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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.36 by dl, Fri Jul 23 13:07:43 2010 UTC vs.
Revision 1.68 by dl, Thu Jan 26 00:08:13 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	<	* When a worker would otherwise be blocked waiting to join a
87	<	* task, it first tries a form of linear helping: Each worker
88	<	* records (in field currentSteal) the most recent task it stole
89	<	* from some other worker. Plus, it records (in field currentJoin)
90	<	* the task it is currently actively joining. Method joinTask uses
91	<	* these markers to try to find a worker to help (i.e., steal back
92	<	* a task from and execute it) that could hasten completion of the
93	<	* actively joined task. In essence, the joiner executes a task
94	<	* that would be on its own local deque had the to-be-joined task
95	<	* not been stolen. This may be seen as a conservative variant of
96	<	* the approach in Wagner & Calder "Leapfrogging: a portable
97	<	* technique for implementing efficient futures" SIGPLAN Notices,
98	<	* 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
99	<	* in that: (1) We only maintain dependency links across workers
100	<	* upon steals, rather than maintain per-task bookkeeping.  This
101	<	* may require a linear scan of workers array to locate stealers,
102	<	* but usually doesn't because stealers leave hints (that may
103	<	* become stale/wrong) of where to locate the kathem. This
104	<	* isolates cost to when it is needed, rather than adding to
105	<	* per-task overhead.  (2) It is "shallow", ignoring nesting and
106	<	* potentially cyclic mutual steals.  (3) It is intentionally
107	<	* racy: field currentJoin is updated only while actively joining,
108	<	* which means that we could miss links in the chain during
109	<	* long-lived tasks, GC stalls etc.  (4) We bound the number of
110	<	* attempts to find work (see MAX_HELP_DEPTH) and fall back to
111	<	* suspending the worker and if necessary replacing it with a
112	<	* spare (see ForkJoinPool.tryAwaitJoin).
113	<	*
114	<	* Efficient implementation of these algorithms currently relies
115	<	* on an uncomfortable amount of "Unsafe" mechanics. To maintain
116	<	* correct orderings, reads and writes of variable base require
117	<	* volatile ordering.  Variable sp does not require volatile
118	<	* writes but still needs store-ordering, which we accomplish by
119	<	* pre-incrementing sp before filling the slot with an ordered
120	<	* store.  (Pre-incrementing also enables backouts used in
121	<	* joinTask.)  Because they are protected by volatile base reads,
122	<	* reads of the queue array and its slots by other threads do not
123	<	* need volatile load semantics, but writes (in push) require
124	<	* store order and CASes (in pop and deq) require (volatile) CAS
125	<	* semantics.  (Michael, Saraswat, and Vechev's algorithm has
126	<	* similar properties, but without support for nulling slots.)
127	<	* Since these combinations aren't supported using ordinary
128	<	* volatiles, the only way to accomplish these efficiently is to
129	<	* use direct Unsafe calls. (Using external AtomicIntegers and
130	<	* AtomicReferenceArrays for the indices and array is
131	<	* significantly slower because of memory locality and indirection
132	<	* effects.)
133	<	*
134	<	* Further, performance on most platforms is very sensitive to
135	<	* placement and sizing of the (resizable) queue array.  Even
136	<	* though these queues don't usually become all that big, the
137	<	* initial size must be large enough to counteract cache
138	<	* contention effects across multiple queues (especially in the
139	<	* presence of GC cardmarking). Also, to improve thread-locality,
140	<	* queues are initialized after starting.  All together, these
141	<	* low-level implementation choices produce as much as a factor of
142	<	* 4 performance improvement compared to naive implementations,
143	<	* and enable the processing of billions of tasks per second,
144	<	* sometimes at the expense of ugliness.
145	<	*/
146	<
147	<	/**
148	<	* Generator for initial random seeds for random victim
149	<	* selection. This is used only to create initial seeds. Random
150	<	* steals use a cheaper xorshift generator per steal attempt. We
151	<	* expect only rare contention on seedGenerator, so just use a
152	<	* plain Random.
153	<	*/
154	<	private static final Random seedGenerator = new Random();
155	<
156	<	/**
157	<	* The timeout value for suspending spares. Spare workers that
158	<	* remain unsignalled for more than this time may be trimmed
159	<	* (killed and removed from pool).  Since our goal is to avoid
160	<	* long-term thread buildup, the exact value of timeout does not
161	<	* matter too much so long as it avoids most false-alarm timeouts
162	<	* under GC stalls or momentarily high system load.
163	<	*/
164	<	private static final long SPARE_KEEPALIVE_NANOS =
165	<	5L * 1000L * 1000L * 1000L; // 5 secs
166	<
167	<	/**
168	<	* The maximum stolen->joining link depth allowed in helpJoinTask.
169	<	* Depths for legitimate chains are unbounded, but we use a fixed
170	<	* constant to avoid (otherwise unchecked) cycles and bound
171	<	* staleness of traversal parameters at the expense of sometimes
172	<	* blocking when we could be helping.
173	<	*/
174	<	private static final int MAX_HELP_DEPTH = 8;
175	<
176	<	/**
177	<	* Capacity of work-stealing queue array upon initialization.
178	<	* Must be a power of two. Initial size must be at least 4, but is
179	<	* padded to minimize cache effects.
180	<	*/
181	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
182	<
183	<	/**
184	<	* Maximum work-stealing queue array size.  Must be less than or
185	<	* equal to 1 << 28 to ensure lack of index wraparound. (This
186	<	* is less than usual bounds, because we need leftshift by 3
187	<	* to be in int range).
188	<	*/
189	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28;
190	<
191	<	/**
192	<	* The pool this thread works in. Accessed directly by ForkJoinTask.
193	<	*/
194	<	final ForkJoinPool pool;
195	<
196	<	/**
197	<	* The work-stealing queue array. Size must be a power of two.
198	<	* Initialized in onStart, to improve memory locality.
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28		*/
200	–	private ForkJoinTask<?>[] queue;
201	–
202	–	/**
203	–	* Index (mod queue.length) of least valid queue slot, which is
204	–	* always the next position to steal from if nonempty.
205	–	*/
206	–	private volatile int base;
29
30	<	/**
31	<	* Index (mod queue.length) of next queue slot to push to or pop
210	<	* from. It is written only by owner thread, and accessed by other
211	<	* threads only after reading (volatile) base.  Both sp and base
212	<	* are allowed to wrap around on overflow, but (sp - base) still
213	<	* estimates size.
214	<	*/
215	<	private int sp;
216	<
217	<	/**
218	<	* The index of most recent stealer, used as a hint to avoid
219	<	* traversal in method helpJoinTask. This is only a hint because a
220	<	* worker might have had multiple steals and this only holds one
221	<	* of them (usually the most current). Declared non-volatile,
222	<	* relying on other prevailing sync to keep reasonably current.
223	<	*/
224	<	private int stealHint;
225	<
226	<	/**
227	<	* Run state of this worker. In addition to the usual run levels,
228	<	* tracks if this worker is suspended as a spare, and if it was
229	<	* killed (trimmed) while suspended. However, "active" status is
230	<	* maintained separately.
231	<	*/
232	<	private volatile int runState;
233	<
234	<	private static final int TERMINATING = 0x01;
235	<	private static final int TERMINATED  = 0x02;
236	<	private static final int SUSPENDED   = 0x04; // inactive spare
237	<	private static final int TRIMMED     = 0x08; // killed while suspended
238	<
239	<	/**
240	<	* Number of LockSupport.park calls to block this thread for
241	<	* suspension or event waits. Used for internal instrumention;
242	<	* currently not exported but included because volatile write upon
243	<	* park also provides a workaround for a JVM bug.
244	<	*/
245	<	volatile int parkCount;
246	<
247	<	/**
248	<	* Number of steals, transferred and reset in pool callbacks pool
249	<	* when idle Accessed directly by pool.
250	<	*/
251	<	int stealCount;
252	<
253	<	/**
254	<	* Seed for random number generator for choosing steal victims.
255	<	* Uses Marsaglia xorshift. Must be initialized as nonzero.
256	<	*/
257	<	private int seed;
258	<
259	<
260	<	/**
261	<	* Activity status. When true, this worker is considered active.
262	<	* Accessed directly by pool.  Must be false upon construction.
263	<	*/
264	<	boolean active;
265	<
266	<	/**
267	<	* True if use local fifo, not default lifo, for local polling.
268	<	* Shadows value from ForkJoinPool, which resets it if changed
269	<	* pool-wide.
270	<	*/
271	<	private final boolean locallyFifo;
272	<
273	<	/**
274	<	* Index of this worker in pool array. Set once by pool before
275	<	* running, and accessed directly by pool to locate this worker in
276	<	* its workers array.
277	<	*/
278	<	int poolIndex;
279	<
280	<	/**
281	<	* The last pool event waited for. Accessed only by pool in
282	<	* callback methods invoked within this thread.
283	<	*/
284	<	int lastEventCount;
285	<
286	<	/**
287	<	* Encoded index and event count of next event waiter. Used only
288	<	* by ForkJoinPool for managing event waiters.
289	<	*/
290	<	volatile long nextWaiter;
291	<
292	<	/**
293	<	* The task currently being joined, set only when actively trying
294	<	* to helpStealer. Written only by current thread, but read by
295	<	* others.
296	<	*/
297	<	private volatile ForkJoinTask<?> currentJoin;
298	<
299	<	/**
300	<	* The task most recently stolen from another worker (or
301	<	* submission queue).  Not volatile because always read/written in
302	<	* presence of related volatiles in those cases where it matters.
303	<	*/
304	<	private ForkJoinTask<?> currentSteal;
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 310 | Line 37 | public class ForkJoinWorkerThread extend
37		* @throws NullPointerException if pool is null
38		*/
39		protected ForkJoinWorkerThread(ForkJoinPool pool) {
40	<	this.pool = pool;
41	<	this.locallyFifo = pool.locallyFifo;
42	<	// To avoid exposing construction details to subclasses,
316	<	// remaining initialization is in start() and onStart()
317	<	}
318	<
319	<	/**
320	<	* Performs additional initialization and starts this thread
321	<	*/
322	<	final void start(int poolIndex, UncaughtExceptionHandler ueh) {
323	<	this.poolIndex = poolIndex;
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	>	this.workQueue = new ForkJoinPool.WorkQueue(this, pool.localMode);
47	>	pool.registerWorker(this);
48		}
49
330	–	// Public/protected methods
331	–
50		/**
51		* Returns the pool hosting this thread.
52		*
#	Line 348 | 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() {
364	–	int rs = seedGenerator.nextInt();
365	–	seed = rs == 0? 1 : rs; // seed must be nonzero
366	–
367	–	// Allocate name string and arrays in this thread
368	–	String pid = Integer.toString(pool.getPoolNumber());
369	–	String wid = Integer.toString(poolIndex);
370	–	setName("ForkJoinPool-" + pid + "-worker-" + wid);
371	–
372	–	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
82		}
83
84		/**
#	Line 381 | Line 90 | public class ForkJoinWorkerThread extend
90		* to an unrecoverable error, or {@code null} if completed normally
91		*/
92		protected void onTermination(Throwable exception) {
384	–	try {
385	–	cancelTasks();
386	–	setTerminated();
387	–	pool.workerTerminated(this);
388	–	} catch (Throwable ex) {        // Shouldn't ever happen
389	–	if (exception == null)      // but if so, at least rethrown
390	–	exception = ex;
391	–	} finally {
392	–	if (exception != null)
393	–	UNSAFE.throwException(exception);
394	–	}
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(this);
105		} catch (Throwable ex) {
106		exception = ex;
107		} finally {
410	–	onTermination(exception);
411	–	}
412	–	}
413	–
414	–	// helpers for run()
415	–
416	–	/**
417	–	* Find and execute tasks and check status while running
418	–	*/
419	–	private void mainLoop() {
420	–	int emptyScans = 0; // consecutive times failed to find work
421	–	ForkJoinPool p = pool;
422	–	for (;;) {
423	–	p.preStep(this, emptyScans);
424	–	if (runState != 0)
425	–	return;
426	–	ForkJoinTask<?> t; // try to get and run stolen or submitted task
427	–	if ((t = scan()) != null || (t = pollSubmission()) != null) {
428	–	t.tryExec();
429	–	if (base != sp)
430	–	runLocalTasks();
431	–	currentSteal = null;
432	–	emptyScans = 0;
433	–	}
434	–	else
435	–	++emptyScans;
436	–	}
437	–	}
438	–
439	–	/**
440	–	* Runs local tasks until queue is empty or shut down.  Call only
441	–	* while active.
442	–	*/
443	–	private void runLocalTasks() {
444	–	while (runState == 0) {
445	–	ForkJoinTask<?> t = locallyFifo? locallyDeqTask() : popTask();
446	–	if (t != null)
447	–	t.tryExec();
448	–	else if (base == sp)
449	–	break;
450	–	}
451	–	}
452	–
453	–	/**
454	–	* If a submission exists, try to activate and take it
455	–	*
456	–	* @return a task, if available
457	–	*/
458	–	private ForkJoinTask<?> pollSubmission() {
459	–	ForkJoinPool p = pool;
460	–	while (p.hasQueuedSubmissions()) {
461	–	if (active || (active = p.tryIncrementActiveCount())) {
462	–	ForkJoinTask<?> t = p.pollSubmission();
463	–	if (t != null) {
464	–	currentSteal = t;
465	–	return t;
466	–	}
467	–	return scan(); // if missed, rescan
468	–	}
469	–	}
470	–	return null;
471	–	}
472	–
473	–	/*
474	–	* Intrinsics-based atomic writes for queue slots. These are
475	–	* basically the same as methods in AtomicObjectArray, but
476	–	* specialized for (1) ForkJoinTask elements (2) requirement that
477	–	* nullness and bounds checks have already been performed by
478	–	* callers and (3) effective offsets are known not to overflow
479	–	* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
480	–	* need corresponding version for reads: plain array reads are OK
481	–	* because they protected by other volatile reads and are
482	–	* confirmed by CASes.
483	–	*
484	–	* Most uses don't actually call these methods, but instead contain
485	–	* inlined forms that enable more predictable optimization.  We
486	–	* don't define the version of write used in pushTask at all, but
487	–	* instead inline there a store-fenced array slot write.
488	–	*/
489	–
490	–	/**
491	–	* CASes slot i of array q from t to null. Caller must ensure q is
492	–	* non-null and index is in range.
493	–	*/
494	–	private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
495	–	ForkJoinTask<?> t) {
496	–	return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
497	–	}
498	–
499	–	/**
500	–	* Performs a volatile write of the given task at given slot of
501	–	* array q.  Caller must ensure q is non-null and index is in
502	–	* range. This method is used only during resets and backouts.
503	–	*/
504	–	private static final void writeSlot(ForkJoinTask<?>[] q, int i,
505	–	ForkJoinTask<?> t) {
506	–	UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t);
507	–	}
508	–
509	–	// queue methods
510	–
511	–	/**
512	–	* Pushes a task. Call only from this thread.
513	–	*
514	–	* @param t the task. Caller must ensure non-null.
515	–	*/
516	–	final void pushTask(ForkJoinTask<?> t) {
517	–	ForkJoinTask<?>[] q = queue;
518	–	int mask = q.length - 1; // implicit assert q != null
519	–	int s = sp++;            // ok to increment sp before slot write
520	–	UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t);
521	–	if ((s -= base) == 0)
522	–	pool.signalWork();   // was empty
523	–	else if (s == mask)
524	–	growQueue();         // is full
525	–	}
526	–
527	–	/**
528	–	* Tries to take a task from the base of the queue, failing if
529	–	* empty or contended. Note: Specializations of this code appear
530	–	* in locallyDeqTask and elsewhere.
531	–	*
532	–	* @return a task, or null if none or contended
533	–	*/
534	–	final ForkJoinTask<?> deqTask() {
535	–	ForkJoinTask<?> t;
536	–	ForkJoinTask<?>[] q;
537	–	int b, i;
538	–	if ((b = base) != sp &&
539	–	(q = queue) != null && // must read q after b
540	–	(t = q[i = (q.length - 1) & b]) != null && base == b &&
541	–	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
542	–	base = b + 1;
543	–	return t;
544	–	}
545	–	return null;
546	–	}
547	–
548	–	/**
549	–	* Tries to take a task from the base of own queue. Assumes active
550	–	* status.  Called only by current thread.
551	–	*
552	–	* @return a task, or null if none
553	–	*/
554	–	final ForkJoinTask<?> locallyDeqTask() {
555	–	ForkJoinTask<?>[] q = queue;
556	–	if (q != null) {
557	–	ForkJoinTask<?> t;
558	–	int b, i;
559	–	while (sp != (b = base)) {
560	–	if ((t = q[i = (q.length - 1) & b]) != null && base == b &&
561	–	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase,
562	–	t, null)) {
563	–	base = b + 1;
564	–	return t;
565	–	}
566	–	}
567	–	}
568	–	return null;
569	–	}
570	–
571	–	/**
572	–	* Returns a popped task, or null if empty. Assumes active status.
573	–	* Called only by current thread.
574	–	*/
575	–	final ForkJoinTask<?> popTask() {
576	–	int s;
577	–	ForkJoinTask<?>[] q;
578	–	if (base != (s = sp) && (q = queue) != null) {
579	–	int i = (q.length - 1) & --s;
580	–	ForkJoinTask<?> t = q[i];
581	–	if (t != null && UNSAFE.compareAndSwapObject
582	–	(q, (i << qShift) + qBase, t, null)) {
583	–	sp = s;
584	–	return t;
585	–	}
586	–	}
587	–	return null;
588	–	}
589	–
590	–	/**
591	–	* Specialized version of popTask to pop only if topmost element
592	–	* is the given task. Called only by current thread while
593	–	* active.
594	–	*
595	–	* @param t the task. Caller must ensure non-null.
596	–	*/
597	–	final boolean unpushTask(ForkJoinTask<?> t) {
598	–	int s;
599	–	ForkJoinTask<?>[] q;
600	–	if (base != (s = sp) && (q = queue) != null &&
601	–	UNSAFE.compareAndSwapObject
602	–	(q, (((q.length - 1) & --s) << qShift) + qBase, t, null)) {
603	–	sp = s;
604	–	return true;
605	–	}
606	–	return false;
607	–	}
608	–
609	–	/**
610	–	* Returns next task or null if empty or contended
611	–	*/
612	–	final ForkJoinTask<?> peekTask() {
613	–	ForkJoinTask<?>[] q = queue;
614	–	if (q == null)
615	–	return null;
616	–	int mask = q.length - 1;
617	–	int i = locallyFifo ? base : (sp - 1);
618	–	return q[i & mask];
619	–	}
620	–
621	–	/**
622	–	* Doubles queue array size. Transfers elements by emulating
623	–	* steals (deqs) from old array and placing, oldest first, into
624	–	* new array.
625	–	*/
626	–	private void growQueue() {
627	–	ForkJoinTask<?>[] oldQ = queue;
628	–	int oldSize = oldQ.length;
629	–	int newSize = oldSize << 1;
630	–	if (newSize > MAXIMUM_QUEUE_CAPACITY)
631	–	throw new RejectedExecutionException("Queue capacity exceeded");
632	–	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
633	–
634	–	int b = base;
635	–	int bf = b + oldSize;
636	–	int oldMask = oldSize - 1;
637	–	int newMask = newSize - 1;
638	–	do {
639	–	int oldIndex = b & oldMask;
640	–	ForkJoinTask<?> t = oldQ[oldIndex];
641	–	if (t != null && !casSlotNull(oldQ, oldIndex, t))
642	–	t = null;
643	–	writeSlot(newQ, b & newMask, t);
644	–	} while (++b != bf);
645	–	pool.signalWork();
646	–	}
647	–
648	–	/**
649	–	* Computes next value for random victim probe in scan().  Scans
650	–	* don't require a very high quality generator, but also not a
651	–	* crummy one.  Marsaglia xor-shift is cheap and works well enough.
652	–	* Note: This is manually inlined in scan()
653	–	*/
654	–	private static final int xorShift(int r) {
655	–	r ^= r << 13;
656	–	r ^= r >>> 17;
657	–	return r ^ (r << 5);
658	–	}
659	–
660	–	/**
661	–	* Tries to steal a task from another worker. Starts at a random
662	–	* index of workers array, and probes workers until finding one
663	–	* with non-empty queue or finding that all are empty.  It
664	–	* randomly selects the first n probes. If these are empty, it
665	–	* resorts to a circular sweep, which is necessary to accurately
666	–	* set active status. (The circular sweep uses steps of
667	–	* approximately half the array size plus 1, to avoid bias
668	–	* stemming from leftmost packing of the array in ForkJoinPool.)
669	–	*
670	–	* This method must be both fast and quiet -- usually avoiding
671	–	* memory accesses that could disrupt cache sharing etc other than
672	–	* those needed to check for and take tasks (or to activate if not
673	–	* already active). This accounts for, among other things,
674	–	* updating random seed in place without storing it until exit.
675	–	*
676	–	* @return a task, or null if none found
677	–	*/
678	–	private ForkJoinTask<?> scan() {
679	–	ForkJoinPool p = pool;
680	–	ForkJoinWorkerThread[] ws;        // worker array
681	–	int n;                            // upper bound of #workers
682	–	if ((ws = p.workers) != null && (n = ws.length) > 1) {
683	–	boolean canSteal = active;    // shadow active status
684	–	int r = seed;                 // extract seed once
685	–	int mask = n - 1;
686	–	int j = -n;                   // loop counter
687	–	int k = r;                    // worker index, random if j < 0
688	–	for (;;) {
689	–	ForkJoinWorkerThread v = ws[k & mask];
690	–	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift
691	–	if (v != null && v.base != v.sp) {
692	–	if (canSteal ||       // ensure active status
693	–	(canSteal = active = p.tryIncrementActiveCount())) {
694	–	int b = v.base;   // inline specialized deqTask
695	–	ForkJoinTask<?>[] q;
696	–	if (b != v.sp && (q = v.queue) != null) {
697	–	ForkJoinTask<?> t;
698	–	int i = (q.length - 1) & b;
699	–	long u = (i << qShift) + qBase; // raw offset
700	–	if ((t = q[i]) != null && v.base == b &&
701	–	UNSAFE.compareAndSwapObject(q, u, t, null)) {
702	–	currentSteal = t;
703	–	v.stealHint = poolIndex;
704	–	v.base = b + 1;
705	–	seed = r;
706	–	++stealCount;
707	–	return t;
708	–	}
709	–	}
710	–	}
711	–	j = -n;
712	–	k = r;                // restart on contention
713	–	}
714	–	else if (++j <= 0)
715	–	k = r;
716	–	else if (j <= n)
717	–	k += (n >>> 1) | 1;
718	–	else
719	–	break;
720	–	}
721	–	}
722	–	return null;
723	–	}
724	–
725	–	// Run State management
726	–
727	–	// status check methods used mainly by ForkJoinPool
728	–	final boolean isTerminating() { return (runState & TERMINATING) != 0; }
729	–	final boolean isTerminated()  { return (runState & TERMINATED) != 0; }
730	–	final boolean isSuspended()   { return (runState & SUSPENDED) != 0; }
731	–	final boolean isTrimmed()     { return (runState & TRIMMED) != 0; }
732	–
733	–	/**
734	–	* Sets state to TERMINATING, also resuming if suspended.
735	–	*/
736	–	final void shutdown() {
737	–	for (;;) {
738	–	int s = runState;
739	–	if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended
740	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
741	–	(s & ~SUSPENDED) |
742	–	(TRIMMED|TERMINATING))) {
743	–	LockSupport.unpark(this);
744	–	break;
745	–	}
746	–	}
747	–	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
748	–	s | TERMINATING))
749	–	break;
750	–	}
751	–	}
752	–
753	–	/**
754	–	* Sets state to TERMINATED. Called only by this thread.
755	–	*/
756	–	private void setTerminated() {
757	–	int s;
758	–	do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset,
759	–	s = runState,
760	–	s | (TERMINATING|TERMINATED)));
761	–	}
762	–
763	–	/**
764	–	* Instrumented version of park used by ForkJoinPool.awaitEvent
765	–	*/
766	–	final void doPark() {
767	–	++parkCount;
768	–	LockSupport.park(this);
769	–	}
770	–
771	–	/**
772	–	* If suspended, tries to set status to unsuspended and unparks.
773	–	*
774	–	* @return true if successful
775	–	*/
776	–	final boolean tryResumeSpare() {
777	–	int s = runState;
778	–	if ((s & SUSPENDED) != 0 &&
779	–	UNSAFE.compareAndSwapInt(this, runStateOffset, s,
780	–	s & ~SUSPENDED)) {
781	–	LockSupport.unpark(this);
782	–	return true;
783	–	}
784	–	return false;
785	–	}
786	–
787	–	/**
788	–	* Sets suspended status and blocks as spare until resumed,
789	–	* shutdown, or timed out.
790	–	*
791	–	* @return false if trimmed
792	–	*/
793	–	final boolean suspendAsSpare() {
794	–	for (;;) {               // set suspended unless terminating
795	–	int s = runState;
796	–	if ((s & TERMINATING) != 0) { // must kill
797	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
798	–	s | (TRIMMED | TERMINATING)))
799	–	return false;
800	–	}
801	–	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
802	–	s | SUSPENDED))
803	–	break;
804	–	}
805	–	boolean timed;
806	–	long nanos;
807	–	long startTime;
808	–	if (poolIndex < pool.parallelism) {
809	–	timed = false;
810	–	nanos = 0L;
811	–	startTime = 0L;
812	–	}
813	–	else {
814	–	timed = true;
815	–	nanos = SPARE_KEEPALIVE_NANOS;
816	–	startTime = System.nanoTime();
817	–	}
818	–	pool.accumulateStealCount(this);
819	–	lastEventCount = 0;      // reset upon resume
820	–	interrupted();           // clear/ignore interrupts
821	–	while ((runState & SUSPENDED) != 0) {
822	–	++parkCount;
823	–	if (!timed)
824	–	LockSupport.park(this);
825	–	else if ((nanos -= (System.nanoTime() - startTime)) > 0)
826	–	LockSupport.parkNanos(this, nanos);
827	–	else { // try to trim on timeout
828	–	int s = runState;
829	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
830	–	(s & ~SUSPENDED) |
831	–	(TRIMMED|TERMINATING)))
832	–	return false;
833	–	}
834	–	}
835	–	return true;
836	–	}
837	–
838	–	// Misc support methods for ForkJoinPool
839	–
840	–	/**
841	–	* Returns an estimate of the number of tasks in the queue.  Also
842	–	* used by ForkJoinTask.
843	–	*/
844	–	final int getQueueSize() {
845	–	return -base + sp;
846	–	}
847	–
848	–	/**
849	–	* Removes and cancels all tasks in queue.  Can be called from any
850	–	* thread.
851	–	*/
852	–	final void cancelTasks() {
853	–	ForkJoinTask<?> cj = currentJoin; // try to kill live tasks
854	–	if (cj != null) {
855	–	currentJoin = null;
856	–	cj.cancelIgnoringExceptions();
857	–	}
858	–	ForkJoinTask<?> cs = currentSteal;
859	–	if (cs != null) {
860	–	currentSteal = null;
861	–	cs.cancelIgnoringExceptions();
862	–	}
863	–	while (base != sp) {
864	–	ForkJoinTask<?> t = deqTask();
865	–	if (t != null)
866	–	t.cancelIgnoringExceptions();
867	–	}
868	–	}
869	–
870	–	/**
871	–	* Drains tasks to given collection c.
872	–	*
873	–	* @return the number of tasks drained
874	–	*/
875	–	final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
876	–	int n = 0;
877	–	while (base != sp) {
878	–	ForkJoinTask<?> t = deqTask();
879	–	if (t != null) {
880	–	c.add(t);
881	–	++n;
882	–	}
883	–	}
884	–	return n;
885	–	}
886	–
887	–	// Support methods for ForkJoinTask
888	–
889	–	/**
890	–	* Gets and removes a local task.
891	–	*
892	–	* @return a task, if available
893	–	*/
894	–	final ForkJoinTask<?> pollLocalTask() {
895	–	while (sp != base) {
896	–	if (active || (active = pool.tryIncrementActiveCount()))
897	–	return locallyFifo? locallyDeqTask() : popTask();
898	–	}
899	–	return null;
900	–	}
901	–
902	–	/**
903	–	* Gets and removes a local or stolen task.
904	–	*
905	–	* @return a task, if available
906	–	*/
907	–	final ForkJoinTask<?> pollTask() {
908	–	ForkJoinTask<?> t;
909	–	return (t = pollLocalTask()) != null ? t : scan();
910	–	}
911	–
912	–	/**
913	–	* Possibly runs some tasks and/or blocks, until task is done.
914	–	* The main body is basically a big spinloop, alternating between
915	–	* calls to helpJoinTask and pool.tryAwaitJoin with increased
916	–	* patience parameters until either the task is done without
917	–	* waiting, or we have, if necessary, created or resumed a
918	–	* replacement for this thread while it blocks.
919	–	*
920	–	* @param joinMe the task to join
921	–	* @return task status on exit
922	–	*/
923	–	final int joinTask(ForkJoinTask<?> joinMe) {
924	–	int stat;
925	–	ForkJoinTask<?> prevJoin = currentJoin;
926	–	currentJoin = joinMe;
927	–	if ((stat = joinMe.status) >= 0 &&
928	–	(sp == base || (stat = localHelpJoinTask(joinMe)) >= 0)) {
929	–	ForkJoinPool p = pool;
930	–	int helpRetries = 2;     // initial patience values
931	–	int awaitRetries = -1;   // -1 is sentinel for replace-check only
932	–	do {
933	–	helpJoinTask(joinMe, helpRetries);
934	–	if ((stat = joinMe.status) < 0)
935	–	break;
936	–	boolean busy = p.tryAwaitJoin(joinMe, awaitRetries);
937	–	if ((stat = joinMe.status) < 0)
938	–	break;
939	–	if (awaitRetries == -1)
940	–	awaitRetries = 0;
941	–	else if (busy)
942	–	++awaitRetries;
943	–	if (helpRetries < p.parallelism)
944	–	helpRetries <<= 1;
945	–	Thread.yield(); // tame unbounded loop
946	–	} while (joinMe.status >= 0);
947	–	}
948	–	currentJoin = prevJoin;
949	–	return stat;
950	–	}
951	–
952	–	/**
953	–	* Run tasks in local queue until given task is done.
954	–	*
955	–	* @param joinMe the task to join
956	–	* @return task status on exit
957	–	*/
958	–	private int localHelpJoinTask(ForkJoinTask<?> joinMe) {
959	–	int stat, s;
960	–	ForkJoinTask<?>[] q;
961	–	while ((stat = joinMe.status) >= 0 &&
962	–	base != (s = sp) && (q = queue) != null) {
963	–	ForkJoinTask<?> t;
964	–	int i = (q.length - 1) & --s;
965	–	long u = (i << qShift) + qBase; // raw offset
966	–	if ((t = q[i]) != null &&
967	–	UNSAFE.compareAndSwapObject(q, u, t, null)) {
968	–	/*
969	–	* This recheck (and similarly in helpJoinTask)
970	–	* handles cases where joinMe is independently
971	–	* cancelled or forced even though there is other work
972	–	* available. Back out of the pop by putting t back
973	–	* into slot before we commit by writing sp.
974	–	*/
975	–	if ((stat = joinMe.status) < 0) {
976	–	UNSAFE.putObjectVolatile(q, u, t);
977	–	break;
978	–	}
979	–	sp = s;
980	–	t.tryExec();
981	–	}
982	–	}
983	–	return stat;
984	–	}
985	–
986	–	/**
987	–	* Tries to locate and help perform tasks for a stealer of the
988	–	* given task, or in turn one of its stealers.  Traces
989	–	* currentSteal->currentJoin links looking for a thread working on
990	–	* a descendant of the given task and with a non-empty queue to
991	–	* steal back and execute tasks from. Restarts search upon
992	–	* encountering chains that are stale, unknown, or of length
993	–	* greater than MAX_HELP_DEPTH links, to avoid unbounded cycles.
994	–	*
995	–	* The implementation is very branchy to cope with the restart
996	–	* cases.  Returns void, not task status (which must be reread by
997	–	* caller anyway) to slightly simplify control paths.
998	–	*
999	–	* @param joinMe the task to join
1000	–	*/
1001	–	final void helpJoinTask(ForkJoinTask<?> joinMe, int retries) {
1002	–	ForkJoinWorkerThread[] ws = pool.workers;
1003	–	int n;
1004	–	if (ws == null || (n = ws.length) <= 1)
1005	–	return;                   // need at least 2 workers
1006	–
1007	–	restart:while (joinMe.status >= 0 && --retries >= 0) {
1008	–	ForkJoinTask<?> task = joinMe;        // base of chain
1009	–	ForkJoinWorkerThread thread = this;   // thread with stolen task
1010	–	for (int depth = 0; depth < MAX_HELP_DEPTH; ++depth) {
1011	–	// Try to find v, the stealer of task, by first using hint
1012	–	ForkJoinWorkerThread v = ws[thread.stealHint & (n - 1)];
1013	–	if (v == null || v.currentSteal != task) {
1014	–	for (int j = 0; ; ++j) {      // search array
1015	–	if (task.status < 0 || j == n)
1016	–	continue restart;     // stale or no stealer
1017	–	if ((v = ws[j]) != null && v.currentSteal == task) {
1018	–	thread.stealHint = j; // save for next time
1019	–	break;
1020	–	}
1021	–	}
1022	–	}
1023	–	// Try to help v, using specialized form of deqTask
1024	–	int b;
1025	–	ForkJoinTask<?>[] q;
1026	–	while ((b = v.base) != v.sp && (q = v.queue) != null) {
1027	–	int i = (q.length - 1) & b;
1028	–	long u = (i << qShift) + qBase;
1029	–	ForkJoinTask<?> t = q[i];
1030	–	if (task.status < 0)          // stale
1031	–	continue restart;
1032	–	if (v.base == b) {            // recheck after reading t
1033	–	if (t == null)            // producer stalled
1034	–	continue restart;     // retry via restart
1035	–	if (UNSAFE.compareAndSwapObject(q, u, t, null)) {
1036	–	if (joinMe.status < 0) {
1037	–	UNSAFE.putObjectVolatile(q, u, t);
1038	–	return;           // back out on cancel
1039	–	}
1040	–	ForkJoinTask<?> prevSteal = currentSteal;
1041	–	currentSteal = t;
1042	–	v.stealHint = poolIndex;
1043	–	v.base = b + 1;
1044	–	t.tryExec();
1045	–	currentSteal = prevSteal;
1046	–	}
1047	–	}
1048	–	if (joinMe.status < 0)
1049	–	return;
1050	–	}
1051	–	// Try to descend to find v's stealer
1052	–	ForkJoinTask<?> next = v.currentJoin;
1053	–	if (next == null || task.status < 0)
1054	–	continue restart;             // no descendent or stale
1055	–	if (joinMe.status < 0)
1056	–	return;
1057	–	task = next;
1058	–	thread = v;
1059	–	}
1060	–	}
1061	–	}
1062	–
1063	–	/**
1064	–	* Returns an estimate of the number of tasks, offset by a
1065	–	* function of number of idle workers.
1066	–	*
1067	–	* This method provides a cheap heuristic guide for task
1068	–	* partitioning when programmers, frameworks, tools, or languages
1069	–	* have little or no idea about task granularity.  In essence by
1070	–	* offering this method, we ask users only about tradeoffs in
1071	–	* overhead vs expected throughput and its variance, rather than
1072	–	* how finely to partition tasks.
1073	–	*
1074	–	* In a steady state strict (tree-structured) computation, each
1075	–	* thread makes available for stealing enough tasks for other
1076	–	* threads to remain active. Inductively, if all threads play by
1077	–	* the same rules, each thread should make available only a
1078	–	* constant number of tasks.
1079	–	*
1080	–	* The minimum useful constant is just 1. But using a value of 1
1081	–	* would require immediate replenishment upon each steal to
1082	–	* maintain enough tasks, which is infeasible.  Further,
1083	–	* partitionings/granularities of offered tasks should minimize
1084	–	* steal rates, which in general means that threads nearer the top
1085	–	* of computation tree should generate more than those nearer the
1086	–	* bottom. In perfect steady state, each thread is at
1087	–	* approximately the same level of computation tree. However,
1088	–	* producing extra tasks amortizes the uncertainty of progress and
1089	–	* diffusion assumptions.
1090	–	*
1091	–	* So, users will want to use values larger, but not much larger
1092	–	* than 1 to both smooth over transient shortages and hedge
1093	–	* against uneven progress; as traded off against the cost of
1094	–	* extra task overhead. We leave the user to pick a threshold
1095	–	* value to compare with the results of this call to guide
1096	–	* decisions, but recommend values such as 3.
1097	–	*
1098	–	* When all threads are active, it is on average OK to estimate
1099	–	* surplus strictly locally. In steady-state, if one thread is
1100	–	* maintaining say 2 surplus tasks, then so are others. So we can
1101	–	* just use estimated queue length (although note that (sp - base)
1102	–	* can be an overestimate because of stealers lagging increments
1103	–	* of base).  However, this strategy alone leads to serious
1104	–	* mis-estimates in some non-steady-state conditions (ramp-up,
1105	–	* ramp-down, other stalls). We can detect many of these by
1106	–	* further considering the number of "idle" threads, that are
1107	–	* known to have zero queued tasks, so compensate by a factor of
1108	–	* (#idle/#active) threads.
1109	–	*/
1110	–	final int getEstimatedSurplusTaskCount() {
1111	–	return sp - base - pool.idlePerActive();
1112	–	}
1113	–
1114	–	/**
1115	–	* Runs tasks until {@code pool.isQuiescent()}.
1116	–	*/
1117	–	final void helpQuiescePool() {
1118	–	for (;;) {
1119	–	ForkJoinTask<?> t = pollLocalTask();
1120	–	if (t != null || (t = scan()) != null) {
1121	–	t.tryExec();
1122	–	currentSteal = null;
1123	–	}
1124	–	else {
1125	–	ForkJoinPool p = pool;
1126	–	if (active) {
1127	–	active = false; // inactivate
1128	–	do {} while (!p.tryDecrementActiveCount());
1129	–	}
1130	–	if (p.isQuiescent()) {
1131	–	active = true; // re-activate
1132	–	do {} while (!p.tryIncrementActiveCount());
1133	–	return;
1134	–	}
1135	–	}
1136	–	}
1137	–	}
1138	–
1139	–	// Unsafe mechanics
1140	–
1141	–	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
1142	–	private static final long runStateOffset =
1143	–	objectFieldOffset("runState", ForkJoinWorkerThread.class);
1144	–	private static final long qBase =
1145	–	UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
1146	–	private static final long threadStatusOffset =
1147	–	objectFieldOffset("threadStatus", Thread.class);
1148	–	private static final int qShift;
1149	–
1150	–	static {
1151	–	int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
1152	–	if ((s & (s-1)) != 0)
1153	–	throw new Error("data type scale not a power of two");
1154	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
1155	–	}
1156	–
1157	–	private static long objectFieldOffset(String field, Class<?> klazz) {
1158	–	try {
1159	–	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1160	–	} catch (NoSuchFieldException e) {
1161	–	// Convert Exception to corresponding Error
1162	–	NoSuchFieldError error = new NoSuchFieldError(field);
1163	–	error.initCause(e);
1164	–	throw error;
1165	–	}
1166	–	}
1167	–
1168	–	/**
1169	–	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
1170	–	* Replace with a simple call to Unsafe.getUnsafe when integrating
1171	–	* into a jdk.
1172	–	*
1173	–	* @return a sun.misc.Unsafe
1174	–	*/
1175	–	private static sun.misc.Unsafe getUnsafe() {
1176	–	try {
1177	–	return sun.misc.Unsafe.getUnsafe();
1178	–	} 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");
1186	<	f.setAccessible(true);
1187	<	return (sun.misc.Unsafe) f.get(null);
1188	<	}});
1189	<	} catch (java.security.PrivilegedActionException e) {
1190	<	throw new RuntimeException("Could not initialize intrinsics",
1191	<	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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