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

Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.4 by dl, Wed Jan 7 20:51:36 2009 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	–	import java.util.*;
9	–	import java.util.concurrent.*;
10	–	import java.util.concurrent.atomic.*;
11	–	import java.util.concurrent.locks.*;
12	–	import sun.misc.Unsafe;
13	–	import java.lang.reflect.*;
8
9		/**
10	<	* A thread managed by a {@link ForkJoinPool}.  This class is
11	<	* subclassable solely for the sake of adding functionality -- there
12	<	* are no overridable methods dealing with scheduling or
13	<	* execution. However, you can override initialization and termination
14	<	* cleanup methods surrounding the main task processing loop.  If you
15	<	* do create such a subclass, you will also need to supply a custom
16	<	* ForkJoinWorkerThreadFactory to use it in a ForkJoinPool.
17	<	*
18	<	* <p>This class also provides methods for generating per-thread
19	<	* random numbers, with the same properties as {@link
20	<	* java.util.Random} but with each generator isolated from those of
21	<	* other threads.
10	>	* A thread managed by a {@link ForkJoinPool}, which executes
11	>	* {@link ForkJoinTask}s.
12	>	* This class is subclassable solely for the sake of adding
13	>	* functionality -- there are no overridable methods dealing with
14	>	* scheduling or execution.  However, you can override initialization
15	>	* and termination methods surrounding the main task processing loop.
16	>	* If you do create such a subclass, you will also need to supply a
17	>	* custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to use it
18	>	* in a {@code ForkJoinPool}.
19	>	*
20	>	* @since 1.7
21	>	* @author Doug Lea
22		*/
23		public class ForkJoinWorkerThread extends Thread {
24		/*
25	<	* Algorithm overview:
26	<	*
27	<	* 1. Work-Stealing: Work-stealing queues are special forms of
34	<	* Deques that support only three of the four possible
35	<	* end-operations -- push, pop, and deq (aka steal), and only do
36	<	* so under the constraints that push and pop are called only from
37	<	* the owning thread, while deq may be called from other threads.
38	<	* (If you are unfamiliar with them, you probably want to read
39	<	* Herlihy and Shavit's book "The Art of Multiprocessor
40	<	* programming", chapter 16 describing these in more detail before
41	<	* proceeding.)  The main work-stealing queue design is roughly
42	<	* similar to "Dynamic Circular Work-Stealing Deque" by David
43	<	* Chase and Yossi Lev, SPAA 2005
44	<	* (http://research.sun.com/scalable/pubs/index.html).  The main
45	<	* difference ultimately stems from gc requirements that we null
46	<	* out taken slots as soon as we can, to maintain as small a
47	<	* footprint as possible even in programs generating huge numbers
48	<	* of tasks. To accomplish this, we shift the CAS arbitrating pop
49	<	* vs deq (steal) from being on the indices ("base" and "sp") to
50	<	* the slots themselves (mainly via method "casSlotNull()"). So,
51	<	* both a successful pop and deq mainly entail CAS'ing a nonnull
52	<	* slot to null.  Because we rely on CASes of references, we do
53	<	* not need tag bits on base or sp.  They are simple ints as used
54	<	* in any circular array-based queue (see for example ArrayDeque).
55	<	* Updates to the indices must still be ordered in a way that
56	<	* guarantees that (sp - base) > 0 means the queue is empty, but
57	<	* otherwise may err on the side of possibly making the queue
58	<	* appear nonempty when a push, pop, or deq have not fully
59	<	* committed. Note that this means that the deq operation,
60	<	* considered individually, is not wait-free. One thief cannot
61	<	* successfully continue until another in-progress one (or, if
62	<	* previously empty, a push) completes.  However, in the
63	<	* aggregate, we ensure at least probablistic non-blockingness. If
64	<	* an attempted steal fails, a thief always chooses a different
65	<	* random victim target to try next. So, in order for one thief to
66	<	* progress, it suffices for any in-progress deq or new push on
67	<	* any empty queue to complete. One reason this works well here is
68	<	* that apparently-nonempty often means soon-to-be-stealable,
69	<	* which gives threads a chance to activate if necessary before
70	<	* stealing (see below).
71	<	*
72	<	* Efficient implementation of this approach currently relies on
73	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
74	<	* correct orderings, reads and writes of variable base require
75	<	* volatile ordering.  Variable sp does not require volatile write
76	<	* but needs cheaper store-ordering on writes.  Because they are
77	<	* protected by volatile base reads, reads of the queue array and
78	<	* its slots do not need volatile load semantics, but writes (in
79	<	* push) require store order and CASes (in pop and deq) require
80	<	* (volatile) CAS semantics. Since these combinations aren't
81	<	* supported using ordinary volatiles, the only way to accomplish
82	<	* these effciently is to use direct Unsafe calls. (Using external
83	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
84	<	* array is significantly slower because of memory locality and
85	<	* indirection effects.) Further, performance on most platforms is
86	<	* very sensitive to placement and sizing of the (resizable) queue
87	<	* array.  Even though these queues don't usually become all that
88	<	* big, the initial size must be large enough to counteract cache
89	<	* contention effects across multiple queues (especially in the
90	<	* presence of GC cardmarking). Also, to improve thread-locality,
91	<	* queues are currently initialized immediately after the thread
92	<	* gets the initial signal to start processing tasks.  However,
93	<	* all queue-related methods except pushTask are written in a way
94	<	* that allows them to instead be lazily allocated and/or disposed
95	<	* of when empty. All together, these low-level implementation
96	<	* choices produce as much as a factor of 4 performance
97	<	* improvement compared to naive implementations, and enable the
98	<	* processing of billions of tasks per second, sometimes at the
99	<	* expense of ugliness.
100	<	*
101	<	* 2. Run control: The primary run control is based on a global
102	<	* counter (activeCount) held by the pool. It uses an algorithm
103	<	* similar to that in Herlihy and Shavit section 17.6 to cause
104	<	* threads to eventually block when all threads declare they are
105	<	* inactive. (See variable "scans".)  For this to work, threads
106	<	* must be declared active when executing tasks, and before
107	<	* stealing a task. They must be inactive before blocking on the
108	<	* Pool Barrier (awaiting a new submission or other Pool
109	<	* event). In between, there is some free play which we take
110	<	* advantage of to avoid contention and rapid flickering of the
111	<	* global activeCount: If inactive, we activate only if a victim
112	<	* queue appears to be nonempty (see above).  Similarly, a thread
113	<	* tries to inactivate only after a full scan of other threads.
114	<	* The net effect is that contention on activeCount is rarely a
115	<	* measurable performance issue. (There are also a few other cases
116	<	* where we scan for work rather than retry/block upon
117	<	* contention.)
118	<	*
119	<	* 3. Selection control. We maintain policy of always choosing to
120	<	* run local tasks rather than stealing, and always trying to
121	<	* steal tasks before trying to run a new submission. All steals
122	<	* are currently performed in randomly-chosen deq-order. It may be
123	<	* worthwhile to bias these with locality / anti-locality
124	<	* information, but doing this well probably requires more
125	<	* lower-level information from JVMs than currently provided.
126	<	*/
127	<
128	<	/**
129	<	* Capacity of work-stealing queue array upon initialization.
130	<	* Must be a power of two. Initial size must be at least 2, but is
131	<	* padded to minimize cache effects.
132	<	*/
133	<	private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
134	<
135	<	/**
136	<	* Maximum work-stealing queue array size.  Must be less than or
137	<	* equal to 1 << 30 to ensure lack of index wraparound.
138	<	*/
139	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 30;
140	<
141	<	/**
142	<	* Generator of seeds for per-thread random numbers.
143	<	*/
144	<	private static final Random randomSeedGenerator = new Random();
145	<
146	<	/**
147	<	* The work-stealing queue array. Size must be a power of two.
148	<	*/
149	<	private ForkJoinTask<?>[] queue;
150	<
151	<	/**
152	<	* Index (mod queue.length) of next queue slot to push to or pop
153	<	* from. It is written only by owner thread, via ordered store.
154	<	* Both sp and base are allowed to wrap around on overflow, but
155	<	* (sp - base) still estimates size.
25	>	* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
26	>	* ForkJoinTasks. For explanation, see the internal documentation
27	>	* of class ForkJoinPool.
28		*/
157	–	private volatile int sp;
29
30	<	/**
31	<	* Index (mod queue.length) of least valid queue slot, which is
161	<	* always the next position to steal from if nonempty.
162	<	*/
163	<	private volatile int base;
164	<
165	<	/**
166	<	* The pool this thread works in.
167	<	*/
168	<	final ForkJoinPool pool;
169	<
170	<	/**
171	<	* Index of this worker in pool array. Set once by pool before
172	<	* running, and accessed directly by pool during cleanup etc
173	<	*/
174	<	int poolIndex;
175	<
176	<	/**
177	<	* Run state of this worker. Supports simple versions of the usual
178	<	* shutdown/shutdownNow control.
179	<	*/
180	<	private volatile int runState;
181	<
182	<	// Runstate values. Order matters
183	<	private static final int RUNNING     = 0;
184	<	private static final int SHUTDOWN    = 1;
185	<	private static final int TERMINATING = 2;
186	<	private static final int TERMINATED  = 3;
187	<
188	<	/**
189	<	* Activity status. When true, this worker is considered active.
190	<	* Must be false upon construction. It must be true when executing
191	<	* tasks, and BEFORE stealing a task. It must be false before
192	<	* blocking on the Pool Barrier.
193	<	*/
194	<	private boolean active;
195	<
196	<	/**
197	<	* Number of steals, transferred to pool when idle
198	<	*/
199	<	private int stealCount;
200	<
201	<	/**
202	<	* Seed for random number generator for choosing steal victims
203	<	*/
204	<	private int randomVictimSeed;
205	<
206	<	/**
207	<	* Seed for embedded Jurandom
208	<	*/
209	<	private long juRandomSeed;
210	<
211	<	/**
212	<	* The last barrier event waited for
213	<	*/
214	<	private long eventCount;
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.
35	+	*
36		* @param pool the pool this thread works in
37		* @throws NullPointerException if pool is null
38		*/
39		protected ForkJoinWorkerThread(ForkJoinPool pool) {
40	<	if (pool == null) throw new NullPointerException();
40	>	super(pool.nextWorkerName());
41	>	setDaemon(true);
42	>	Thread.UncaughtExceptionHandler ueh = pool.ueh;
43	>	if (ueh != null)
44	>	setUncaughtExceptionHandler(ueh);
45		this.pool = pool;
46	<	// remaining initialization deferred to onStart
46	>	pool.registerWorker(this.workQueue = new ForkJoinPool.WorkQueue
47	>	(pool, this, pool.localMode));
48		}
49
227	–	// public access methods
228	–
50		/**
51	<	* Returns the pool hosting this thread
51	>	* Returns the pool hosting this thread.
52	>	*
53		* @return the pool
54		*/
55		public ForkJoinPool getPool() {
#	Line 239 | Line 61 | public class ForkJoinWorkerThread extend
61		* returned value ranges from zero to the maximum number of
62		* threads (minus one) that have ever been created in the pool.
63		* This method may be useful for applications that track status or
64	<	* collect results on a per-worker basis.
65	<	* @return the index number.
64	>	* collect results per-worker rather than per-task.
65	>	*
66	>	* @return the index number
67		*/
68		public int getPoolIndex() {
69	<	return poolIndex;
247	<	}
248	<
249	<	//  Access methods used by Pool
250	<
251	<	/**
252	<	* Get and clear steal count for accumulation by pool.  Called
253	<	* only when known to be idle (in pool.sync and termination).
254	<	*/
255	<	final int getAndClearStealCount() {
256	<	int sc = stealCount;
257	<	stealCount = 0;
258	<	return sc;
259	<	}
260	<
261	<	/**
262	<	* Returns estimate of the number of tasks in the queue, without
263	<	* correcting for transient negative values
264	<	*/
265	<	final int getRawQueueSize() {
266	<	return sp - base;
267	<	}
268	<
269	<	// Intrinsics-based support for queue operations.
270	<	// Currently these three (setSp, setSlot, casSlotNull) are
271	<	// usually manually inlined to improve performance
272	<
273	<	/**
274	<	* Sets sp in store-order.
275	<	*/
276	<	private void setSp(int s) {
277	<	_unsafe.putOrderedInt(this, spOffset, s);
278	<	}
279	<
280	<	/**
281	<	* Add in store-order the given task at given slot of q to
282	<	* null. Caller must ensure q is nonnull and index is in range.
283	<	*/
284	<	private static void setSlot(ForkJoinTask<?>[] q, int i,
285	<	ForkJoinTask<?> t){
286	<	_unsafe.putOrderedObject(q, (i << qShift) + qBase, t);
287	<	}
288	<
289	<	/**
290	<	* CAS given slot of q to null. Caller must ensure q is nonnull
291	<	* and index is in range.
292	<	*/
293	<	private static boolean casSlotNull(ForkJoinTask<?>[] q, int i,
294	<	ForkJoinTask<?> t) {
295	<	return _unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
296	<	}
297	<
298	<	// Main queue methods
299	<
300	<	/**
301	<	* Pushes a task. Called only by current thread.
302	<	* @param t the task. Caller must ensure nonnull
303	<	*/
304	<	final void pushTask(ForkJoinTask<?> t) {
305	<	ForkJoinTask<?>[] q = queue;
306	<	int mask = q.length - 1;
307	<	int s = sp;
308	<	_unsafe.putOrderedObject(q, ((s & mask) << qShift) + qBase, t);
309	<	_unsafe.putOrderedInt(this, spOffset, ++s);
310	<	if ((s -= base) == 1)
311	<	pool.signalNonEmptyWorkerQueue();
312	<	else if (s >= mask)
313	<	growQueue();
314	<	}
315	<
316	<	/**
317	<	* Tries to take a task from the base of the queue, failing if
318	<	* either empty or contended.
319	<	* @return a task, or null if none or contended.
320	<	*/
321	<	private ForkJoinTask<?> deqTask() {
322	<	ForkJoinTask<?>[] q;
323	<	ForkJoinTask<?> t;
324	<	int i;
325	<	int b;
326	<	if (sp != (b = base) &&
327	<	(q = queue) != null && // must read q after b
328	<	(t = q[i = (q.length - 1) & b]) != null &&
329	<	_unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
330	<	base = b + 1;
331	<	return t;
332	<	}
333	<	return null;
69	>	return workQueue.poolIndex;
70		}
71
72		/**
337	–	* Returns a popped task, or null if empty.  Called only by
338	–	* current thread.
339	–	*/
340	–	final ForkJoinTask<?> popTask() {
341	–	ForkJoinTask<?> t;
342	–	int i;
343	–	ForkJoinTask<?>[] q = queue;
344	–	int mask = q.length - 1;
345	–	int s = sp;
346	–	if (s != base &&
347	–	(t = q[i = (s - 1) & mask]) != null &&
348	–	_unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
349	–	_unsafe.putOrderedInt(this, spOffset, s - 1);
350	–	return t;
351	–	}
352	–	return null;
353	–	}
354	–
355	–	/**
356	–	* Specialized version of popTask to pop only if
357	–	* topmost element is the given task. Called only
358	–	* by current thread.
359	–	* @param t the task. Caller must ensure nonnull
360	–	*/
361	–	final boolean unpushTask(ForkJoinTask<?> t) {
362	–	ForkJoinTask<?>[] q = queue;
363	–	int mask = q.length - 1;
364	–	int s = sp - 1;
365	–	if (_unsafe.compareAndSwapObject(q, ((s & mask) << qShift) + qBase,
366	–	t, null)) {
367	–	_unsafe.putOrderedInt(this, spOffset, s);
368	–	return true;
369	–	}
370	–	return false;
371	–	}
372	–
373	–	/**
374	–	* Returns next task to pop.
375	–	*/
376	–	final ForkJoinTask<?> peekTask() {
377	–	ForkJoinTask<?>[] q = queue;
378	–	return q == null? null : q[(sp - 1) & (q.length - 1)];
379	–	}
380	–
381	–	/**
382	–	* Doubles queue array size. Transfers elements by emulating
383	–	* steals (deqs) from old array and placing, oldest first, into
384	–	* new array.
385	–	*/
386	–	private void growQueue() {
387	–	ForkJoinTask<?>[] oldQ = queue;
388	–	int oldSize = oldQ.length;
389	–	int newSize = oldSize << 1;
390	–	if (newSize > MAXIMUM_QUEUE_CAPACITY)
391	–	throw new RejectedExecutionException("Queue capacity exceeded");
392	–	ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
393	–
394	–	int b = base;
395	–	int bf = b + oldSize;
396	–	int oldMask = oldSize - 1;
397	–	int newMask = newSize - 1;
398	–	do {
399	–	int oldIndex = b & oldMask;
400	–	ForkJoinTask<?> t = oldQ[oldIndex];
401	–	if (t != null && !casSlotNull(oldQ, oldIndex, t))
402	–	t = null;
403	–	setSlot(newQ, b & newMask, t);
404	–	} while (++b != bf);
405	–	pool.signalIdleWorkers(false);
406	–	}
407	–
408	–	// Runstate management
409	–
410	–	final boolean isShutdown()    { return runState >= SHUTDOWN;  }
411	–	final boolean isTerminating() { return runState >= TERMINATING;  }
412	–	final boolean isTerminated()  { return runState == TERMINATED; }
413	–	final boolean shutdown()      { return transitionRunStateTo(SHUTDOWN); }
414	–	final boolean shutdownNow()   { return transitionRunStateTo(TERMINATING); }
415	–
416	–	/**
417	–	* Transition to at least the given state. Return true if not
418	–	* already at least given state.
419	–	*/
420	–	private boolean transitionRunStateTo(int state) {
421	–	for (;;) {
422	–	int s = runState;
423	–	if (s >= state)
424	–	return false;
425	–	if (_unsafe.compareAndSwapInt(this, runStateOffset, s, state))
426	–	return true;
427	–	}
428	–	}
429	–
430	–	/**
431	–	* Ensure status is active and if necessary adjust pool active count
432	–	*/
433	–	final void activate() {
434	–	if (!active) {
435	–	active = true;
436	–	pool.incrementActiveCount();
437	–	}
438	–	}
439	–
440	–	/**
441	–	* Ensure status is inactive and if necessary adjust pool active count
442	–	*/
443	–	final void inactivate() {
444	–	if (active) {
445	–	active = false;
446	–	pool.decrementActiveCount();
447	–	}
448	–	}
449	–
450	–	// Lifecycle methods
451	–
452	–	/**
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() {
462	–	juRandomSeed = randomSeedGenerator.nextLong();
463	–	do;while((randomVictimSeed = nextRandomInt()) == 0); // must be nonzero
464	–	if (queue == null)
465	–	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
466	–
467	–	// Heuristically allow one initial thread to warm up; others wait
468	–	if (poolIndex < pool.getParallelism() - 1) {
469	–	eventCount = pool.sync(this, 0);
470	–	activate();
471	–	}
82		}
83
84		/**
85	<	* Perform cleanup associated with termination of this worker
85	>	* Performs cleanup associated with termination of this worker
86		* thread.  If you override this method, you must invoke
87	<	* super.onTermination at the end of the overridden method.
87	>	* {@code super.onTermination} at the end of the overridden method.
88		*
89		* @param exception the exception causing this thread to abort due
90	<	* to an unrecoverable error, or null if completed normally.
90	>	* to an unrecoverable error, or {@code null} if completed normally
91		*/
92		protected void onTermination(Throwable exception) {
483	–	try {
484	–	clearLocalTasks();
485	–	inactivate();
486	–	cancelTasks();
487	–	} finally {
488	–	terminate(exception);
489	–	}
490	–	}
491	–
492	–	/**
493	–	* Notify pool of termination and, if exception is nonnull,
494	–	* rethrow it to trigger this thread's uncaughtExceptionHandler
495	–	*/
496	–	private void terminate(Throwable exception) {
497	–	transitionRunStateTo(TERMINATED);
498	–	try {
499	–	pool.workerTerminated(this);
500	–	} finally {
501	–	if (exception != null)
502	–	ForkJoinTask.rethrowException(exception);
503	–	}
504	–	}
505	–
506	–	/**
507	–	* Run local tasks on exit from main.
508	–	*/
509	–	private void clearLocalTasks() {
510	–	while (base != sp && !pool.isTerminating()) {
511	–	ForkJoinTask<?> t = popTask();
512	–	if (t != null) {
513	–	activate(); // ensure active status
514	–	t.quietlyExec();
515	–	}
516	–	}
517	–	}
518	–
519	–	/**
520	–	* Removes and cancels all tasks in queue.  Can be called from any
521	–	* thread.
522	–	*/
523	–	final void cancelTasks() {
524	–	while (base != sp) {
525	–	ForkJoinTask<?> t = deqTask();
526	–	if (t != null)
527	–	t.cancelIgnoringExceptions();
528	–	}
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	<	while (!isShutdown())
541	<	step();
104	>	pool.runWorker(workQueue);
105		} catch (Throwable ex) {
106		exception = ex;
107		} finally {
108	<	onTermination(exception);
109	<	}
110	<	}
111	<
112	<	/**
113	<	* Main top-level action.
114	<	*/
552	<	private void step() {
553	<	ForkJoinTask<?> t = sp != base? popTask() : null;
554	<	if (t != null || (t = scan(null, true)) != null) {
555	<	activate();
556	<	t.quietlyExec();
557	<	}
558	<	else {
559	<	inactivate();
560	<	eventCount = pool.sync(this, eventCount);
561	<	}
562	<	}
563	<
564	<	// scanning for and stealing tasks
565	<
566	<	/**
567	<	* Computes next value for random victim probe. Scans don't
568	<	* require a very high quality generator, but also not a crummy
569	<	* one. Marsaglia xor-shift is cheap and works well.
570	<	*
571	<	* This is currently unused, and manually inlined
572	<	*/
573	<	private static int xorShift(int r) {
574	<	r ^= r << 1;
575	<	r ^= r >>> 3;
576	<	r ^= r << 10;
577	<	return r;
578	<	}
579	<
580	<	/**
581	<	* Tries to steal a task from another worker and/or, if enabled,
582	<	* submission queue. Starts at a random index of workers array,
583	<	* and probes workers until finding one with non-empty queue or
584	<	* finding that all are empty.  It randomly selects the first n-1
585	<	* probes. If these are empty, it resorts to full circular
586	<	* traversal, which is necessary to accurately set active status
587	<	* by caller. Also restarts if pool barrier has tripped since last
588	<	* scan, which forces refresh of workers array, in case barrier
589	<	* was associated with resize.
590	<	*
591	<	* This method must be both fast and quiet -- usually avoiding
592	<	* memory accesses that could disrupt cache sharing etc other than
593	<	* those needed to check for and take tasks. This accounts for,
594	<	* among other things, updating random seed in place without
595	<	* storing it until exit. (Note that we only need to store it if
596	<	* we found a task; otherwise it doesn't matter if we start at the
597	<	* same place next time.)
598	<	*
599	<	* @param joinMe if non null; exit early if done
600	<	* @param checkSubmissions true if OK to take submissions
601	<	* @return a task, or null if none found
602	<	*/
603	<	private ForkJoinTask<?> scan(ForkJoinTask<?> joinMe,
604	<	boolean checkSubmissions) {
605	<	ForkJoinPool p = pool;
606	<	if (p == null)                    // Never null, but avoids
607	<	return null;                  //   implicit nullchecks below
608	<	int r = randomVictimSeed;         // extract once to keep scan quiet
609	<	restart:                          // outer loop refreshes ws array
610	<	while (joinMe == null || joinMe.status >= 0) {
611	<	int mask;
612	<	ForkJoinWorkerThread[] ws = p.workers;
613	<	if (ws != null && (mask = ws.length - 1) > 0) {
614	<	int probes = -mask;       // use random index while negative
615	<	int idx = r;
616	<	for (;;) {
617	<	ForkJoinWorkerThread v;
618	<	// inlined xorshift to update seed
619	<	r ^= r << 1;  r ^= r >>> 3; r ^= r << 10;
620	<	if ((v = ws[mask & idx]) != null && v.sp != v.base) {
621	<	ForkJoinTask<?> t;
622	<	activate();
623	<	if ((joinMe == null || joinMe.status >= 0) &&
624	<	(t = v.deqTask()) != null) {
625	<	randomVictimSeed = r;
626	<	++stealCount;
627	<	return t;
628	<	}
629	<	continue restart; // restart on contention
630	<	}
631	<	if ((probes >> 1) <= mask) // n-1 random then circular
632	<	idx = (probes++ < 0)? r : (idx + 1);
633	<	else
634	<	break;
635	<	}
636	<	}
637	<	if (checkSubmissions && p.hasQueuedSubmissions()) {
638	<	activate();
639	<	ForkJoinTask<?> t = p.pollSubmission();
640	<	if (t != null)
641	<	return t;
642	<	}
643	<	else {
644	<	long ec = eventCount;     // restart on pool event
645	<	if ((eventCount = p.getEventCount()) == ec)
646	<	break;
108	>	try {
109	>	onTermination(exception);
110	>	} catch (Throwable ex) {
111	>	if (exception == null)
112	>	exception = ex;
113	>	} finally {
114	>	pool.deregisterWorker(this, exception);
115		}
116		}
649	–	return null;
650	–	}
651	–
652	–	/**
653	–	* Callback from pool.sync to rescan before blocking.  If a
654	–	* task is found, it is pushed so it can be executed upon return.
655	–	* @return true if found and pushed a task
656	–	*/
657	–	final boolean prescan() {
658	–	ForkJoinTask<?> t = scan(null, true);
659	–	if (t != null) {
660	–	pushTask(t);
661	–	return true;
662	–	}
663	–	else {
664	–	inactivate();
665	–	return false;
666	–	}
667	–	}
668	–
669	–	// Support for ForkJoinTask methods
670	–
671	–	/**
672	–	* Scan, returning early if joinMe done
673	–	*/
674	–	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
675	–	ForkJoinTask<?> t = scan(joinMe, false);
676	–	if (t != null && joinMe.status < 0 && sp == base) {
677	–	pushTask(t); // unsteal if done and this task would be stealable
678	–	t = null;
679	–	}
680	–	return t;
681	–	}
682	–
683	–	/**
684	–	* Pops or steals a task
685	–	* @return task, or null if none available
686	–	*/
687	–	final ForkJoinTask<?> pollLocalOrStolenTask() {
688	–	ForkJoinTask<?> t;
689	–	return (t = popTask()) == null? scan(null, false) : t;
690	–	}
691	–
692	–	/**
693	–	* Runs tasks until pool isQuiescent
694	–	*/
695	–	final void helpQuiescePool() {
696	–	for (;;) {
697	–	ForkJoinTask<?> t = pollLocalOrStolenTask();
698	–	if (t != null) {
699	–	activate();
700	–	t.quietlyExec();
701	–	}
702	–	else {
703	–	inactivate();
704	–	if (pool.isQuiescent()) {
705	–	activate(); // re-activate on exit
706	–	break;
707	–	}
708	–	}
709	–	}
710	–	}
711	–
712	–	/**
713	–	* Returns an estimate of the number of tasks in the queue.
714	–	*/
715	–	final int getQueueSize() {
716	–	int n = sp - base;
717	–	return n <= 0? 0 : n; // suppress momentarily negative values
718	–	}
719	–
720	–	/**
721	–	* Returns an estimate of the number of tasks, offset by a
722	–	* function of number of idle workers.
723	–	*/
724	–	final int getEstimatedSurplusTaskCount() {
725	–	// The halving approximates weighting idle vs non-idle workers
726	–	return (sp - base) - (pool.getIdleThreadCount() >>> 1);
727	–	}
728	–
729	–	// Per-worker exported random numbers
730	–
731	–	// Same constants as java.util.Random
732	–	final static long JURandomMultiplier = 0x5DEECE66DL;
733	–	final static long JURandomAddend = 0xBL;
734	–	final static long JURandomMask = (1L << 48) - 1;
735	–
736	–	private final int nextJURandom(int bits) {
737	–	long next = (juRandomSeed * JURandomMultiplier + JURandomAddend) &
738	–	JURandomMask;
739	–	juRandomSeed = next;
740	–	return (int)(next >>> (48 - bits));
741	–	}
742	–
743	–	private final int nextJURandomInt(int n) {
744	–	if (n <= 0)
745	–	throw new IllegalArgumentException("n must be positive");
746	–	int bits = nextJURandom(31);
747	–	if ((n & -n) == n)
748	–	return (int)((n * (long)bits) >> 31);
749	–
750	–	for (;;) {
751	–	int val = bits % n;
752	–	if (bits - val + (n-1) >= 0)
753	–	return val;
754	–	bits = nextJURandom(31);
755	–	}
756	–	}
757	–
758	–	private final long nextJURandomLong() {
759	–	return ((long)(nextJURandom(32)) << 32) + nextJURandom(32);
760	–	}
761	–
762	–	private final long nextJURandomLong(long n) {
763	–	if (n <= 0)
764	–	throw new IllegalArgumentException("n must be positive");
765	–	long offset = 0;
766	–	while (n >= Integer.MAX_VALUE) { // randomly pick half range
767	–	int bits = nextJURandom(2); // 2nd bit for odd vs even split
768	–	long half = n >>> 1;
769	–	long nextn = ((bits & 2) == 0)? half : n - half;
770	–	if ((bits & 1) == 0)
771	–	offset += n - nextn;
772	–	n = nextn;
773	–	}
774	–	return offset + nextJURandomInt((int)n);
775	–	}
776	–
777	–	private final double nextJURandomDouble() {
778	–	return (((long)(nextJURandom(26)) << 27) + nextJURandom(27))
779	–	/ (double)(1L << 53);
780	–	}
781	–
782	–	/**
783	–	* Returns a random integer using a per-worker random
784	–	* number generator with the same properties as
785	–	* {@link java.util.Random#nextInt}
786	–	* @return the next pseudorandom, uniformly distributed {@code int}
787	–	*         value from this worker's random number generator's sequence
788	–	*/
789	–	public static int nextRandomInt() {
790	–	return ((ForkJoinWorkerThread)(Thread.currentThread())).
791	–	nextJURandom(32);
792	–	}
793	–
794	–	/**
795	–	* Returns a random integer using a per-worker random
796	–	* number generator with the same properties as
797	–	* {@link java.util.Random#nextInt(int)}
798	–	* @param n the bound on the random number to be returned.  Must be
799	–	*        positive.
800	–	* @return the next pseudorandom, uniformly distributed {@code int}
801	–	*         value between {@code 0} (inclusive) and {@code n} (exclusive)
802	–	*         from this worker's random number generator's sequence
803	–	* @throws IllegalArgumentException if n is not positive
804	–	*/
805	–	public static int nextRandomInt(int n) {
806	–	return ((ForkJoinWorkerThread)(Thread.currentThread())).
807	–	nextJURandomInt(n);
808	–	}
809	–
810	–	/**
811	–	* Returns a random long using a per-worker random
812	–	* number generator with the same properties as
813	–	* {@link java.util.Random#nextLong}
814	–	* @return the next pseudorandom, uniformly distributed {@code long}
815	–	*         value from this worker's random number generator's sequence
816	–	*/
817	–	public static long nextRandomLong() {
818	–	return ((ForkJoinWorkerThread)(Thread.currentThread())).
819	–	nextJURandomLong();
820	–	}
821	–
822	–	/**
823	–	* Returns a random integer using a per-worker random
824	–	* number generator with the same properties as
825	–	* {@link java.util.Random#nextInt(int)}
826	–	* @param n the bound on the random number to be returned.  Must be
827	–	*        positive.
828	–	* @return the next pseudorandom, uniformly distributed {@code int}
829	–	*         value between {@code 0} (inclusive) and {@code n} (exclusive)
830	–	*         from this worker's random number generator's sequence
831	–	* @throws IllegalArgumentException if n is not positive
832	–	*/
833	–	public static long nextRandomLong(long n) {
834	–	return ((ForkJoinWorkerThread)(Thread.currentThread())).
835	–	nextJURandomLong(n);
836	–	}
837	–
838	–	/**
839	–	* Returns a random double using a per-worker random
840	–	* number generator with the same properties as
841	–	* {@link java.util.Random#nextDouble}
842	–	* @return the next pseudorandom, uniformly distributed {@code double}
843	–	*         value between {@code 0.0} and {@code 1.0} from this
844	–	*         worker's random number generator's sequence
845	–	*/
846	–	public static double nextRandomDouble() {
847	–	return ((ForkJoinWorkerThread)(Thread.currentThread())).
848	–	nextJURandomDouble();
849	–	}
850	–
851	–	// Temporary Unsafe mechanics for preliminary release
852	–
853	–	static final Unsafe _unsafe;
854	–	static final long baseOffset;
855	–	static final long spOffset;
856	–	static final long qBase;
857	–	static final int qShift;
858	–	static final long runStateOffset;
859	–	static {
860	–	try {
861	–	if (ForkJoinWorkerThread.class.getClassLoader() != null) {
862	–	Field f = Unsafe.class.getDeclaredField("theUnsafe");
863	–	f.setAccessible(true);
864	–	_unsafe = (Unsafe)f.get(null);
865	–	}
866	–	else
867	–	_unsafe = Unsafe.getUnsafe();
868	–	baseOffset = _unsafe.objectFieldOffset
869	–	(ForkJoinWorkerThread.class.getDeclaredField("base"));
870	–	spOffset = _unsafe.objectFieldOffset
871	–	(ForkJoinWorkerThread.class.getDeclaredField("sp"));
872	–	runStateOffset = _unsafe.objectFieldOffset
873	–	(ForkJoinWorkerThread.class.getDeclaredField("runState"));
874	–	qBase = _unsafe.arrayBaseOffset(ForkJoinTask[].class);
875	–	int s = _unsafe.arrayIndexScale(ForkJoinTask[].class);
876	–	if ((s & (s-1)) != 0)
877	–	throw new Error("data type scale not a power of two");
878	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
879	–	} catch (Exception e) {
880	–	throw new RuntimeException("Could not initialize intrinsics", e);
881	–	}
117		}
118		}
119	+

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