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

Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.2 by dl, Wed Jan 7 16:07:37 2009 UTC vs.
Revision 1.71 by dl, Wed Nov 14 17:20:38 2012 UTC

#	Line 1 | Line 1
1		/*
2		* Written by Doug Lea with assistance from members of JCP JSR-166
3		* Expert Group and released to the public domain, as explained at
4	<	* http://creativecommons.org/licenses/publicdomain
4	>	* http://creativecommons.org/publicdomain/zero/1.0/
5		*/
6
7		package jsr166y;
8	–	import java.util.*;
9	–	import java.util.concurrent.*;
10	–	import java.util.concurrent.atomic.*;
11	–	import java.util.concurrent.locks.*;
12	–	import sun.misc.Unsafe;
13	–	import java.lang.reflect.*;
8
9		/**
10	<	* A thread managed by a {@link ForkJoinPool}.  This class is
11	<	* subclassable solely for the sake of adding functionality -- there
12	<	* are no overridable methods dealing with scheduling or
13	<	* execution. However, you can override initialization and termination
14	<	* 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;
30	>	final ForkJoinPool.WorkQueue workQueue; // Work-stealing mechanics
31	>	final ForkJoinPool pool;                // the pool this thread works in
32
33		/**
34	<	* The pool this thread works in.
34	>	* An initial name for a newly constructed worker, used until
35	>	* onStart can establish a useful name. This removes need to
36	>	* establish a name from worker startup path.
37		*/
38	<	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;
38	>	static final String provisionalName = "aForkJoinWorkerThread";
39
40		/**
41		* Creates a ForkJoinWorkerThread operating in the given pool.
42	+	*
43		* @param pool the pool this thread works in
44		* @throws NullPointerException if pool is null
45		*/
46		protected ForkJoinWorkerThread(ForkJoinPool pool) {
47	<	if (pool == null) throw new NullPointerException();
47	>	super(provisionalName); // bootstrap name
48	>	Thread.UncaughtExceptionHandler ueh = pool.ueh;
49	>	if (ueh != null)
50	>	setUncaughtExceptionHandler(ueh);
51	>	setDaemon(true);
52		this.pool = pool;
53	<	// remaining initialization deferred to onStart
53	>	pool.registerWorker(this.workQueue = new ForkJoinPool.WorkQueue
54	>	(pool, this, pool.localMode));
55		}
56
227	–	// public access methods
228	–
57		/**
58	<	* Returns the pool hosting the current task execution.
58	>	* Returns the pool hosting this thread.
59	>	*
60		* @return the pool
61		*/
62	<	public static ForkJoinPool getPool() {
63	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).pool;
235	<	}
236	<
237	<	/**
238	<	* Returns the index number of the current worker thread in its
239	<	* pool.  The returned value ranges from zero to the maximum
240	<	* number of threads (minus one) that have ever been created in
241	<	* the pool.  This method may be useful for applications that
242	<	* track status or collect results on a per-worker basis.
243	<	* @return the index number.
244	<	*/
245	<	public static int getPoolIndex() {
246	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).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;
62	>	public ForkJoinPool getPool() {
63	>	return pool;
64		}
65
66		/**
67	<	* Returns estimate of the number of tasks in the queue, without
68	<	* correcting for transient negative values
69	<	*/
70	<	final int getRawQueueSize() {
71	<	return sp - base;
72	<	}
73	<
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;
334	<	}
335	<
336	<	/**
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
67	>	* Returns the index number of this thread in its pool.  The
68	>	* returned value ranges from zero to the maximum number of
69	>	* threads (minus one) that have ever been created in the pool.
70	>	* This method may be useful for applications that track status or
71	>	* collect results per-worker rather than per-task.
72	>	*
73	>	* @return the index number
74		*/
75	<	final void inactivate() {
76	<	if (active) {
445	<	active = false;
446	<	pool.decrementActiveCount();
447	<	}
75	>	public int getPoolIndex() {
76	>	return workQueue.poolIndex;
77		}
78
450	–	// Lifecycle methods
451	–
79		/**
80		* Initializes internal state after construction but before
81		* processing any tasks. If you override this method, you must
82	<	* invoke super.onStart() at the beginning of the method.
82	>	* invoke {@code super.onStart()} at the beginning of the method.
83		* Initialization requires care: Most fields must have legal
84		* default values, to ensure that attempted accesses from other
85		* threads work correctly even before this thread starts
86		* processing tasks.
87		*/
88		protected void onStart() {
89	<	juRandomSeed = randomSeedGenerator.nextLong();
90	<	do;while((randomVictimSeed = nextRandomInt()) == 0); // must be nonzero
91	<	if (queue == null)
92	<	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	<	}
89	>	String pref; // replace bootstrap name
90	>	if (provisionalName.equals(getName()) &&
91	>	(pref = pool.workerNamePrefix) != null)
92	>	setName(pref.concat(Long.toString(getId())));
93		}
94
95		/**
96	<	* Perform cleanup associated with termination of this worker
96	>	* Performs cleanup associated with termination of this worker
97		* thread.  If you override this method, you must invoke
98	<	* super.onTermination at the end of the overridden method.
98	>	* {@code super.onTermination} at the end of the overridden method.
99		*
100		* @param exception the exception causing this thread to abort due
101	<	* to an unrecoverable error, or null if completed normally.
101	>	* to an unrecoverable error, or {@code null} if completed normally
102		*/
103		protected void onTermination(Throwable exception) {
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.cancelIgnoreExceptions();
528	–	}
104		}
105
106		/**
107		* This method is required to be public, but should never be
108		* called explicitly. It performs the main run loop to execute
109	<	* ForkJoinTasks.
109	>	* {@link ForkJoinTask}s.
110		*/
111		public void run() {
112		Throwable exception = null;
113		try {
114		onStart();
115	<	while (!isShutdown())
541	<	step();
115	>	pool.runWorker(workQueue);
116		} catch (Throwable ex) {
117		exception = ex;
118		} finally {
119	<	onTermination(exception);
120	<	}
121	<	}
122	<
123	<	/**
124	<	* Main top-level action.
125	<	*/
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;
647	<	}
648	<	}
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	<	* Implements ForkJoinTask.helpJoin
673	<	*/
674	<	final int helpJoinTask(ForkJoinTask<?> joinMe) {
675	<	ForkJoinTask<?> t = null;
676	<	int s;
677	<	while ((s = joinMe.status) >= 0) {
678	<	if (t == null) {
679	<	if ((t = scan(joinMe, false)) == null)  // block if no work
680	<	return joinMe.awaitDone(this, false);
681	<	// else recheck status before exec
682	<	}
683	<	else {
684	<	t.quietlyExec();
685	<	t = null;
686	<	}
687	<	}
688	<	if (t != null) // unsteal
689	<	pushTask(t);
690	<	return s;
691	<	}
692	<
693	<	/**
694	<	* Pops or steals a task
695	<	* @return task, or null if none available
696	<	*/
697	<	final ForkJoinTask<?> getLocalOrStolenTask() {
698	<	ForkJoinTask<?> t = popTask();
699	<	return t != null? t : scan(null, false);
700	<	}
701	<
702	<	/**
703	<	* Runs tasks until pool isQuiescent
704	<	*/
705	<	final void helpQuiescePool() {
706	<	for (;;) {
707	<	ForkJoinTask<?> t = getLocalOrStolenTask();
708	<	if (t != null) {
709	<	activate();
710	<	t.quietlyExec();
711	<	}
712	<	else {
713	<	inactivate();
714	<	if (pool.isQuiescent()) {
715	<	activate(); // re-activate on exit
716	<	break;
717	<	}
718	<	}
719	<	}
720	<	}
721	<
722	<	/**
723	<	* Returns an estimate of the number of tasks in the queue.
724	<	*/
725	<	final int getQueueSize() {
726	<	int b = base;
727	<	int n = sp - b;
728	<	return n <= 0? 0 : n; // suppress momentarily negative values
729	<	}
730	<
731	<	/**
732	<	* Returns an estimate of the number of tasks, offset by a
733	<	* function of number of idle workers.
734	<	*/
735	<	final int getEstimatedSurplusTaskCount() {
736	<	return (sp - base) - (pool.getIdleThreadCount() >>> 1);
737	<	}
738	<
739	<	// Per-worker exported random numbers
740	<
741	<	// Same constants as java.util.Random
742	<	final static long JURandomMultiplier = 0x5DEECE66DL;
743	<	final static long JURandomAddend = 0xBL;
744	<	final static long JURandomMask = (1L << 48) - 1;
745	<
746	<	private final int nextJURandom(int bits) {
747	<	long next = (juRandomSeed * JURandomMultiplier + JURandomAddend) &
748	<	JURandomMask;
749	<	juRandomSeed = next;
750	<	return (int)(next >>> (48 - bits));
751	<	}
752	<
753	<	private final int nextJURandomInt(int n) {
754	<	if (n <= 0)
755	<	throw new IllegalArgumentException("n must be positive");
756	<	int bits = nextJURandom(31);
757	<	if ((n & -n) == n)
758	<	return (int)((n * (long)bits) >> 31);
759	<
760	<	for (;;) {
761	<	int val = bits % n;
762	<	if (bits - val + (n-1) >= 0)
763	<	return val;
764	<	bits = nextJURandom(31);
765	<	}
766	<	}
767	<
768	<	private final long nextJURandomLong() {
769	<	return ((long)(nextJURandom(32)) << 32) + nextJURandom(32);
770	<	}
771	<
772	<	private final long nextJURandomLong(long n) {
773	<	if (n <= 0)
774	<	throw new IllegalArgumentException("n must be positive");
775	<	long offset = 0;
776	<	while (n >= Integer.MAX_VALUE) { // randomly pick half range
777	<	int bits = nextJURandom(2); // 2nd bit for odd vs even split
778	<	long half = n >>> 1;
779	<	long nextn = ((bits & 2) == 0)? half : n - half;
780	<	if ((bits & 1) == 0)
781	<	offset += n - nextn;
782	<	n = nextn;
783	<	}
784	<	return offset + nextJURandomInt((int)n);
785	<	}
786	<
787	<	private final double nextJURandomDouble() {
788	<	return (((long)(nextJURandom(26)) << 27) + nextJURandom(27))
789	<	/ (double)(1L << 53);
790	<	}
791	<
792	<	/**
793	<	* Returns a random integer using a per-worker random
794	<	* number generator with the same properties as
795	<	* {@link java.util.Random#nextInt}
796	<	* @return the next pseudorandom, uniformly distributed {@code int}
797	<	*         value from this worker's random number generator's sequence
798	<	*/
799	<	public static int nextRandomInt() {
800	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).
801	<	nextJURandom(32);
802	<	}
803	<
804	<	/**
805	<	* Returns a random integer using a per-worker random
806	<	* number generator with the same properties as
807	<	* {@link java.util.Random#nextInt(int)}
808	<	* @param n the bound on the random number to be returned.  Must be
809	<	*        positive.
810	<	* @return the next pseudorandom, uniformly distributed {@code int}
811	<	*         value between {@code 0} (inclusive) and {@code n} (exclusive)
812	<	*         from this worker's random number generator's sequence
813	<	* @throws IllegalArgumentException if n is not positive
814	<	*/
815	<	public static int nextRandomInt(int n) {
816	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).
817	<	nextJURandomInt(n);
818	<	}
819	<
820	<	/**
821	<	* Returns a random long using a per-worker random
822	<	* number generator with the same properties as
823	<	* {@link java.util.Random#nextLong}
824	<	* @return the next pseudorandom, uniformly distributed {@code long}
825	<	*         value from this worker's random number generator's sequence
826	<	*/
827	<	public static long nextRandomLong() {
828	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).
829	<	nextJURandomLong();
830	<	}
831	<
832	<	/**
833	<	* Returns a random integer using a per-worker random
834	<	* number generator with the same properties as
835	<	* {@link java.util.Random#nextInt(int)}
836	<	* @param n the bound on the random number to be returned.  Must be
837	<	*        positive.
838	<	* @return the next pseudorandom, uniformly distributed {@code int}
839	<	*         value between {@code 0} (inclusive) and {@code n} (exclusive)
840	<	*         from this worker's random number generator's sequence
841	<	* @throws IllegalArgumentException if n is not positive
842	<	*/
843	<	public static long nextRandomLong(long n) {
844	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).
845	<	nextJURandomLong(n);
846	<	}
847	<
848	<	/**
849	<	* Returns a random double using a per-worker random
850	<	* number generator with the same properties as
851	<	* {@link java.util.Random#nextDouble}
852	<	* @return the next pseudorandom, uniformly distributed {@code double}
853	<	*         value between {@code 0.0} and {@code 1.0} from this
854	<	*         worker's random number generator's sequence
855	<	*/
856	<	public static double nextRandomDouble() {
857	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).
858	<	nextJURandomDouble();
859	<	}
860	<
861	<	// Temporary Unsafe mechanics for preliminary release
862	<
863	<	static final Unsafe _unsafe;
864	<	static final long baseOffset;
865	<	static final long spOffset;
866	<	static final long qBase;
867	<	static final int qShift;
868	<	static final long runStateOffset;
869	<	static {
870	<	try {
871	<	if (ForkJoinWorkerThread.class.getClassLoader() != null) {
872	<	Field f = Unsafe.class.getDeclaredField("theUnsafe");
873	<	f.setAccessible(true);
874	<	_unsafe = (Unsafe)f.get(null);
119	>	try {
120	>	onTermination(exception);
121	>	} catch (Throwable ex) {
122	>	if (exception == null)
123	>	exception = ex;
124	>	} finally {
125	>	pool.deregisterWorker(this, exception);
126		}
876	–	else
877	–	_unsafe = Unsafe.getUnsafe();
878	–	baseOffset = _unsafe.objectFieldOffset
879	–	(ForkJoinWorkerThread.class.getDeclaredField("base"));
880	–	spOffset = _unsafe.objectFieldOffset
881	–	(ForkJoinWorkerThread.class.getDeclaredField("sp"));
882	–	runStateOffset = _unsafe.objectFieldOffset
883	–	(ForkJoinWorkerThread.class.getDeclaredField("runState"));
884	–	qBase = _unsafe.arrayBaseOffset(ForkJoinTask[].class);
885	–	int s = _unsafe.arrayIndexScale(ForkJoinTask[].class);
886	–	if ((s & (s-1)) != 0)
887	–	throw new Error("data type scale not a power of two");
888	–	qShift = 31 - Integer.numberOfLeadingZeros(s);
889	–	} catch (Exception e) {
890	–	throw new RuntimeException("Could not initialize intrinsics", e);
127		}
128		}
129		}

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