seed;
212	<
213	<	/**
214	<	* Activity status. When true, this worker is considered active.
215	<	* Accessed directly by pool.  Must be false upon construction.
216	<	*/
217	<	boolean active;
218	<
219	<	/**
220	<	* True if use local fifo, not default lifo, for local polling.
221	<	* Shadows value from ForkJoinPool, which resets it if changed
222	<	* pool-wide.
223	<	*/
224	<	private boolean locallyFifo;
225	<
226	<	/**
227	<	* Index of this worker in pool array. Set once by pool before
228	<	* running, and accessed directly by pool to locate this worker in
229	<	* its workers array.
230	<	*/
231	<	int poolIndex;
232	<
233	<	/**
234	<	* The last pool event waited for. Accessed only by pool in
235	<	* callback methods invoked within this thread.
236	<	*/
237	<	int lastEventCount;
238	<
239	<	/**
240	<	* Encoded index and event count of next event waiter. Used only
241	<	* by ForkJoinPool for managing event waiters.
242	<	*/
243	<	volatile long nextWaiter;
30	>	final ForkJoinPool.WorkQueue workQueue; // Work-stealing mechanics
31	>	final ForkJoinPool pool;                // the pool this thread works in
32
33		/**
34		* Creates a ForkJoinWorkerThread operating in the given pool.
#	Line 249 | Line 37 | public class ForkJoinWorkerThread extend
37		* @throws NullPointerException if pool is null
38		*/
39		protected ForkJoinWorkerThread(ForkJoinPool pool) {
40	<	if (pool == null) throw new NullPointerException();
41	<	this.pool = pool;
42	<	// To avoid exposing construction details to subclasses,
255	<	// remaining initialization is in start() and onStart()
256	<	}
257	<
258	<	/**
259	<	* Performs additional initialization and starts this thread
260	<	*/
261	<	final void start(int poolIndex, boolean locallyFifo,
262	<	UncaughtExceptionHandler ueh) {
263	<	this.poolIndex = poolIndex;
264	<	this.locallyFifo = locallyFifo;
40	>	super(pool.nextWorkerName());
41	>	setDaemon(true);
42	>	Thread.UncaughtExceptionHandler ueh = pool.ueh;
43		if (ueh != null)
44		setUncaughtExceptionHandler(ueh);
45	<	setDaemon(true);
46	<	start();
45	>	this.pool = pool;
46	>	pool.registerWorker(this.workQueue = new ForkJoinPool.WorkQueue
47	>	(pool, this, pool.localMode));
48		}
49
271	–	// Public/protected methods
272	–
50		/**
51		* Returns the pool hosting this thread.
52		*
#	Line 289 | Line 66 | public class ForkJoinWorkerThread extend
66		* @return the index number
67		*/
68		public int getPoolIndex() {
69	<	return poolIndex;
69	>	return workQueue.poolIndex;
70		}
71
72		/**
73		* Initializes internal state after construction but before
74		* processing any tasks. If you override this method, you must
75	<	* invoke super.onStart() at the beginning of the method.
75	>	* invoke {@code super.onStart()} at the beginning of the method.
76		* Initialization requires care: Most fields must have legal
77		* default values, to ensure that attempted accesses from other
78		* threads work correctly even before this thread starts
79		* processing tasks.
80		*/
81		protected void onStart() {
305	–	int rs = seedGenerator.nextInt();
306	–	seed = rs == 0? 1 : rs; // seed must be nonzero
307	–
308	–	// Allocate name string and queue array in this thread
309	–	String pid = Integer.toString(pool.getPoolNumber());
310	–	String wid = Integer.toString(poolIndex);
311	–	setName("ForkJoinPool-" + pid + "-worker-" + wid);
312	–
313	–	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
82		}
83
84		/**
#	Line 322 | Line 90 | public class ForkJoinWorkerThread extend
90		* to an unrecoverable error, or {@code null} if completed normally
91		*/
92		protected void onTermination(Throwable exception) {
325	–	try {
326	–	cancelTasks();
327	–	setTerminated();
328	–	pool.workerTerminated(this);
329	–	} catch (Throwable ex) {        // Shouldn't ever happen
330	–	if (exception == null)      // but if so, at least rethrown
331	–	exception = ex;
332	–	} finally {
333	–	if (exception != null)
334	–	UNSAFE.throwException(exception);
335	–	}
93		}
94
95		/**
96		* This method is required to be public, but should never be
97		* called explicitly. It performs the main run loop to execute
98	<	* ForkJoinTasks.
98	>	* {@link ForkJoinTask}s.
99		*/
100		public void run() {
101		Throwable exception = null;
102		try {
103		onStart();
104	<	mainLoop();
104	>	pool.runWorker(workQueue);
105		} catch (Throwable ex) {
106		exception = ex;
107		} finally {
351	–	onTermination(exception);
352	–	}
353	–	}
354	–
355	–	// helpers for run()
356	–
357	–	/**
358	–	* Find and execute tasks and check status while running
359	–	*/
360	–	private void mainLoop() {
361	–	boolean ran = false; // true if ran task on previous step
362	–	ForkJoinPool p = pool;
363	–	for (;;) {
364	–	p.preStep(this, ran);
365	–	if (runState != 0)
366	–	return;
367	–	ForkJoinTask<?> t; // try to get and run stolen or submitted task
368	–	if (ran = (t = scan()) != null || (t = pollSubmission()) != null) {
369	–	t.tryExec();
370	–	if (base != sp)
371	–	runLocalTasks();
372	–	}
373	–	}
374	–	}
375	–
376	–	/**
377	–	* Runs local tasks until queue is empty or shut down.  Call only
378	–	* while active.
379	–	*/
380	–	private void runLocalTasks() {
381	–	while (runState == 0) {
382	–	ForkJoinTask<?> t = locallyFifo? locallyDeqTask() : popTask();
383	–	if (t != null)
384	–	t.tryExec();
385	–	else if (base == sp)
386	–	break;
387	–	}
388	–	}
389	–
390	–	/**
391	–	* If a submission exists, try to activate and take it
392	–	*
393	–	* @return a task, if available
394	–	*/
395	–	private ForkJoinTask<?> pollSubmission() {
396	–	ForkJoinPool p = pool;
397	–	while (p.hasQueuedSubmissions()) {
398	–	if (active || (active = p.tryIncrementActiveCount())) {
399	–	ForkJoinTask<?> t = p.pollSubmission();
400	–	return t != null ? t : scan(); // if missed, rescan
401	–	}
402	–	}
403	–	return null;
404	–	}
405	–
406	–	/*
407	–	* Intrinsics-based atomic writes for queue slots. These are
408	–	* basically the same as methods in AtomicObjectArray, but
409	–	* specialized for (1) ForkJoinTask elements (2) requirement that
410	–	* nullness and bounds checks have already been performed by
411	–	* callers and (3) effective offsets are known not to overflow
412	–	* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
413	–	* need corresponding version for reads: plain array reads are OK
414	–	* because they protected by other volatile reads and are
415	–	* confirmed by CASes.
416	–	*
417	–	* Most uses don't actually call these methods, but instead contain
418	–	* inlined forms that enable more predictable optimization.  We
419	–	* don't define the version of write used in pushTask at all, but
420	–	* instead inline there a store-fenced array slot write.
421	–	*/
422	–
423	–	/**
424	–	* CASes slot i of array q from t to null. Caller must ensure q is
425	–	* non-null and index is in range.
426	–	*/
427	–	private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
428	–	ForkJoinTask<?> t) {
429	–	return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
430	–	}
431	–
432	–	/**
433	–	* Performs a volatile write of the given task at given slot of
434	–	* array q.  Caller must ensure q is non-null and index is in
435	–	* range. This method is used only during resets and backouts.
436	–	*/
437	–	private static final void writeSlot(ForkJoinTask<?>[] q, int i,
438	–	ForkJoinTask<?> t) {
439	–	UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t);
440	–	}
441	–
442	–	// queue methods
443	–
444	–	/**
445	–	* Pushes a task. Call only from this thread.
446	–	*
447	–	* @param t the task. Caller must ensure non-null.
448	–	*/
449	–	final void pushTask(ForkJoinTask<?> t) {
450	–	int s;
451	–	ForkJoinTask<?>[] q = queue;
452	–	int mask = q.length - 1; // implicit assert q != null
453	–	UNSAFE.putOrderedObject(q, (((s = sp++) & mask) << qShift) + qBase, t);
454	–	if ((s -= base) <= 0)
455	–	pool.signalWork();
456	–	else if (s + 1 >= mask)
457	–	growQueue();
458	–	}
459	–
460	–	/**
461	–	* Tries to take a task from the base of the queue, failing if
462	–	* empty or contended. Note: Specializations of this code appear
463	–	* in scan and scanWhileJoining.
464	–	*
465	–	* @return a task, or null if none or contended
466	–	*/
467	–	final ForkJoinTask<?> deqTask() {
468	–	ForkJoinTask<?> t;
469	–	ForkJoinTask<?>[] q;
470	–	int b, i;
471	–	if ((b = base) != sp &&
472	–	(q = queue) != null && // must read q after b
473	–	(t = q[i = (q.length - 1) & b]) != null &&
474	–	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
475	–	base = b + 1;
476	–	return t;
477	–	}
478	–	return null;
479	–	}
480	–
481	–	/**
482	–	* Tries to take a task from the base of own queue. Assumes active
483	–	* status.  Called only by current thread.
484	–	*
485	–	* @return a task, or null if none
486	–	*/
487	–	final ForkJoinTask<?> locallyDeqTask() {
488	–	ForkJoinTask<?>[] q = queue;
489	–	if (q != null) {
490	–	ForkJoinTask<?> t;
491	–	int b, i;
492	–	while (sp != (b = base)) {
493	–	if ((t = q[i = (q.length - 1) & b]) != null &&
494	–	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase,
495	–	t, null)) {
496	–	base = b + 1;
497	–	return t;
498	–	}
499	–	}
500	–	}
501	–	return null;
502	–	}
503	–
504	–	/**
505	–	* Returns a popped task, or null if empty. Assumes active status.
506	–	* Called only by current thread. (Note: a specialization of this
507	–	* code appears in scanWhileJoining.)
508	–	*/
509	–	final ForkJoinTask<?> popTask() {
510	–	int s;
511	–	ForkJoinTask<?>[] q = queue;
512	–	if (q != null && (s = sp) != base) {
513	–	int i = (q.length - 1) & --s;
514	–	ForkJoinTask<?> t = q[i];
515	–	if (t != null && UNSAFE.compareAndSwapObject
516	–	(q, (i << qShift) + qBase, t, null)) {
517	–	sp = s;
518	–	return t;
519	–	}
520	–	}
521	–	return null;
522	–	}
523	–
524	–	/**
525	–	* Specialized version of popTask to pop only if
526	–	* topmost element is the given task. Called only
527	–	* by current thread while active.
528	–	*
529	–	* @param t the task. Caller must ensure non-null.
530	–	*/
531	–	final boolean unpushTask(ForkJoinTask<?> t) {
532	–	int s;
533	–	ForkJoinTask<?>[] q = queue;
534	–	if (q != null && UNSAFE.compareAndSwapObject
535	–	(q, (((q.length - 1) & (s = sp - 1)) << qShift) + qBase, t, null)){
536	–	sp = s;
537	–	return true;
538	–	}
539	–	return false;
540	–	}
541	–
542	–	/**
543	–	* Returns next task or null if empty or contended
544	–	*/
545	–	final ForkJoinTask<?> peekTask() {
546	–	ForkJoinTask<?>[] q = queue;
547	–	if (q == null)
548	–	return null;
549	–	int mask = q.length - 1;
550	–	int i = locallyFifo ? base : (sp - 1);
551	–	return q[i & mask];
552	–	}
553	–
554	–	/**
555	–	* Doubles queue array size. Transfers elements by emulating
556	–	* steals (deqs) from old array and placing, oldest first, into
557	–	* new array.
558	–	*/
559	–	private void growQueue() {
560	–	ForkJoinTask<?>[] oldQ = queue;
561	–	int oldSize = oldQ.length;
562	–	int newSize = oldSize << 1;
563	–	if (newSize > MAXIMUM_QUEUE_CAPACITY)
564	–	throw new RejectedExecutionException("Queue capacity exceeded");
565	–	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
566	–
567	–	int b = base;
568	–	int bf = b + oldSize;
569	–	int oldMask = oldSize - 1;
570	–	int newMask = newSize - 1;
571	–	do {
572	–	int oldIndex = b & oldMask;
573	–	ForkJoinTask<?> t = oldQ[oldIndex];
574	–	if (t != null && !casSlotNull(oldQ, oldIndex, t))
575	–	t = null;
576	–	writeSlot(newQ, b & newMask, t);
577	–	} while (++b != bf);
578	–	pool.signalWork();
579	–	}
580	–
581	–	/**
582	–	* Computes next value for random victim probe in scan().  Scans
583	–	* don't require a very high quality generator, but also not a
584	–	* crummy one.  Marsaglia xor-shift is cheap and works well enough.
585	–	* Note: This is manually inlined in scan()
586	–	*/
587	–	private static final int xorShift(int r) {
588	–	r ^= r << 13;
589	–	r ^= r >>> 17;
590	–	return r ^ (r << 5);
591	–	}
592	–
593	–	/**
594	–	* Tries to steal a task from another worker. Starts at a random
595	–	* index of workers array, and probes workers until finding one
596	–	* with non-empty queue or finding that all are empty.  It
597	–	* randomly selects the first n probes. If these are empty, it
598	–	* resorts to a circular sweep, which is necessary to accurately
599	–	* set active status. (The circular sweep uses steps of
600	–	* approximately half the array size plus 1, to avoid bias
601	–	* stemming from leftmost packing of the array in ForkJoinPool.)
602	–	*
603	–	* This method must be both fast and quiet -- usually avoiding
604	–	* memory accesses that could disrupt cache sharing etc other than
605	–	* those needed to check for and take tasks (or to activate if not
606	–	* already active). This accounts for, among other things,
607	–	* updating random seed in place without storing it until exit.
608	–	*
609	–	* @return a task, or null if none found
610	–	*/
611	–	private ForkJoinTask<?> scan() {
612	–	ForkJoinPool p = pool;
613	–	ForkJoinWorkerThread[] ws = p.workers;
614	–	int n = ws.length;            // upper bound of #workers
615	–	boolean canSteal = active;    // shadow active status
616	–	int r = seed;                 // extract seed once
617	–	int k = r;                    // index: random if j<0 else step
618	–	for (int j = -n; j < n; ++j) {
619	–	ForkJoinWorkerThread v = ws[k & (n - 1)];
620	–	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
621	–	if (v != null && v.base != v.sp) {
622	–	if (canSteal ||       // ensure active status
623	–	(canSteal = active = p.tryIncrementActiveCount())) {
624	–	int b, i;         // inlined specialization of deqTask
625	–	ForkJoinTask<?> t;
626	–	ForkJoinTask<?>[] q;
627	–	if ((b = v.base) != v.sp &&  // recheck
628	–	(q = v.queue) != null &&
629	–	(t = q[i = (q.length - 1) & b]) != null &&
630	–	UNSAFE.compareAndSwapObject
631	–	(q, (i << qShift) + qBase, t, null)) {
632	–	v.base = b + 1;
633	–	seed = r;
634	–	++stealCount;
635	–	return t;
636	–	}
637	–	}
638	–	j = -n;               // reset on contention
639	–	}
640	–	k = j >= 0? k + ((n >>> 1) | 1) : r;
641	–	}
642	–	return null;
643	–	}
644	–
645	–	// Run State management
646	–
647	–	// status check methods used mainly by ForkJoinPool
648	–	final boolean isTerminating() { return (runState & TERMINATING) != 0; }
649	–	final boolean isTerminated()  { return (runState & TERMINATED) != 0; }
650	–	final boolean isSuspended()   { return (runState & SUSPENDED) != 0; }
651	–	final boolean isTrimmed()     { return (runState & TRIMMED) != 0; }
652	–
653	–	/**
654	–	* Sets state to TERMINATING, also resuming if suspended.
655	–	*/
656	–	final void shutdown() {
657	–	for (;;) {
658	–	int s = runState;
659	–	if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended
660	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
661	–	(s & ~SUSPENDED) |
662	–	(TRIMMED|TERMINATING))) {
663	–	LockSupport.unpark(this);
664	–	break;
665	–	}
666	–	}
667	–	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
668	–	s | TERMINATING))
669	–	break;
670	–	}
671	–	}
672	–
673	–	/**
674	–	* Sets state to TERMINATED. Called only by this thread.
675	–	*/
676	–	private void setTerminated() {
677	–	int s;
678	–	do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset,
679	–	s = runState,
680	–	s | (TERMINATING|TERMINATED)));
681	–	}
682	–
683	–	/**
684	–	* Instrumented version of park. Also used by ForkJoinPool.awaitEvent
685	–	*/
686	–	final void doPark() {
687	–	++parkCount;
688	–	LockSupport.park(this);
689	–	}
690	–
691	–	/**
692	–	* If suspended, tries to set status to unsuspended.
693	–	* Caller must unpark to actually resume
694	–	*
695	–	* @return true if successful
696	–	*/
697	–	final boolean tryUnsuspend() {
698	–	int s;
699	–	return (((s = runState) & SUSPENDED) != 0 &&
700	–	UNSAFE.compareAndSwapInt(this, runStateOffset, s,
701	–	s & ~SUSPENDED));
702	–	}
703	–
704	–	/**
705	–	* Sets suspended status and blocks as spare until resumed,
706	–	* shutdown, or timed out.
707	–	*
708	–	* @return false if trimmed
709	–	*/
710	–	final boolean suspendAsSpare() {
711	–	for (;;) {               // set suspended unless terminating
712	–	int s = runState;
713	–	if ((s & TERMINATING) != 0) { // must kill
714	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
715	–	s | (TRIMMED | TERMINATING)))
716	–	return false;
717	–	}
718	–	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
719	–	s | SUSPENDED))
720	–	break;
721	–	}
722	–	lastEventCount = 0;      // reset upon resume
723	–	ForkJoinPool p = pool;
724	–	p.releaseWaiters();      // help others progress
725	–	p.accumulateStealCount(this);
726	–	interrupted();           // clear/ignore interrupts
727	–	if (poolIndex < p.getParallelism()) { // untimed wait
728	–	while ((runState & SUSPENDED) != 0)
729	–	doPark();
730	–	return true;
731	–	}
732	–	return timedSuspend();   // timed wait if apparently non-core
733	–	}
734	–
735	–	/**
736	–	* Blocks as spare until resumed or timed out
737	–	* @return false if trimmed
738	–	*/
739	–	private boolean timedSuspend() {
740	–	long nanos = SPARE_KEEPALIVE_NANOS;
741	–	long startTime = System.nanoTime();
742	–	while ((runState & SUSPENDED) != 0) {
743	–	++parkCount;
744	–	if ((nanos -= (System.nanoTime() - startTime)) > 0)
745	–	LockSupport.parkNanos(this, nanos);
746	–	else { // try to trim on timeout
747	–	int s = runState;
748	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
749	–	(s & ~SUSPENDED) |
750	–	(TRIMMED|TERMINATING)))
751	–	return false;
752	–	}
753	–	}
754	–	return true;
755	–	}
756	–
757	–	// Misc support methods for ForkJoinPool
758	–
759	–	/**
760	–	* Returns an estimate of the number of tasks in the queue.  Also
761	–	* used by ForkJoinTask.
762	–	*/
763	–	final int getQueueSize() {
764	–	return -base + sp;
765	–	}
766	–
767	–	/**
768	–	* Set locallyFifo mode. Called only by ForkJoinPool
769	–	*/
770	–	final void setAsyncMode(boolean async) {
771	–	locallyFifo = async;
772	–	}
773	–
774	–	/**
775	–	* Removes and cancels all tasks in queue.  Can be called from any
776	–	* thread.
777	–	*/
778	–	final void cancelTasks() {
779	–	while (base != sp) {
780	–	ForkJoinTask<?> t = deqTask();
781	–	if (t != null)
782	–	t.cancelIgnoringExceptions();
783	–	}
784	–	}
785	–
786	–	/**
787	–	* Drains tasks to given collection c.
788	–	*
789	–	* @return the number of tasks drained
790	–	*/
791	–	final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
792	–	int n = 0;
793	–	while (base != sp) {
794	–	ForkJoinTask<?> t = deqTask();
795	–	if (t != null) {
796	–	c.add(t);
797	–	++n;
798	–	}
799	–	}
800	–	return n;
801	–	}
802	–
803	–	// Support methods for ForkJoinTask
804	–
805	–	/**
806	–	* Returns an estimate of the number of tasks, offset by a
807	–	* function of number of idle workers.
808	–	*
809	–	* This method provides a cheap heuristic guide for task
810	–	* partitioning when programmers, frameworks, tools, or languages
811	–	* have little or no idea about task granularity.  In essence by
812	–	* offering this method, we ask users only about tradeoffs in
813	–	* overhead vs expected throughput and its variance, rather than
814	–	* how finely to partition tasks.
815	–	*
816	–	* In a steady state strict (tree-structured) computation, each
817	–	* thread makes available for stealing enough tasks for other
818	–	* threads to remain active. Inductively, if all threads play by
819	–	* the same rules, each thread should make available only a
820	–	* constant number of tasks.
821	–	*
822	–	* The minimum useful constant is just 1. But using a value of 1
823	–	* would require immediate replenishment upon each steal to
824	–	* maintain enough tasks, which is infeasible.  Further,
825	–	* partitionings/granularities of offered tasks should minimize
826	–	* steal rates, which in general means that threads nearer the top
827	–	* of computation tree should generate more than those nearer the
828	–	* bottom. In perfect steady state, each thread is at
829	–	* approximately the same level of computation tree. However,
830	–	* producing extra tasks amortizes the uncertainty of progress and
831	–	* diffusion assumptions.
832	–	*
833	–	* So, users will want to use values larger, but not much larger
834	–	* than 1 to both smooth over transient shortages and hedge
835	–	* against uneven progress; as traded off against the cost of
836	–	* extra task overhead. We leave the user to pick a threshold
837	–	* value to compare with the results of this call to guide
838	–	* decisions, but recommend values such as 3.
839	–	*
840	–	* When all threads are active, it is on average OK to estimate
841	–	* surplus strictly locally. In steady-state, if one thread is
842	–	* maintaining say 2 surplus tasks, then so are others. So we can
843	–	* just use estimated queue length (although note that (sp - base)
844	–	* can be an overestimate because of stealers lagging increments
845	–	* of base).  However, this strategy alone leads to serious
846	–	* mis-estimates in some non-steady-state conditions (ramp-up,
847	–	* ramp-down, other stalls). We can detect many of these by
848	–	* further considering the number of "idle" threads, that are
849	–	* known to have zero queued tasks, so compensate by a factor of
850	–	* (#idle/#active) threads.
851	–	*/
852	–	final int getEstimatedSurplusTaskCount() {
853	–	return sp - base - pool.idlePerActive();
854	–	}
855	–
856	–	/**
857	–	* Gets and removes a local task.
858	–	*
859	–	* @return a task, if available
860	–	*/
861	–	final ForkJoinTask<?> pollLocalTask() {
862	–	while (base != sp) {
863	–	if (active || (active = pool.tryIncrementActiveCount()))
864	–	return locallyFifo? locallyDeqTask() : popTask();
865	–	}
866	–	return null;
867	–	}
868	–
869	–	/**
870	–	* Gets and removes a local or stolen task.
871	–	*
872	–	* @return a task, if available
873	–	*/
874	–	final ForkJoinTask<?> pollTask() {
875	–	ForkJoinTask<?> t;
876	–	return (t = pollLocalTask()) != null ? t : scan();
877	–	}
878	–
879	–	/**
880	–	* Returns a stolen task, if available, unless joinMe is done
881	–	*
882	–	* This method is intrinsically nonmodular. To maintain the
883	–	* property that tasks are never stolen if the awaited task is
884	–	* ready, we must interleave mechanics of scan with status
885	–	* checks. We rely here on the commit points of deq that allow us
886	–	* to cancel a steal even after CASing slot to null, but before
887	–	* adjusting base index: If, after the CAS, we see that joinMe is
888	–	* ready, we can back out by placing the task back into the slot,
889	–	* without adjusting index. The scan loop is otherwise the same as
890	–	* in scan.
891	–	*
892	–	* The outer loop cannot be allowed to run forever, because it
893	–	* could lead to a form of deadlock if all threads are executing
894	–	* this method. However, we must also be patient before giving up,
895	–	* to cope with GC stalls, transient high loads, etc. The loop
896	–	* terminates (causing caller to possibly block this thread and
897	–	* create a replacement) only after #workers clean sweeps during
898	–	* which all running threads are active.
899	–	*/
900	–	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
901	–	int sweeps = 0;
902	–	int r = seed;
903	–	ForkJoinPool p = pool;
904	–	p.releaseWaiters(); // help other threads progress
905	–	while (joinMe.status >= 0) {
906	–	ForkJoinWorkerThread[] ws = p.workers;
907	–	int n = ws.length;
908	–	int k = r;
909	–	for (int j = -n; j < n; ++j) {
910	–	ForkJoinWorkerThread v = ws[k & (n - 1)];
911	–	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
912	–	if (v != null) {
913	–	int b = v.base;
914	–	ForkJoinTask<?>[] q;
915	–	if (b != v.sp && (q = v.queue) != null) {
916	–	int i = (q.length - 1) & b;
917	–	ForkJoinTask<?> t = q[i];
918	–	if (t != null) {
919	–	if (joinMe.status < 0)
920	–	return null;
921	–	if (UNSAFE.compareAndSwapObject
922	–	(q, (i << qShift) + qBase, t, null)) {
923	–	if (joinMe.status < 0) {
924	–	writeSlot(q, i, t); // back out
925	–	return null;
926	–	}
927	–	v.base = b + 1;
928	–	seed = r;
929	–	++stealCount;
930	–	return t;
931	–	}
932	–	}
933	–	sweeps = 0; // ensure rescan on contention
934	–	}
935	–	}
936	–	k = j >= 0? k + ((n >>> 1) | 1) : r;
937	–	if ((j & 7) == 0 && joinMe.status < 0) // periodically recheck
938	–	return null;
939	–	}
940	–	if ((sweeps = p.inactiveCount() == 0 ? sweeps + 1 : 0) > n)
941	–	return null;
942	–	}
943	–	return null;
944	–	}
945	–
946	–	/**
947	–	* Runs tasks until {@code pool.isQuiescent()}.
948	–	*/
949	–	final void helpQuiescePool() {
950	–	for (;;) {
951	–	ForkJoinTask<?> t = pollLocalTask();
952	–	if (t != null || (t = scan()) != null)
953	–	t.tryExec();
954	–	else {
955	–	ForkJoinPool p = pool;
956	–	if (active) {
957	–	active = false; // inactivate
958	–	do {} while (!p.tryDecrementActiveCount());
959	–	}
960	–	if (p.isQuiescent()) {
961	–	active = true; // re-activate
962	–	do {} while (!p.tryIncrementActiveCount());
963	–	return;
964	–	}
965	–	}
966	–	}
967	–	}
968	–
969	–	// Unsafe mechanics
970	–
971	–	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
972	–	private static final long runStateOffset =
973	–	objectFieldOffset("runState", ForkJoinWorkerThread.class);
974	–	private static final long qBase =
975	–	UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
976	–	private static final int qShift;
977	–
978	–	static {
979	–	int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
980	–	if ((s & (s-1)) != 0)
981	–	throw new Error("data type scale not a power of two");
982	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
983	–	}
984	–
985	–	private static long objectFieldOffset(String field, Class<?> klazz) {
986	–	try {
987	–	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
988	–	} catch (NoSuchFieldException e) {
989	–	// Convert Exception to corresponding Error
990	–	NoSuchFieldError error = new NoSuchFieldError(field);
991	–	error.initCause(e);
992	–	throw error;
993	–	}
994	–	}
995	–
996	–	/**
997	–	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
998	–	* Replace with a simple call to Unsafe.getUnsafe when integrating
999	–	* into a jdk.
1000	–	*
1001	–	* @return a sun.misc.Unsafe
1002	–	*/
1003	–	private static sun.misc.Unsafe getUnsafe() {
1004	–	try {
1005	–	return sun.misc.Unsafe.getUnsafe();
1006	–	} catch (SecurityException se) {
108		try {
109	<	return java.security.AccessController.doPrivileged
110	<	(new java.security
111	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
112	<	public sun.misc.Unsafe run() throws Exception {
113	<	java.lang.reflect.Field f = sun.misc
114	<	.Unsafe.class.getDeclaredField("theUnsafe");
1014	<	f.setAccessible(true);
1015	<	return (sun.misc.Unsafe) f.get(null);
1016	<	}});
1017	<	} catch (java.security.PrivilegedActionException e) {
1018	<	throw new RuntimeException("Could not initialize intrinsics",
1019	<	e.getCause());
109	>	onTermination(exception);
110	>	} catch (Throwable ex) {
111	>	if (exception == null)
112	>	exception = ex;
113	>	} finally {
114	>	pool.deregisterWorker(this, exception);
115		}
116		}
117		}
118		}
119	+

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