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

Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.35 by dl, Wed Jul 7 19:52:32 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 stolen) the most recent task it stole
89	<	* from some other worker. Plus, it records (in field joining) the
90	<	* 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	<	* requires a linear scan of workers array to locate stealers,
102	<	* which isolates cost to when it is needed, rather than adding to
103	<	* per-task overhead.  (2) It is "shallow", ignoring nesting and
104	<	* potentially cyclic mutual steals.  (3) It is intentionally
105	<	* racy: field joining is updated only while actively joining,
106	<	* which means that we could miss links in the chain during
107	<	* long-lived tasks, GC stalls etc.  (4) We fall back to
108	<	* suspending the worker and if necessary replacing it with a
109	<	* spare (see ForkJoinPool.tryAwaitJoin).
110	<	*
111	<	* Efficient implementation of these algorithms currently relies on
112	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
113	<	* correct orderings, reads and writes of variable base require
114	<	* volatile ordering.  Variable sp does not require volatile
115	<	* writes but still needs store-ordering, which we accomplish by
116	<	* pre-incrementing sp before filling the slot with an ordered
117	<	* store.  (Pre-incrementing also enables backouts used in
118	<	* scanWhileJoining.)  Because they are protected by volatile base
119	<	* reads, reads of the queue array and its slots by other threads
120	<	* do not need volatile load semantics, but writes (in push)
121	<	* require store order and CASes (in pop and deq) require
122	<	* (volatile) CAS semantics.  (Michael, Saraswat, and Vechev's
123	<	* algorithm has similar properties, but without support for
124	<	* nulling slots.)  Since these combinations aren't supported
125	<	* using ordinary volatiles, the only way to accomplish these
126	<	* efficiently is to use direct Unsafe calls. (Using external
127	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
128	<	* array is significantly slower because of memory locality and
129	<	* indirection effects.)
130	<	*
131	<	* Further, performance on most platforms is very sensitive to
132	<	* placement and sizing of the (resizable) queue array.  Even
133	<	* though these queues don't usually become all that big, the
134	<	* initial size must be large enough to counteract cache
135	<	* contention effects across multiple queues (especially in the
136	<	* presence of GC cardmarking). Also, to improve thread-locality,
137	<	* queues are initialized after starting.  All together, these
138	<	* low-level implementation choices produce as much as a factor of
139	<	* 4 performance improvement compared to naive implementations,
140	<	* and enable the processing of billions of tasks per second,
141	<	* sometimes at the expense of ugliness.
142	<	*/
143	<
144	<	/**
145	<	* Generator for initial random seeds for random victim
146	<	* selection. This is used only to create initial seeds. Random
147	<	* steals use a cheaper xorshift generator per steal attempt. We
148	<	* expect only rare contention on seedGenerator, so just use a
149	<	* plain Random.
150	<	*/
151	<	private static final Random seedGenerator = new Random();
152	<
153	<	/**
154	<	* The timeout value for suspending spares. Spare workers that
155	<	* remain unsignalled for more than this time may be trimmed
156	<	* (killed and removed from pool).  Since our goal is to avoid
157	<	* long-term thread buildup, the exact value of timeout does not
158	<	* matter too much so long as it avoids most false-alarm timeouts
159	<	* under GC stalls or momentarily high system load.
160	<	*/
161	<	private static final long SPARE_KEEPALIVE_NANOS =
162	<	5L * 1000L * 1000L * 1000L; // 5 secs
163	<
164	<	/**
165	<	* Capacity of work-stealing queue array upon initialization.
166	<	* Must be a power of two. Initial size must be at least 4, but is
167	<	* padded to minimize cache effects.
168	<	*/
169	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
170	<
171	<	/**
172	<	* Maximum work-stealing queue array size.  Must be less than or
173	<	* equal to 1 << 28 to ensure lack of index wraparound. (This
174	<	* is less than usual bounds, because we need leftshift by 3
175	<	* to be in int range).
176	<	*/
177	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28;
178	<
179	<	/**
180	<	* The pool this thread works in. Accessed directly by ForkJoinTask.
181	<	*/
182	<	final ForkJoinPool pool;
183	<
184	<	/**
185	<	* The task most recently stolen from another worker
186	<	*/
187	<	private volatile ForkJoinTask<?> stolen;
188	<
189	<	/**
190	<	* The task currently being joined, set only when actively
191	<	* trying to helpStealer.
192	<	*/
193	<	private volatile ForkJoinTask<?> joining;
194	<
195	<	/**
196	<	* The work-stealing queue array. Size must be a power of two.
197	<	* Initialized in onStart, to improve memory locality.
198	<	*/
199	<	private ForkJoinTask<?>[] queue;
200	<
201	<	/**
202	<	* Index (mod queue.length) of least valid queue slot, which is
203	<	* always the next position to steal from if nonempty.
204	<	*/
205	<	private volatile int base;
206	<
207	<	/**
208	<	* Index (mod queue.length) of next queue slot to push to or pop
209	<	* from. It is written only by owner thread, and accessed by other
210	<	* threads only after reading (volatile) base.  Both sp and base
211	<	* are allowed to wrap around on overflow, but (sp - base) still
212	<	* estimates size.
213	<	*/
214	<	private int sp;
215	<
216	<	/**
217	<	* Run state of this worker. In addition to the usual run levels,
218	<	* tracks if this worker is suspended as a spare, and if it was
219	<	* killed (trimmed) while suspended. However, "active" status is
220	<	* maintained separately.
221	<	*/
222	<	private volatile int runState;
223	<
224	<	private static final int TERMINATING = 0x01;
225	<	private static final int TERMINATED  = 0x02;
226	<	private static final int SUSPENDED   = 0x04; // inactive spare
227	<	private static final int TRIMMED     = 0x08; // killed while suspended
228	<
229	<	/**
230	<	* Number of LockSupport.park calls to block this thread for
231	<	* suspension or event waits. Used for internal instrumention;
232	<	* currently not exported but included because volatile write upon
233	<	* park also provides a workaround for a JVM bug.
234	<	*/
235	<	volatile int parkCount;
236	<
237	<	/**
238	<	* Number of steals, transferred and reset in pool callbacks pool
239	<	* when idle Accessed directly by pool.
240	<	*/
241	<	int stealCount;
242	<
243	<	/**
244	<	* Seed for random number generator for choosing steal victims.
245	<	* Uses Marsaglia xorshift. Must be initialized as nonzero.
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28		*/
247	–	private int seed;
29
30	<	/**
31	<	* Activity status. When true, this worker is considered active.
251	<	* Accessed directly by pool.  Must be false upon construction.
252	<	*/
253	<	boolean active;
254	<
255	<	/**
256	<	* True if use local fifo, not default lifo, for local polling.
257	<	* Shadows value from ForkJoinPool, which resets it if changed
258	<	* pool-wide.
259	<	*/
260	<	private final boolean locallyFifo;
261	<
262	<	/**
263	<	* Index of this worker in pool array. Set once by pool before
264	<	* running, and accessed directly by pool to locate this worker in
265	<	* its workers array.
266	<	*/
267	<	int poolIndex;
268	<
269	<	/**
270	<	* The last pool event waited for. Accessed only by pool in
271	<	* callback methods invoked within this thread.
272	<	*/
273	<	int lastEventCount;
274	<
275	<	/**
276	<	* Encoded index and event count of next event waiter. Used only
277	<	* by ForkJoinPool for managing event waiters.
278	<	*/
279	<	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 285 | 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,
291	<	// remaining initialization is in start() and onStart()
292	<	}
293	<
294	<	/**
295	<	* Performs additional initialization and starts this thread
296	<	*/
297	<	final void start(int poolIndex, UncaughtExceptionHandler ueh) {
298	<	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
305	–	// Public/protected methods
306	–
50		/**
51		* Returns the pool hosting this thread.
52		*
#	Line 323 | 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() {
339	–	int rs = seedGenerator.nextInt();
340	–	seed = rs == 0? 1 : rs; // seed must be nonzero
341	–
342	–	// Allocate name string and arrays in this thread
343	–	String pid = Integer.toString(pool.getPoolNumber());
344	–	String wid = Integer.toString(poolIndex);
345	–	setName("ForkJoinPool-" + pid + "-worker-" + wid);
346	–
347	–	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
82		}
83
84		/**
#	Line 356 | Line 90 | public class ForkJoinWorkerThread extend
90		* to an unrecoverable error, or {@code null} if completed normally
91		*/
92		protected void onTermination(Throwable exception) {
359	–	try {
360	–	stolen = null;
361	–	joining = null;
362	–	cancelTasks();
363	–	setTerminated();
364	–	pool.workerTerminated(this);
365	–	} catch (Throwable ex) {        // Shouldn't ever happen
366	–	if (exception == null)      // but if so, at least rethrown
367	–	exception = ex;
368	–	} finally {
369	–	if (exception != null)
370	–	UNSAFE.throwException(exception);
371	–	}
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 {
387	–	onTermination(exception);
388	–	}
389	–	}
390	–
391	–	// helpers for run()
392	–
393	–	/**
394	–	* Find and execute tasks and check status while running
395	–	*/
396	–	private void mainLoop() {
397	–	boolean ran = false;      // true if ran task in last loop iter
398	–	boolean prevRan = false;  // true if ran on last or previous step
399	–	ForkJoinPool p = pool;
400	–	for (;;) {
401	–	p.preStep(this, prevRan);
402	–	if (runState != 0)
403	–	return;
404	–	ForkJoinTask<?> t; // try to get and run stolen or submitted task
405	–	if ((t = scan()) != null || (t = pollSubmission()) != null) {
406	–	t.tryExec();
407	–	if (base != sp)
408	–	runLocalTasks();
409	–	stolen = null;
410	–	prevRan = ran = true;
411	–	}
412	–	else {
413	–	prevRan = ran;
414	–	ran = false;
415	–	}
416	–	}
417	–	}
418	–
419	–	/**
420	–	* Runs local tasks until queue is empty or shut down.  Call only
421	–	* while active.
422	–	*/
423	–	private void runLocalTasks() {
424	–	while (runState == 0) {
425	–	ForkJoinTask<?> t = locallyFifo? locallyDeqTask() : popTask();
426	–	if (t != null)
427	–	t.tryExec();
428	–	else if (base == sp)
429	–	break;
430	–	}
431	–	}
432	–
433	–	/**
434	–	* If a submission exists, try to activate and take it
435	–	*
436	–	* @return a task, if available
437	–	*/
438	–	private ForkJoinTask<?> pollSubmission() {
439	–	ForkJoinPool p = pool;
440	–	while (p.hasQueuedSubmissions()) {
441	–	if (active || (active = p.tryIncrementActiveCount())) {
442	–	ForkJoinTask<?> t = p.pollSubmission();
443	–	return t != null ? t : scan(); // if missed, rescan
444	–	}
445	–	}
446	–	return null;
447	–	}
448	–
449	–	/*
450	–	* Intrinsics-based atomic writes for queue slots. These are
451	–	* basically the same as methods in AtomicObjectArray, but
452	–	* specialized for (1) ForkJoinTask elements (2) requirement that
453	–	* nullness and bounds checks have already been performed by
454	–	* callers and (3) effective offsets are known not to overflow
455	–	* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
456	–	* need corresponding version for reads: plain array reads are OK
457	–	* because they protected by other volatile reads and are
458	–	* confirmed by CASes.
459	–	*
460	–	* Most uses don't actually call these methods, but instead contain
461	–	* inlined forms that enable more predictable optimization.  We
462	–	* don't define the version of write used in pushTask at all, but
463	–	* instead inline there a store-fenced array slot write.
464	–	*/
465	–
466	–	/**
467	–	* CASes slot i of array q from t to null. Caller must ensure q is
468	–	* non-null and index is in range.
469	–	*/
470	–	private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
471	–	ForkJoinTask<?> t) {
472	–	return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
473	–	}
474	–
475	–	/**
476	–	* Performs a volatile write of the given task at given slot of
477	–	* array q.  Caller must ensure q is non-null and index is in
478	–	* range. This method is used only during resets and backouts.
479	–	*/
480	–	private static final void writeSlot(ForkJoinTask<?>[] q, int i,
481	–	ForkJoinTask<?> t) {
482	–	UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t);
483	–	}
484	–
485	–	// queue methods
486	–
487	–	/**
488	–	* Pushes a task. Call only from this thread.
489	–	*
490	–	* @param t the task. Caller must ensure non-null.
491	–	*/
492	–	final void pushTask(ForkJoinTask<?> t) {
493	–	ForkJoinTask<?>[] q = queue;
494	–	int mask = q.length - 1; // implicit assert q != null
495	–	int s = sp++;            // ok to increment sp before slot write
496	–	UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t);
497	–	if ((s -= base) == 0)
498	–	pool.signalWork();   // was empty
499	–	else if (s == mask)
500	–	growQueue();         // is full
501	–	}
502	–
503	–	/**
504	–	* Tries to take a task from the base of the queue, failing if
505	–	* empty or contended. Note: Specializations of this code appear
506	–	* in locallyDeqTask and elsewhere.
507	–	*
508	–	* @return a task, or null if none or contended
509	–	*/
510	–	final ForkJoinTask<?> deqTask() {
511	–	ForkJoinTask<?> t;
512	–	ForkJoinTask<?>[] q;
513	–	int b, i;
514	–	if ((b = base) != sp &&
515	–	(q = queue) != null && // must read q after b
516	–	(t = q[i = (q.length - 1) & b]) != null && base == b &&
517	–	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
518	–	base = b + 1;
519	–	return t;
520	–	}
521	–	return null;
522	–	}
523	–
524	–	/**
525	–	* Tries to take a task from the base of own queue. Assumes active
526	–	* status.  Called only by current thread.
527	–	*
528	–	* @return a task, or null if none
529	–	*/
530	–	final ForkJoinTask<?> locallyDeqTask() {
531	–	ForkJoinTask<?>[] q = queue;
532	–	if (q != null) {
533	–	ForkJoinTask<?> t;
534	–	int b, i;
535	–	while (sp != (b = base)) {
536	–	if ((t = q[i = (q.length - 1) & b]) != null && base == b &&
537	–	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase,
538	–	t, null)) {
539	–	base = b + 1;
540	–	return t;
541	–	}
542	–	}
543	–	}
544	–	return null;
545	–	}
546	–
547	–	/**
548	–	* Returns a popped task, or null if empty. Assumes active status.
549	–	* Called only by current thread. (Note: a specialization of this
550	–	* code appears in popWhileJoining.)
551	–	*/
552	–	final ForkJoinTask<?> popTask() {
553	–	int s;
554	–	ForkJoinTask<?>[] q;
555	–	if (base != (s = sp) && (q = queue) != null) {
556	–	int i = (q.length - 1) & --s;
557	–	ForkJoinTask<?> t = q[i];
558	–	if (t != null && UNSAFE.compareAndSwapObject
559	–	(q, (i << qShift) + qBase, t, null)) {
560	–	sp = s;
561	–	return t;
562	–	}
563	–	}
564	–	return null;
565	–	}
566	–
567	–	/**
568	–	* Specialized version of popTask to pop only if topmost element
569	–	* is the given task. Called only by current thread while
570	–	* active.
571	–	*
572	–	* @param t the task. Caller must ensure non-null.
573	–	*/
574	–	final boolean unpushTask(ForkJoinTask<?> t) {
575	–	int s;
576	–	ForkJoinTask<?>[] q;
577	–	if (base != (s = sp) && (q = queue) != null &&
578	–	UNSAFE.compareAndSwapObject
579	–	(q, (((q.length - 1) & --s) << qShift) + qBase, t, null)) {
580	–	sp = s;
581	–	return true;
582	–	}
583	–	return false;
584	–	}
585	–
586	–	/**
587	–	* Returns next task or null if empty or contended
588	–	*/
589	–	final ForkJoinTask<?> peekTask() {
590	–	ForkJoinTask<?>[] q = queue;
591	–	if (q == null)
592	–	return null;
593	–	int mask = q.length - 1;
594	–	int i = locallyFifo ? base : (sp - 1);
595	–	return q[i & mask];
596	–	}
597	–
598	–	/**
599	–	* Doubles queue array size. Transfers elements by emulating
600	–	* steals (deqs) from old array and placing, oldest first, into
601	–	* new array.
602	–	*/
603	–	private void growQueue() {
604	–	ForkJoinTask<?>[] oldQ = queue;
605	–	int oldSize = oldQ.length;
606	–	int newSize = oldSize << 1;
607	–	if (newSize > MAXIMUM_QUEUE_CAPACITY)
608	–	throw new RejectedExecutionException("Queue capacity exceeded");
609	–	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
610	–
611	–	int b = base;
612	–	int bf = b + oldSize;
613	–	int oldMask = oldSize - 1;
614	–	int newMask = newSize - 1;
615	–	do {
616	–	int oldIndex = b & oldMask;
617	–	ForkJoinTask<?> t = oldQ[oldIndex];
618	–	if (t != null && !casSlotNull(oldQ, oldIndex, t))
619	–	t = null;
620	–	writeSlot(newQ, b & newMask, t);
621	–	} while (++b != bf);
622	–	pool.signalWork();
623	–	}
624	–
625	–	/**
626	–	* Computes next value for random victim probe in scan().  Scans
627	–	* don't require a very high quality generator, but also not a
628	–	* crummy one.  Marsaglia xor-shift is cheap and works well enough.
629	–	* Note: This is manually inlined in scan()
630	–	*/
631	–	private static final int xorShift(int r) {
632	–	r ^= r << 13;
633	–	r ^= r >>> 17;
634	–	return r ^ (r << 5);
635	–	}
636	–
637	–	/**
638	–	* Tries to steal a task from another worker. Starts at a random
639	–	* index of workers array, and probes workers until finding one
640	–	* with non-empty queue or finding that all are empty.  It
641	–	* randomly selects the first n probes. If these are empty, it
642	–	* resorts to a circular sweep, which is necessary to accurately
643	–	* set active status. (The circular sweep uses steps of
644	–	* approximately half the array size plus 1, to avoid bias
645	–	* stemming from leftmost packing of the array in ForkJoinPool.)
646	–	*
647	–	* This method must be both fast and quiet -- usually avoiding
648	–	* memory accesses that could disrupt cache sharing etc other than
649	–	* those needed to check for and take tasks (or to activate if not
650	–	* already active). This accounts for, among other things,
651	–	* updating random seed in place without storing it until exit.
652	–	*
653	–	* @return a task, or null if none found
654	–	*/
655	–	private ForkJoinTask<?> scan() {
656	–	ForkJoinPool p = pool;
657	–	ForkJoinWorkerThread[] ws;        // worker array
658	–	int n;                            // upper bound of #workers
659	–	if ((ws = p.workers) != null && (n = ws.length) > 1) {
660	–	boolean canSteal = active;    // shadow active status
661	–	int r = seed;                 // extract seed once
662	–	int mask = n - 1;
663	–	int j = -n;                   // loop counter
664	–	int k = r;                    // worker index, random if j < 0
665	–	for (;;) {
666	–	ForkJoinWorkerThread v = ws[k & mask];
667	–	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift
668	–	if (v != null && v.base != v.sp) {
669	–	if (canSteal ||       // ensure active status
670	–	(canSteal = active = p.tryIncrementActiveCount())) {
671	–	int b = v.base;   // inline specialized deqTask
672	–	ForkJoinTask<?>[] q;
673	–	if (b != v.sp && (q = v.queue) != null) {
674	–	ForkJoinTask<?> t;
675	–	int i = (q.length - 1) & b;
676	–	long u = (i << qShift) + qBase; // raw offset
677	–	if ((t = q[i]) != null && v.base == b &&
678	–	UNSAFE.compareAndSwapObject(q, u, t, null)) {
679	–	stolen = t;
680	–	v.base = b + 1;
681	–	seed = r;
682	–	++stealCount;
683	–	return t;
684	–	}
685	–	}
686	–	}
687	–	j = -n;
688	–	k = r;                // restart on contention
689	–	}
690	–	else if (++j <= 0)
691	–	k = r;
692	–	else if (j <= n)
693	–	k += (n >>> 1) | 1;
694	–	else
695	–	break;
696	–	}
697	–	}
698	–	return null;
699	–	}
700	–
701	–	// Run State management
702	–
703	–	// status check methods used mainly by ForkJoinPool
704	–	final boolean isTerminating() { return (runState & TERMINATING) != 0; }
705	–	final boolean isTerminated()  { return (runState & TERMINATED) != 0; }
706	–	final boolean isSuspended()   { return (runState & SUSPENDED) != 0; }
707	–	final boolean isTrimmed()     { return (runState & TRIMMED) != 0; }
708	–
709	–	/**
710	–	* Sets state to TERMINATING, also resuming if suspended.
711	–	*/
712	–	final void shutdown() {
713	–	for (;;) {
714	–	int s = runState;
715	–	if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended
716	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
717	–	(s & ~SUSPENDED) |
718	–	(TRIMMED|TERMINATING))) {
719	–	LockSupport.unpark(this);
720	–	break;
721	–	}
722	–	}
723	–	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
724	–	s | TERMINATING))
725	–	break;
726	–	}
727	–	}
728	–
729	–	/**
730	–	* Sets state to TERMINATED. Called only by this thread.
731	–	*/
732	–	private void setTerminated() {
733	–	int s;
734	–	do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset,
735	–	s = runState,
736	–	s | (TERMINATING|TERMINATED)));
737	–	}
738	–
739	–	/**
740	–	* Instrumented version of park used by ForkJoinPool.awaitEvent
741	–	*/
742	–	final void doPark() {
743	–	++parkCount;
744	–	LockSupport.park(this);
745	–	}
746	–
747	–	/**
748	–	* If suspended, tries to set status to unsuspended.
749	–	* Caller must unpark to actually resume
750	–	*
751	–	* @return true if successful
752	–	*/
753	–	final boolean tryUnsuspend() {
754	–	int s = runState;
755	–	if ((s & SUSPENDED) != 0)
756	–	return UNSAFE.compareAndSwapInt(this, runStateOffset, s,
757	–	s & ~SUSPENDED);
758	–	return false;
759	–	}
760	–
761	–	/**
762	–	* Sets suspended status and blocks as spare until resumed,
763	–	* shutdown, or timed out.
764	–	*
765	–	* @return false if trimmed
766	–	*/
767	–	final boolean suspendAsSpare() {
768	–	for (;;) {               // set suspended unless terminating
769	–	int s = runState;
770	–	if ((s & TERMINATING) != 0) { // must kill
771	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
772	–	s | (TRIMMED | TERMINATING)))
773	–	return false;
774	–	}
775	–	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
776	–	s | SUSPENDED))
777	–	break;
778	–	}
779	–	boolean timed;
780	–	long nanos;
781	–	long startTime;
782	–	if (poolIndex < pool.parallelism) {
783	–	timed = false;
784	–	nanos = 0L;
785	–	startTime = 0L;
786	–	}
787	–	else {
788	–	timed = true;
789	–	nanos = SPARE_KEEPALIVE_NANOS;
790	–	startTime = System.nanoTime();
791	–	}
792	–	pool.accumulateStealCount(this);
793	–	lastEventCount = 0;      // reset upon resume
794	–	interrupted();           // clear/ignore interrupts
795	–	while ((runState & SUSPENDED) != 0) {
796	–	++parkCount;
797	–	if (!timed)
798	–	LockSupport.park(this);
799	–	else if ((nanos -= (System.nanoTime() - startTime)) > 0)
800	–	LockSupport.parkNanos(this, nanos);
801	–	else { // try to trim on timeout
802	–	int s = runState;
803	–	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
804	–	(s & ~SUSPENDED) |
805	–	(TRIMMED|TERMINATING)))
806	–	return false;
807	–	}
808	–	}
809	–	return true;
810	–	}
811	–
812	–	// Misc support methods for ForkJoinPool
813	–
814	–	/**
815	–	* Returns an estimate of the number of tasks in the queue.  Also
816	–	* used by ForkJoinTask.
817	–	*/
818	–	final int getQueueSize() {
819	–	return -base + sp;
820	–	}
821	–
822	–	/**
823	–	* Removes and cancels all tasks in queue.  Can be called from any
824	–	* thread.
825	–	*/
826	–	final void cancelTasks() {
827	–	while (base != sp) {
828	–	ForkJoinTask<?> t = deqTask();
829	–	if (t != null)
830	–	t.cancelIgnoringExceptions();
831	–	}
832	–	}
833	–
834	–	/**
835	–	* Drains tasks to given collection c.
836	–	*
837	–	* @return the number of tasks drained
838	–	*/
839	–	final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
840	–	int n = 0;
841	–	while (base != sp) {
842	–	ForkJoinTask<?> t = deqTask();
843	–	if (t != null) {
844	–	c.add(t);
845	–	++n;
846	–	}
847	–	}
848	–	return n;
849	–	}
850	–
851	–	// Support methods for ForkJoinTask
852	–
853	–	/**
854	–	* Possibly runs some tasks and/or blocks, until task is done.
855	–	*
856	–	* @param joinMe the task to join
857	–	*/
858	–	final void joinTask(ForkJoinTask<?> joinMe) {
859	–	ForkJoinTask<?> prevJoining = joining;
860	–	joining = joinMe;
861	–	while (joinMe.status >= 0) {
862	–	int s = sp;
863	–	if (s == base) {
864	–	nonlocalJoinTask(joinMe);
865	–	break;
866	–	}
867	–	// process local task
868	–	ForkJoinTask<?> t;
869	–	ForkJoinTask<?>[] q = queue;
870	–	int i = (q.length - 1) & --s;
871	–	long u = (i << qShift) + qBase; // raw offset
872	–	if ((t = q[i]) != null &&
873	–	UNSAFE.compareAndSwapObject(q, u, t, null)) {
874	–	/*
875	–	* This recheck (and similarly in nonlocalJoinTask)
876	–	* handles cases where joinMe is independently
877	–	* cancelled or forced even though there is other work
878	–	* available. Back out of the pop by putting t back
879	–	* into slot before we commit by setting sp.
880	–	*/
881	–	if (joinMe.status < 0) {
882	–	UNSAFE.putObjectVolatile(q, u, t);
883	–	break;
884	–	}
885	–	sp = s;
886	–	t.tryExec();
887	–	}
888	–	}
889	–	joining = prevJoining;
890	–	}
891	–
892	–	/**
893	–	* Tries to locate and help perform tasks for a stealer of the
894	–	* given task (or in turn one of its stealers), blocking (via
895	–	* pool.tryAwaitJoin) upon failure to find work.  Traces
896	–	* stolen->joining links looking for a thread working on
897	–	* a descendant of the given task and with a non-empty queue to
898	–	* steal back and execute tasks from. Inhibits mutual steal chains
899	–	* and scans on outer joins upon nesting to avoid unbounded
900	–	* growth.  Restarts search upon encountering inconsistencies.
901	–	* Tries to block if two passes agree that there are no remaining
902	–	* targets.
903	–	*
904	–	* @param joinMe the task to join
905	–	*/
906	–	private void nonlocalJoinTask(ForkJoinTask<?> joinMe) {
907	–	ForkJoinPool p = pool;
908	–	int scans = p.parallelism;       // give up if too many retries
909	–	ForkJoinTask<?> bottom = null;   // target seen when can't descend
910	–	restart: while (joinMe.status >= 0) {
911	–	ForkJoinTask<?> target = null;
912	–	ForkJoinTask<?> next = joinMe;
913	–	while (scans >= 0 && next != null) {
914	–	--scans;
915	–	target = next;
916	–	next = null;
917	–	ForkJoinWorkerThread v = null;
918	–	ForkJoinWorkerThread[] ws = p.workers;
919	–	int n = ws.length;
920	–	for (int j = 0; j < n; ++j) {
921	–	ForkJoinWorkerThread w = ws[j];
922	–	if (w != null && w.stolen == target) {
923	–	v = w;
924	–	break;
925	–	}
926	–	}
927	–	if (v != null && v != this) {
928	–	ForkJoinTask<?> prevStolen = stolen;
929	–	int b;
930	–	ForkJoinTask<?>[] q;
931	–	while ((b = v.base) != v.sp && (q = v.queue) != null) {
932	–	int i = (q.length - 1) & b;
933	–	long u = (i << qShift) + qBase;
934	–	ForkJoinTask<?> t = q[i];
935	–	if (target.status < 0)
936	–	continue restart;
937	–	if (t != null && v.base == b &&
938	–	UNSAFE.compareAndSwapObject(q, u, t, null)) {
939	–	if (joinMe.status < 0) {
940	–	UNSAFE.putObjectVolatile(q, u, t);
941	–	return; // back out
942	–	}
943	–	stolen = t;
944	–	v.base = b + 1;
945	–	t.tryExec();
946	–	stolen = prevStolen;
947	–	}
948	–	if (joinMe.status < 0)
949	–	return;
950	–	}
951	–	next = v.joining;
952	–	}
953	–	if (target.status < 0)
954	–	continue restart;  // inconsistent
955	–	if (joinMe.status < 0)
956	–	return;
957	–	}
958	–
959	–	if (bottom != target)
960	–	bottom = target;    // recheck landing spot
961	–	else if (p.tryAwaitJoin(joinMe) < 0)
962	–	return;             // successfully blocked
963	–	Thread.yield();         // tame spin in case too many active
964	–	}
965	–	}
966	–
967	–	/**
968	–	* Returns an estimate of the number of tasks, offset by a
969	–	* function of number of idle workers.
970	–	*
971	–	* This method provides a cheap heuristic guide for task
972	–	* partitioning when programmers, frameworks, tools, or languages
973	–	* have little or no idea about task granularity.  In essence by
974	–	* offering this method, we ask users only about tradeoffs in
975	–	* overhead vs expected throughput and its variance, rather than
976	–	* how finely to partition tasks.
977	–	*
978	–	* In a steady state strict (tree-structured) computation, each
979	–	* thread makes available for stealing enough tasks for other
980	–	* threads to remain active. Inductively, if all threads play by
981	–	* the same rules, each thread should make available only a
982	–	* constant number of tasks.
983	–	*
984	–	* The minimum useful constant is just 1. But using a value of 1
985	–	* would require immediate replenishment upon each steal to
986	–	* maintain enough tasks, which is infeasible.  Further,
987	–	* partitionings/granularities of offered tasks should minimize
988	–	* steal rates, which in general means that threads nearer the top
989	–	* of computation tree should generate more than those nearer the
990	–	* bottom. In perfect steady state, each thread is at
991	–	* approximately the same level of computation tree. However,
992	–	* producing extra tasks amortizes the uncertainty of progress and
993	–	* diffusion assumptions.
994	–	*
995	–	* So, users will want to use values larger, but not much larger
996	–	* than 1 to both smooth over transient shortages and hedge
997	–	* against uneven progress; as traded off against the cost of
998	–	* extra task overhead. We leave the user to pick a threshold
999	–	* value to compare with the results of this call to guide
1000	–	* decisions, but recommend values such as 3.
1001	–	*
1002	–	* When all threads are active, it is on average OK to estimate
1003	–	* surplus strictly locally. In steady-state, if one thread is
1004	–	* maintaining say 2 surplus tasks, then so are others. So we can
1005	–	* just use estimated queue length (although note that (sp - base)
1006	–	* can be an overestimate because of stealers lagging increments
1007	–	* of base).  However, this strategy alone leads to serious
1008	–	* mis-estimates in some non-steady-state conditions (ramp-up,
1009	–	* ramp-down, other stalls). We can detect many of these by
1010	–	* further considering the number of "idle" threads, that are
1011	–	* known to have zero queued tasks, so compensate by a factor of
1012	–	* (#idle/#active) threads.
1013	–	*/
1014	–	final int getEstimatedSurplusTaskCount() {
1015	–	return sp - base - pool.idlePerActive();
1016	–	}
1017	–
1018	–	/**
1019	–	* Gets and removes a local task.
1020	–	*
1021	–	* @return a task, if available
1022	–	*/
1023	–	final ForkJoinTask<?> pollLocalTask() {
1024	–	while (sp != base) {
1025	–	if (active || (active = pool.tryIncrementActiveCount()))
1026	–	return locallyFifo? locallyDeqTask() : popTask();
1027	–	}
1028	–	return null;
1029	–	}
1030	–
1031	–	/**
1032	–	* Gets and removes a local or stolen task.
1033	–	*
1034	–	* @return a task, if available
1035	–	*/
1036	–	final ForkJoinTask<?> pollTask() {
1037	–	ForkJoinTask<?> t;
1038	–	return (t = pollLocalTask()) != null ? t : scan();
1039	–	}
1040	–
1041	–	/**
1042	–	* Runs tasks until {@code pool.isQuiescent()}.
1043	–	*/
1044	–	final void helpQuiescePool() {
1045	–	for (;;) {
1046	–	ForkJoinTask<?> t = pollLocalTask();
1047	–	if (t != null || (t = scan()) != null) {
1048	–	t.tryExec();
1049	–	stolen = null;
1050	–	}
1051	–	else {
1052	–	ForkJoinPool p = pool;
1053	–	if (active) {
1054	–	active = false; // inactivate
1055	–	do {} while (!p.tryDecrementActiveCount());
1056	–	}
1057	–	if (p.isQuiescent()) {
1058	–	active = true; // re-activate
1059	–	do {} while (!p.tryIncrementActiveCount());
1060	–	return;
1061	–	}
1062	–	}
1063	–	}
1064	–	}
1065	–
1066	–	// Unsafe mechanics
1067	–
1068	–	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
1069	–	private static final long runStateOffset =
1070	–	objectFieldOffset("runState", ForkJoinWorkerThread.class);
1071	–	private static final long qBase =
1072	–	UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
1073	–	private static final int qShift;
1074	–
1075	–	static {
1076	–	int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
1077	–	if ((s & (s-1)) != 0)
1078	–	throw new Error("data type scale not a power of two");
1079	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
1080	–	}
1081	–
1082	–	private static long objectFieldOffset(String field, Class<?> klazz) {
1083	–	try {
1084	–	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1085	–	} catch (NoSuchFieldException e) {
1086	–	// Convert Exception to corresponding Error
1087	–	NoSuchFieldError error = new NoSuchFieldError(field);
1088	–	error.initCause(e);
1089	–	throw error;
1090	–	}
1091	–	}
1092	–
1093	–	/**
1094	–	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
1095	–	* Replace with a simple call to Unsafe.getUnsafe when integrating
1096	–	* into a jdk.
1097	–	*
1098	–	* @return a sun.misc.Unsafe
1099	–	*/
1100	–	private static sun.misc.Unsafe getUnsafe() {
1101	–	try {
1102	–	return sun.misc.Unsafe.getUnsafe();
1103	–	} 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");
1111	<	f.setAccessible(true);
1112	<	return (sun.misc.Unsafe) f.get(null);
1113	<	}});
1114	<	} catch (java.security.PrivilegedActionException e) {
1115	<	throw new RuntimeException("Could not initialize intrinsics",
1116	<	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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