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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.2 by dl, Wed Jan 7 16:07:37 2009 UTC vs.
Revision 1.31 by dl, Mon Apr 5 15:52:26 2010 UTC

#	Line 5 | Line 5
5		*/
6
7		package jsr166y;
8	<	import java.util.*;
8	>
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.*;
10	>
11	>	import java.util.Random;
12	>	import java.util.Collection;
13	>	import java.util.concurrent.locks.LockSupport;
14
15		/**
16		* A thread managed by a {@link ForkJoinPool}.  This class is
17		* subclassable solely for the sake of adding functionality -- there
18	<	* are no overridable methods dealing with scheduling or
19	<	* execution. However, you can override initialization and termination
20	<	* cleanup methods surrounding the main task processing loop.  If you
21	<	* do create such a subclass, you will also need to supply a custom
22	<	* ForkJoinWorkerThreadFactory to use it in a ForkJoinPool.
23	<	*
24	<	* <p>This class also provides methods for generating per-thread
25	<	* random numbers, with the same properties as {@link
26	<	* java.util.Random} but with each generator isolated from those of
27	<	* other threads.
18	>	* are no overridable methods dealing with scheduling or execution.
19	>	* However, you can override initialization and termination methods
20	>	* surrounding the main task processing loop.  If you do create such a
21	>	* subclass, you will also need to supply a custom {@link
22	>	* ForkJoinPool.ForkJoinWorkerThreadFactory} to use it in a {@code
23	>	* ForkJoinPool}.
24	>	*
25	>	* @since 1.7
26	>	* @author Doug Lea
27		*/
28		public class ForkJoinWorkerThread extends Thread {
29		/*
30	<	* Algorithm overview:
30	>	* Overview:
31	>	*
32	>	* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
33	>	* ForkJoinTasks. This class includes bookkeeping in support of
34	>	* 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	<	* 1. Work-Stealing: Work-stealing queues are special forms of
41	<	* Deques that support only three of the four possible
42	<	* end-operations -- push, pop, and deq (aka steal), and only do
43	<	* so under the constraints that push and pop are called only from
44	<	* the owning thread, while deq may be called from other threads.
45	<	* (If you are unfamiliar with them, you probably want to read
46	<	* Herlihy and Shavit's book "The Art of Multiprocessor
47	<	* programming", chapter 16 describing these in more detail before
48	<	* proceeding.)  The main work-stealing queue design is roughly
49	<	* similar to "Dynamic Circular Work-Stealing Deque" by David
50	<	* Chase and Yossi Lev, SPAA 2005
51	<	* (http://research.sun.com/scalable/pubs/index.html).  The main
52	<	* difference ultimately stems from gc requirements that we null
53	<	* out taken slots as soon as we can, to maintain as small a
54	<	* footprint as possible even in programs generating huge numbers
55	<	* of tasks. To accomplish this, we shift the CAS arbitrating pop
56	<	* vs deq (steal) from being on the indices ("base" and "sp") to
57	<	* the slots themselves (mainly via method "casSlotNull()"). So,
58	<	* both a successful pop and deq mainly entail CAS'ing a nonnull
59	<	* slot to null.  Because we rely on CASes of references, we do
60	<	* not need tag bits on base or sp.  They are simple ints as used
61	<	* in any circular array-based queue (see for example ArrayDeque).
62	<	* Updates to the indices must still be ordered in a way that
63	<	* guarantees that (sp - base) > 0 means the queue is empty, but
64	<	* otherwise may err on the side of possibly making the queue
65	<	* appear nonempty when a push, pop, or deq have not fully
66	<	* committed. Note that this means that the deq operation,
67	<	* considered individually, is not wait-free. One thief cannot
68	<	* successfully continue until another in-progress one (or, if
69	<	* previously empty, a push) completes.  However, in the
70	<	* aggregate, we ensure at least probablistic non-blockingness. If
71	<	* an attempted steal fails, a thief always chooses a different
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 activate if necessary before
78	<	* stealing (see below).
77	>	* which gives threads a chance to set activation status if
78	>	* necessary before stealing.
79	>	*
80	>	* This approach also enables support for "async mode" where local
81	>	* task processing is in FIFO, not LIFO order; simply by using a
82	>	* version of deq rather than pop when locallyFifo is true (as set
83	>	* by the ForkJoinPool).  This allows use in message-passing
84	>	* frameworks in which tasks are never joined.
85		*
86		* Efficient implementation of this approach currently relies on
87		* an uncomfortable amount of "Unsafe" mechanics. To maintain
88		* correct orderings, reads and writes of variable base require
89	<	* volatile ordering.  Variable sp does not require volatile write
90	<	* but needs cheaper store-ordering on writes.  Because they are
91	<	* protected by volatile base reads, reads of the queue array and
92	<	* its slots do not need volatile load semantics, but writes (in
93	<	* push) require store order and CASes (in pop and deq) require
94	<	* (volatile) CAS semantics. Since these combinations aren't
95	<	* supported using ordinary volatiles, the only way to accomplish
96	<	* these effciently is to use direct Unsafe calls. (Using external
89	>	* volatile ordering.  Variable sp does not require volatile
90	>	* writes but still needs store-ordering, which we accomplish by
91	>	* pre-incrementing sp before filling the slot with an ordered
92	>	* store.  (Pre-incrementing also enables backouts used in
93	>	* scanWhileJoining.)  Because they are protected by volatile base
94	>	* reads, reads of the queue array and its slots by other threads
95	>	* do not need volatile load semantics, but writes (in push)
96	>	* require store order and CASes (in pop and deq) require
97	>	* (volatile) CAS semantics.  (Michael, Saraswat, and Vechev's
98	>	* algorithm has similar properties, but without support for
99	>	* nulling slots.)  Since these combinations aren't supported
100	>	* using ordinary volatiles, the only way to accomplish these
101	>	* efficiently is to use direct Unsafe calls. (Using external
102		* AtomicIntegers and AtomicReferenceArrays for the indices and
103		* array is significantly slower because of memory locality and
104	<	* indirection effects.) Further, performance on most platforms is
105	<	* very sensitive to placement and sizing of the (resizable) queue
106	<	* array.  Even though these queues don't usually become all that
107	<	* big, the initial size must be large enough to counteract cache
104	>	* indirection effects.)
105	>	*
106	>	* Further, performance on most platforms is very sensitive to
107	>	* placement and sizing of the (resizable) queue array.  Even
108	>	* though these queues don't usually become all that big, the
109	>	* initial size must be large enough to counteract cache
110		* contention effects across multiple queues (especially in the
111		* presence of GC cardmarking). Also, to improve thread-locality,
112	<	* queues are currently initialized immediately after the thread
113	<	* gets the initial signal to start processing tasks.  However,
114	<	* all queue-related methods except pushTask are written in a way
115	<	* that allows them to instead be lazily allocated and/or disposed
116	<	* 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.
112	>	* queues are initialized after starting.  All together, these
113	>	* low-level implementation choices produce as much as a factor of
114	>	* 4 performance improvement compared to naive implementations,
115	>	* and enable the processing of billions of tasks per second,
116	>	* sometimes at the expense of ugliness.
117		*/
118
119		/**
120	+	* Generator for initial random seeds for random victim
121	+	* selection. This is used only to create initial seeds. Random
122	+	* steals use a cheaper xorshift generator per steal attempt. We
123	+	* expect only rare contention on seedGenerator, so just use a
124	+	* plain Random.
125	+	*/
126	+	private static final Random seedGenerator = new Random();
127	+
128	+	/**
129	+	* The timeout value for suspending spares. Spare workers that
130	+	* remain unsignalled for more than this time may be trimmed
131	+	* (killed and removed from pool).  Since our goal is to avoid
132	+	* long-term thread buildup, the exact value of timeout does not
133	+	* matter too much so long as it avoids most false-alarm timeouts
134	+	* under GC stalls or momentarily high system load.
135	+	*/
136	+	private static final long SPARE_KEEPALIVE_NANOS =
137	+	5L * 1000L * 1000L * 1000L; // 5 secs
138	+
139	+	/**
140		* Capacity of work-stealing queue array upon initialization.
141		* Must be a power of two. Initial size must be at least 2, but is
142		* padded to minimize cache effects.
#	Line 134 | Line 145 | public class ForkJoinWorkerThread extend
145
146		/**
147		* Maximum work-stealing queue array size.  Must be less than or
148	<	* equal to 1 << 30 to ensure lack of index wraparound.
148	>	* equal to 1 << 28 to ensure lack of index wraparound. (This
149	>	* is less than usual bounds, because we need leftshift by 3
150	>	* to be in int range).
151		*/
152	<	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 30;
152	>	private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28;
153
154		/**
155	<	* Generator of seeds for per-thread random numbers.
155	>	* The pool this thread works in. Accessed directly by ForkJoinTask.
156		*/
157	<	private static final Random randomSeedGenerator = new Random();
157	>	final ForkJoinPool pool;
158
159		/**
160		* The work-stealing queue array. Size must be a power of two.
161	+	* Initialized in onStart, to improve memory locality.
162		*/
163		private ForkJoinTask<?>[] queue;
164
165		/**
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.
156	–	*/
157	–	private volatile int sp;
158	–
159	–	/**
166		* Index (mod queue.length) of least valid queue slot, which is
167		* always the next position to steal from if nonempty.
168		*/
169		private volatile int base;
170
171		/**
172	<	* The pool this thread works in.
172	>	* Index (mod queue.length) of next queue slot to push to or pop
173	>	* from. It is written only by owner thread, and accessed by other
174	>	* threads only after reading (volatile) base.  Both sp and base
175	>	* are allowed to wrap around on overflow, but (sp - base) still
176	>	* estimates size.
177	>	*/
178	>	private int sp;
179	>
180	>	/**
181	>	* Run state of this worker. In addition to the usual run levels,
182	>	* tracks if this worker is suspended as a spare, and if it was
183	>	* killed (trimmed) while suspended. However, "active" status is
184	>	* maintained separately.
185		*/
186	<	final ForkJoinPool pool;
186	>	private volatile int runState;
187	>
188	>	private static final int TERMINATING = 0x01;
189	>	private static final int TERMINATED  = 0x02;
190	>	private static final int SUSPENDED   = 0x04; // inactive spare
191	>	private static final int TRIMMED     = 0x08; // killed while suspended
192
193		/**
194	<	* Index of this worker in pool array. Set once by pool before
195	<	* running, and accessed directly by pool during cleanup etc
194	>	* Number of LockSupport.park calls to block this thread for
195	>	* suspension or event waits. Used for internal instrumention;
196	>	* currently not exported but included because volatile write upon
197	>	* park also provides a workaround for a JVM bug.
198		*/
199	<	int poolIndex;
199	>	private volatile int parkCount;
200
201		/**
202	<	* Run state of this worker. Supports simple versions of the usual
203	<	* shutdown/shutdownNow control.
202	>	* Number of steals, transferred and reset in pool callbacks pool
203	>	* when idle Accessed directly by pool.
204		*/
205	<	private volatile int runState;
205	>	int stealCount;
206
207	<	// Runstate values. Order matters
208	<	private static final int RUNNING     = 0;
209	<	private static final int SHUTDOWN    = 1;
210	<	private static final int TERMINATING = 2;
211	<	private static final int TERMINATED  = 3;
207	>	/**
208	>	* Seed for random number generator for choosing steal victims.
209	>	* Uses Marsaglia xorshift. Must be initialized as nonzero.
210	>	*/
211	>	private int seed;
212
213		/**
214		* Activity status. When true, this worker is considered active.
215	<	* 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.
215	>	* Accessed directly by pool.  Must be false upon construction.
216		*/
217	<	private boolean active;
217	>	boolean active;
218
219		/**
220	<	* Number of steals, transferred to pool when idle
220	>	* True if use local fifo, not default lifo, for local polling.
221	>	* Shadows value from ForkJoinPool, which resets it if changed
222	>	* pool-wide.
223		*/
224	<	private int stealCount;
224	>	private boolean locallyFifo;
225
226		/**
227	<	* Seed for random number generator for choosing steal victims
227	>	* Index of this worker in pool array. Set once by pool before
228	>	* running, and accessed directly by pool to locate this worker in
229	>	* its workers array.
230		*/
231	<	private int randomVictimSeed;
231	>	int poolIndex;
232
233		/**
234	<	* Seed for embedded Jurandom
234	>	* The last pool event waited for. Accessed only by pool in
235	>	* callback methods invoked within this thread.
236		*/
237	<	private long juRandomSeed;
237	>	int lastEventCount;
238
239		/**
240	<	* The last barrier event waited for
240	>	* Encoded index and event count of next event waiter. Used only
241	>	* by ForkJoinPool for managing event waiters.
242		*/
243	<	private long eventCount;
243	>	volatile long nextWaiter;
244
245		/**
246		* Creates a ForkJoinWorkerThread operating in the given pool.
247	+	*
248		* @param pool the pool this thread works in
249		* @throws NullPointerException if pool is null
250		*/
251		protected ForkJoinWorkerThread(ForkJoinPool pool) {
252		if (pool == null) throw new NullPointerException();
253		this.pool = pool;
254	<	// remaining initialization deferred to onStart
254	>	// To avoid exposing construction details to subclasses,
255	>	// remaining initialization is in start() and onStart()
256	>	}
257	>
258	>	/**
259	>	* Performs additional initialization and starts this thread
260	>	*/
261	>	final void start(int poolIndex, boolean locallyFifo,
262	>	UncaughtExceptionHandler ueh) {
263	>	this.poolIndex = poolIndex;
264	>	this.locallyFifo = locallyFifo;
265	>	if (ueh != null)
266	>	setUncaughtExceptionHandler(ueh);
267	>	setDaemon(true);
268	>	start();
269		}
270
271	<	// public access methods
271	>	// Public/protected methods
272
273		/**
274	<	* Returns the pool hosting the current task execution.
274	>	* Returns the pool hosting this thread.
275	>	*
276		* @return the pool
277		*/
278	<	public static ForkJoinPool getPool() {
279	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).pool;
278	>	public ForkJoinPool getPool() {
279	>	return pool;
280		}
281
282		/**
283	<	* Returns the index number of the current worker thread in its
284	<	* pool.  The returned value ranges from zero to the maximum
285	<	* number of threads (minus one) that have ever been created in
286	<	* the pool.  This method may be useful for applications that
287	<	* track status or collect results on a per-worker basis.
288	<	* @return the index number.
283	>	* Returns the index number of this thread in its pool.  The
284	>	* returned value ranges from zero to the maximum number of
285	>	* threads (minus one) that have ever been created in the pool.
286	>	* This method may be useful for applications that track status or
287	>	* collect results per-worker rather than per-task.
288	>	*
289	>	* @return the index number
290		*/
291	<	public static int getPoolIndex() {
292	<	return ((ForkJoinWorkerThread)(Thread.currentThread())).poolIndex;
291	>	public int getPoolIndex() {
292	>	return poolIndex;
293		}
294
295	<	//  Access methods used by Pool
295	>	/**
296	>	* Initializes internal state after construction but before
297	>	* processing any tasks. If you override this method, you must
298	>	* invoke super.onStart() at the beginning of the method.
299	>	* Initialization requires care: Most fields must have legal
300	>	* default values, to ensure that attempted accesses from other
301	>	* threads work correctly even before this thread starts
302	>	* processing tasks.
303	>	*/
304	>	protected void onStart() {
305	>	int rs = seedGenerator.nextInt();
306	>	seed = rs == 0? 1 : rs; // seed must be nonzero
307	>
308	>	// Allocate name string and queue array in this thread
309	>	String pid = Integer.toString(pool.getPoolNumber());
310	>	String wid = Integer.toString(poolIndex);
311	>	setName("ForkJoinPool-" + pid + "-worker-" + wid);
312	>
313	>	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
314	>	}
315
316		/**
317	<	* Get and clear steal count for accumulation by pool.  Called
318	<	* only when known to be idle (in pool.sync and termination).
317	>	* Performs cleanup associated with termination of this worker
318	>	* thread.  If you override this method, you must invoke
319	>	* {@code super.onTermination} at the end of the overridden method.
320	>	*
321	>	* @param exception the exception causing this thread to abort due
322	>	* to an unrecoverable error, or {@code null} if completed normally
323		*/
324	<	final int getAndClearStealCount() {
325	<	int sc = stealCount;
326	<	stealCount = 0;
327	<	return sc;
324	>	protected void onTermination(Throwable exception) {
325	>	try {
326	>	cancelTasks();
327	>	setTerminated();
328	>	pool.workerTerminated(this);
329	>	} catch (Throwable ex) {        // Shouldn't ever happen
330	>	if (exception == null)      // but if so, at least rethrown
331	>	exception = ex;
332	>	} finally {
333	>	if (exception != null)
334	>	UNSAFE.throwException(exception);
335	>	}
336	>	}
337	>
338	>	/**
339	>	* This method is required to be public, but should never be
340	>	* called explicitly. It performs the main run loop to execute
341	>	* ForkJoinTasks.
342	>	*/
343	>	public void run() {
344	>	Throwable exception = null;
345	>	try {
346	>	onStart();
347	>	mainLoop();
348	>	} catch (Throwable ex) {
349	>	exception = ex;
350	>	} finally {
351	>	onTermination(exception);
352	>	}
353		}
354
355	+	// helpers for run()
356	+
357		/**
358	<	* Returns estimate of the number of tasks in the queue, without
263	<	* correcting for transient negative values
358	>	* Find and execute tasks and check status while running
359		*/
360	<	final int getRawQueueSize() {
361	<	return sp - base;
360	>	private void mainLoop() {
361	>	boolean ran = false; // true if ran task on previous step
362	>	ForkJoinPool p = pool;
363	>	for (;;) {
364	>	p.preStep(this, ran);
365	>	if (runState != 0)
366	>	return;
367	>	ForkJoinTask<?> t; // try to get and run stolen or submitted task
368	>	if (ran = (t = scan()) != null || (t = pollSubmission()) != null) {
369	>	t.tryExec();
370	>	if (base != sp)
371	>	runLocalTasks();
372	>	}
373	>	}
374		}
375
376	<	// Intrinsics-based support for queue operations.
377	<	// Currently these three (setSp, setSlot, casSlotNull) are
378	<	// usually manually inlined to improve performance
376	>	/**
377	>	* Runs local tasks until queue is empty or shut down.  Call only
378	>	* while active.
379	>	*/
380	>	private void runLocalTasks() {
381	>	while (runState == 0) {
382	>	ForkJoinTask<?> t = locallyFifo? locallyDeqTask() : popTask();
383	>	if (t != null)
384	>	t.tryExec();
385	>	else if (base == sp)
386	>	break;
387	>	}
388	>	}
389
390		/**
391	<	* Sets sp in store-order.
391	>	* If a submission exists, try to activate and take it
392	>	*
393	>	* @return a task, if available
394		*/
395	<	private void setSp(int s) {
396	<	_unsafe.putOrderedInt(this, spOffset, s);
395	>	private ForkJoinTask<?> pollSubmission() {
396	>	ForkJoinPool p = pool;
397	>	while (p.hasQueuedSubmissions()) {
398	>	if (active || (active = p.tryIncrementActiveCount())) {
399	>	ForkJoinTask<?> t = p.pollSubmission();
400	>	return t != null ? t : scan(); // if missed, rescan
401	>	}
402	>	}
403	>	return null;
404		}
405
406	+	/*
407	+	* Intrinsics-based atomic writes for queue slots. These are
408	+	* basically the same as methods in AtomicObjectArray, but
409	+	* specialized for (1) ForkJoinTask elements (2) requirement that
410	+	* nullness and bounds checks have already been performed by
411	+	* callers and (3) effective offsets are known not to overflow
412	+	* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
413	+	* need corresponding version for reads: plain array reads are OK
414	+	* because they protected by other volatile reads and are
415	+	* confirmed by CASes.
416	+	*
417	+	* Most uses don't actually call these methods, but instead contain
418	+	* inlined forms that enable more predictable optimization.  We
419	+	* don't define the version of write used in pushTask at all, but
420	+	* instead inline there a store-fenced array slot write.
421	+	*/
422	+
423		/**
424	<	* Add in store-order the given task at given slot of q to
425	<	* null. Caller must ensure q is nonnull and index is in range.
424	>	* CASes slot i of array q from t to null. Caller must ensure q is
425	>	* non-null and index is in range.
426		*/
427	<	private static void setSlot(ForkJoinTask<?>[] q, int i,
428	<	ForkJoinTask<?> t){
429	<	_unsafe.putOrderedObject(q, (i << qShift) + qBase, t);
427	>	private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
428	>	ForkJoinTask<?> t) {
429	>	return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
430		}
431
432		/**
433	<	* CAS given slot of q to null. Caller must ensure q is nonnull
434	<	* and index is in range.
433	>	* Performs a volatile write of the given task at given slot of
434	>	* array q.  Caller must ensure q is non-null and index is in
435	>	* range. This method is used only during resets and backouts.
436		*/
437	<	private static boolean casSlotNull(ForkJoinTask<?>[] q, int i,
438	<	ForkJoinTask<?> t) {
439	<	return _unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
437	>	private static final void writeSlot(ForkJoinTask<?>[] q, int i,
438	>	ForkJoinTask<?> t) {
439	>	UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t);
440		}
441
442	<	// Main queue methods
442	>	// queue methods
443
444		/**
445	<	* Pushes a task. Called only by current thread.
446	<	* @param t the task. Caller must ensure nonnull
445	>	* Pushes a task. Call only from this thread.
446	>	*
447	>	* @param t the task. Caller must ensure non-null.
448		*/
449		final void pushTask(ForkJoinTask<?> t) {
450	+	int s;
451		ForkJoinTask<?>[] q = queue;
452	<	int mask = q.length - 1;
453	<	int s = sp;
454	<	_unsafe.putOrderedObject(q, ((s & mask) << qShift) + qBase, t);
455	<	_unsafe.putOrderedInt(this, spOffset, ++s);
456	<	if ((s -= base) == 1)
311	<	pool.signalNonEmptyWorkerQueue();
312	<	else if (s >= mask)
452	>	int mask = q.length - 1; // implicit assert q != null
453	>	UNSAFE.putOrderedObject(q, (((s = sp++) & mask) << qShift) + qBase, t);
454	>	if ((s -= base) <= 0)
455	>	pool.signalWork();
456	>	else if (s + 1 >= mask)
457		growQueue();
458		}
459
460		/**
461		* Tries to take a task from the base of the queue, failing if
462	<	* either empty or contended.
463	<	* @return a task, or null if none or contended.
462	>	* empty or contended. Note: Specializations of this code appear
463	>	* in scan and scanWhileJoining.
464	>	*
465	>	* @return a task, or null if none or contended
466		*/
467	<	private ForkJoinTask<?> deqTask() {
322	<	ForkJoinTask<?>[] q;
467	>	final ForkJoinTask<?> deqTask() {
468		ForkJoinTask<?> t;
469	<	int i;
470	<	int b;
471	<	if (sp != (b = base) &&
469	>	ForkJoinTask<?>[] q;
470	>	int b, i;
471	>	if ((b = base) != sp &&
472		(q = queue) != null && // must read q after b
473		(t = q[i = (q.length - 1) & b]) != null &&
474	<	_unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
474	>	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
475		base = b + 1;
476		return t;
477		}
#	Line 334 | Line 479 | public class ForkJoinWorkerThread extend
479		}
480
481		/**
482	<	* Returns a popped task, or null if empty.  Called only by
483	<	* current thread.
482	>	* Tries to take a task from the base of own queue. Assumes active
483	>	* status.  Called only by current thread.
484	>	*
485	>	* @return a task, or null if none
486	>	*/
487	>	final ForkJoinTask<?> locallyDeqTask() {
488	>	ForkJoinTask<?>[] q = queue;
489	>	if (q != null) {
490	>	ForkJoinTask<?> t;
491	>	int b, i;
492	>	while (sp != (b = base)) {
493	>	if ((t = q[i = (q.length - 1) & b]) != null &&
494	>	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase,
495	>	t, null)) {
496	>	base = b + 1;
497	>	return t;
498	>	}
499	>	}
500	>	}
501	>	return null;
502	>	}
503	>
504	>	/**
505	>	* Returns a popped task, or null if empty. Assumes active status.
506	>	* Called only by current thread. (Note: a specialization of this
507	>	* code appears in scanWhileJoining.)
508		*/
509		final ForkJoinTask<?> popTask() {
510	<	ForkJoinTask<?> t;
342	<	int i;
510	>	int s;
511		ForkJoinTask<?>[] q = queue;
512	<	int mask = q.length - 1;
513	<	int s = sp;
514	<	if (s != base &&
515	<	(t = q[i = (s - 1) & mask]) != null &&
516	<	_unsafe.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
517	<	_unsafe.putOrderedInt(this, spOffset, s - 1);
518	<	return t;
512	>	if (q != null && (s = sp) != base) {
513	>	int i = (q.length - 1) & --s;
514	>	ForkJoinTask<?> t = q[i];
515	>	if (t != null && UNSAFE.compareAndSwapObject
516	>	(q, (i << qShift) + qBase, t, null)) {
517	>	sp = s;
518	>	return t;
519	>	}
520		}
521		return null;
522		}
#	Line 355 | Line 524 | public class ForkJoinWorkerThread extend
524		/**
525		* Specialized version of popTask to pop only if
526		* topmost element is the given task. Called only
527	<	* by current thread.
528	<	* @param t the task. Caller must ensure nonnull
527	>	* by current thread while active.
528	>	*
529	>	* @param t the task. Caller must ensure non-null.
530		*/
531		final boolean unpushTask(ForkJoinTask<?> t) {
532	+	int s;
533		ForkJoinTask<?>[] q = queue;
534	<	int mask = q.length - 1;
535	<	int s = sp - 1;
536	<	if (_unsafe.compareAndSwapObject(q, ((s & mask) << qShift) + qBase,
366	<	t, null)) {
367	<	_unsafe.putOrderedInt(this, spOffset, s);
534	>	if (q != null && UNSAFE.compareAndSwapObject
535	>	(q, (((q.length - 1) & (s = sp - 1)) << qShift) + qBase, t, null)){
536	>	sp = s;
537		return true;
538		}
539		return false;
540		}
541
542		/**
543	<	* Returns next task to pop.
543	>	* Returns next task or null if empty or contended
544		*/
545		final ForkJoinTask<?> peekTask() {
546		ForkJoinTask<?>[] q = queue;
547	<	return q == null? null : q[(sp - 1) & (q.length - 1)];
547	>	if (q == null)
548	>	return null;
549	>	int mask = q.length - 1;
550	>	int i = locallyFifo ? base : (sp - 1);
551	>	return q[i & mask];
552		}
553
554		/**
#	Line 400 | Line 573 | public class ForkJoinWorkerThread extend
573		ForkJoinTask<?> t = oldQ[oldIndex];
574		if (t != null && !casSlotNull(oldQ, oldIndex, t))
575		t = null;
576	<	setSlot(newQ, b & newMask, t);
576	>	writeSlot(newQ, b & newMask, t);
577		} while (++b != bf);
578	<	pool.signalIdleWorkers(false);
578	>	pool.signalWork();
579		}
580
581	<	// Runstate management
581	>	/**
582	>	* Computes next value for random victim probe in scan().  Scans
583	>	* don't require a very high quality generator, but also not a
584	>	* crummy one.  Marsaglia xor-shift is cheap and works well enough.
585	>	* Note: This is manually inlined in scan()
586	>	*/
587	>	private static final int xorShift(int r) {
588	>	r ^= r << 13;
589	>	r ^= r >>> 17;
590	>	return r ^ (r << 5);
591	>	}
592
593	<	final boolean isShutdown()    { return runState >= SHUTDOWN;  }
594	<	final boolean isTerminating() { return runState >= TERMINATING;  }
595	<	final boolean isTerminated()  { return runState == TERMINATED; }
596	<	final boolean shutdown()      { return transitionRunStateTo(SHUTDOWN); }
597	<	final boolean shutdownNow()   { return transitionRunStateTo(TERMINATING); }
593	>	/**
594	>	* Tries to steal a task from another worker. Starts at a random
595	>	* index of workers array, and probes workers until finding one
596	>	* with non-empty queue or finding that all are empty.  It
597	>	* randomly selects the first n probes. If these are empty, it
598	>	* resorts to a circular sweep, which is necessary to accurately
599	>	* set active status. (The circular sweep uses steps of
600	>	* approximately half the array size plus 1, to avoid bias
601	>	* stemming from leftmost packing of the array in ForkJoinPool.)
602	>	*
603	>	* This method must be both fast and quiet -- usually avoiding
604	>	* memory accesses that could disrupt cache sharing etc other than
605	>	* those needed to check for and take tasks (or to activate if not
606	>	* already active). This accounts for, among other things,
607	>	* updating random seed in place without storing it until exit.
608	>	*
609	>	* @return a task, or null if none found
610	>	*/
611	>	private ForkJoinTask<?> scan() {
612	>	ForkJoinPool p = pool;
613	>	ForkJoinWorkerThread[] ws = p.workers;
614	>	int n = ws.length;            // upper bound of #workers
615	>	boolean canSteal = active;    // shadow active status
616	>	int r = seed;                 // extract seed once
617	>	int k = r;                    // index: random if j<0 else step
618	>	for (int j = -n; j < n; ++j) {
619	>	ForkJoinWorkerThread v = ws[k & (n - 1)];
620	>	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
621	>	if (v != null && v.base != v.sp) {
622	>	if (canSteal ||       // ensure active status
623	>	(canSteal = active = p.tryIncrementActiveCount())) {
624	>	int b, i;         // inlined specialization of deqTask
625	>	ForkJoinTask<?> t;
626	>	ForkJoinTask<?>[] q;
627	>	if ((b = v.base) != v.sp &&  // recheck
628	>	(q = v.queue) != null &&
629	>	(t = q[i = (q.length - 1) & b]) != null &&
630	>	UNSAFE.compareAndSwapObject
631	>	(q, (i << qShift) + qBase, t, null)) {
632	>	v.base = b + 1;
633	>	seed = r;
634	>	++stealCount;
635	>	return t;
636	>	}
637	>	}
638	>	j = -n;               // reset on contention
639	>	}
640	>	k = j >= 0? k + ((n >>> 1) | 1) : r;
641	>	}
642	>	return null;
643	>	}
644	>
645	>	// Run State management
646	>
647	>	// status check methods used mainly by ForkJoinPool
648	>	final boolean isTerminating() { return (runState & TERMINATING) != 0; }
649	>	final boolean isTerminated()  { return (runState & TERMINATED) != 0; }
650	>	final boolean isSuspended()   { return (runState & SUSPENDED) != 0; }
651	>	final boolean isTrimmed()     { return (runState & TRIMMED) != 0; }
652
653		/**
654	<	* Transition to at least the given state. Return true if not
418	<	* already at least given state.
654	>	* Sets state to TERMINATING, also resuming if suspended.
655		*/
656	<	private boolean transitionRunStateTo(int state) {
656	>	final void shutdown() {
657		for (;;) {
658		int s = runState;
659	<	if (s >= state)
660	<	return false;
661	<	if (_unsafe.compareAndSwapInt(this, runStateOffset, s, state))
662	<	return true;
659	>	if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended
660	>	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
661	>	(s & ~SUSPENDED) |
662	>	(TRIMMED|TERMINATING))) {
663	>	LockSupport.unpark(this);
664	>	break;
665	>	}
666	>	}
667	>	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
668	>	s | TERMINATING))
669	>	break;
670		}
671		}
672
673		/**
674	<	* Ensure status is active and if necessary adjust pool active count
674	>	* Sets state to TERMINATED. Called only by this thread.
675		*/
676	<	final void activate() {
677	<	if (!active) {
678	<	active = true;
679	<	pool.incrementActiveCount();
680	<	}
676	>	private void setTerminated() {
677	>	int s;
678	>	do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset,
679	>	s = runState,
680	>	s | (TERMINATING|TERMINATED)));
681		}
682
683		/**
684	<	* Ensure status is inactive and if necessary adjust pool active count
684	>	* Instrumented version of park. Also used by ForkJoinPool.awaitEvent
685		*/
686	<	final void inactivate() {
687	<	if (active) {
688	<	active = false;
446	<	pool.decrementActiveCount();
447	<	}
686	>	final void doPark() {
687	>	++parkCount;
688	>	LockSupport.park(this);
689		}
690
450	–	// Lifecycle methods
451	–
691		/**
692	<	* Initializes internal state after construction but before
693	<	* processing any tasks. If you override this method, you must
694	<	* invoke super.onStart() at the beginning of the method.
695	<	* Initialization requires care: Most fields must have legal
457	<	* default values, to ensure that attempted accesses from other
458	<	* threads work correctly even before this thread starts
459	<	* processing tasks.
692	>	* If suspended, tries to set status to unsuspended.
693	>	* Caller must unpark to actually resume
694	>	*
695	>	* @return true if successful
696		*/
697	<	protected void onStart() {
698	<	juRandomSeed = randomSeedGenerator.nextLong();
699	<	do;while((randomVictimSeed = nextRandomInt()) == 0); // must be nonzero
700	<	if (queue == null)
701	<	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	<	}
697	>	final boolean tryUnsuspend() {
698	>	int s;
699	>	return (((s = runState) & SUSPENDED) != 0 &&
700	>	UNSAFE.compareAndSwapInt(this, runStateOffset, s,
701	>	s & ~SUSPENDED));
702		}
703
704		/**
705	<	* Perform cleanup associated with termination of this worker
706	<	* thread.  If you override this method, you must invoke
477	<	* super.onTermination at the end of the overridden method.
705	>	* Sets suspended status and blocks as spare until resumed,
706	>	* shutdown, or timed out.
707		*
708	<	* @param exception the exception causing this thread to abort due
480	<	* to an unrecoverable error, or null if completed normally.
708	>	* @return false if trimmed
709		*/
710	<	protected void onTermination(Throwable exception) {
711	<	try {
712	<	clearLocalTasks();
713	<	inactivate();
714	<	cancelTasks();
715	<	} finally {
716	<	terminate(exception);
710	>	final boolean suspendAsSpare() {
711	>	for (;;) {               // set suspended unless terminating
712	>	int s = runState;
713	>	if ((s & TERMINATING) != 0) { // must kill
714	>	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
715	>	s | (TRIMMED | TERMINATING)))
716	>	return false;
717	>	}
718	>	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
719	>	s | SUSPENDED))
720	>	break;
721	>	}
722	>	lastEventCount = 0;      // reset upon resume
723	>	ForkJoinPool p = pool;
724	>	p.releaseWaiters();      // help others progress
725	>	p.accumulateStealCount(this);
726	>	interrupted();           // clear/ignore interrupts
727	>	if (poolIndex < p.getParallelism()) { // untimed wait
728	>	while ((runState & SUSPENDED) != 0)
729	>	doPark();
730	>	return true;
731		}
732	+	return timedSuspend();   // timed wait if apparently non-core
733		}
734
735		/**
736	<	* Notify pool of termination and, if exception is nonnull,
737	<	* rethrow it to trigger this thread's uncaughtExceptionHandler
736	>	* Blocks as spare until resumed or timed out
737	>	* @return false if trimmed
738		*/
739	<	private void terminate(Throwable exception) {
740	<	transitionRunStateTo(TERMINATED);
741	<	try {
742	<	pool.workerTerminated(this);
743	<	} finally {
744	<	if (exception != null)
745	<	ForkJoinTask.rethrowException(exception);
739	>	private boolean timedSuspend() {
740	>	long nanos = SPARE_KEEPALIVE_NANOS;
741	>	long startTime = System.nanoTime();
742	>	while ((runState & SUSPENDED) != 0) {
743	>	++parkCount;
744	>	if ((nanos -= (System.nanoTime() - startTime)) > 0)
745	>	LockSupport.parkNanos(this, nanos);
746	>	else { // try to trim on timeout
747	>	int s = runState;
748	>	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
749	>	(s & ~SUSPENDED) |
750	>	(TRIMMED|TERMINATING)))
751	>	return false;
752	>	}
753		}
754	+	return true;
755		}
756
757	+	// Misc support methods for ForkJoinPool
758	+
759		/**
760	<	* Run local tasks on exit from main.
760	>	* Returns an estimate of the number of tasks in the queue.  Also
761	>	* used by ForkJoinTask.
762		*/
763	<	private void clearLocalTasks() {
764	<	while (base != sp && !pool.isTerminating()) {
765	<	ForkJoinTask<?> t = popTask();
766	<	if (t != null) {
767	<	activate(); // ensure active status
768	<	t.quietlyExec();
769	<	}
770	<	}
763	>	final int getQueueSize() {
764	>	return -base + sp;
765	>	}
766	>
767	>	/**
768	>	* Set locallyFifo mode. Called only by ForkJoinPool
769	>	*/
770	>	final void setAsyncMode(boolean async) {
771	>	locallyFifo = async;
772		}
773
774		/**
#	Line 524 | Line 779 | public class ForkJoinWorkerThread extend
779		while (base != sp) {
780		ForkJoinTask<?> t = deqTask();
781		if (t != null)
782	<	t.cancelIgnoreExceptions();
782	>	t.cancelIgnoringExceptions();
783		}
784		}
785
786		/**
787	<	* This method is required to be public, but should never be
788	<	* called explicitly. It performs the main run loop to execute
789	<	* ForkJoinTasks.
787	>	* Drains tasks to given collection c.
788	>	*
789	>	* @return the number of tasks drained
790		*/
791	<	public void run() {
792	<	Throwable exception = null;
793	<	try {
794	<	onStart();
795	<	while (!isShutdown())
796	<	step();
797	<	} catch (Throwable ex) {
798	<	exception = ex;
544	<	} finally {
545	<	onTermination(exception);
791	>	final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
792	>	int n = 0;
793	>	while (base != sp) {
794	>	ForkJoinTask<?> t = deqTask();
795	>	if (t != null) {
796	>	c.add(t);
797	>	++n;
798	>	}
799		}
800	+	return n;
801		}
802
803	+	// Support methods for ForkJoinTask
804	+
805		/**
806	<	* Main top-level action.
806	>	* Returns an estimate of the number of tasks, offset by a
807	>	* function of number of idle workers.
808	>	*
809	>	* This method provides a cheap heuristic guide for task
810	>	* partitioning when programmers, frameworks, tools, or languages
811	>	* have little or no idea about task granularity.  In essence by
812	>	* offering this method, we ask users only about tradeoffs in
813	>	* overhead vs expected throughput and its variance, rather than
814	>	* how finely to partition tasks.
815	>	*
816	>	* In a steady state strict (tree-structured) computation, each
817	>	* thread makes available for stealing enough tasks for other
818	>	* threads to remain active. Inductively, if all threads play by
819	>	* the same rules, each thread should make available only a
820	>	* constant number of tasks.
821	>	*
822	>	* The minimum useful constant is just 1. But using a value of 1
823	>	* would require immediate replenishment upon each steal to
824	>	* maintain enough tasks, which is infeasible.  Further,
825	>	* partitionings/granularities of offered tasks should minimize
826	>	* steal rates, which in general means that threads nearer the top
827	>	* of computation tree should generate more than those nearer the
828	>	* bottom. In perfect steady state, each thread is at
829	>	* approximately the same level of computation tree. However,
830	>	* producing extra tasks amortizes the uncertainty of progress and
831	>	* diffusion assumptions.
832	>	*
833	>	* So, users will want to use values larger, but not much larger
834	>	* than 1 to both smooth over transient shortages and hedge
835	>	* against uneven progress; as traded off against the cost of
836	>	* extra task overhead. We leave the user to pick a threshold
837	>	* value to compare with the results of this call to guide
838	>	* decisions, but recommend values such as 3.
839	>	*
840	>	* When all threads are active, it is on average OK to estimate
841	>	* surplus strictly locally. In steady-state, if one thread is
842	>	* maintaining say 2 surplus tasks, then so are others. So we can
843	>	* just use estimated queue length (although note that (sp - base)
844	>	* can be an overestimate because of stealers lagging increments
845	>	* of base).  However, this strategy alone leads to serious
846	>	* mis-estimates in some non-steady-state conditions (ramp-up,
847	>	* ramp-down, other stalls). We can detect many of these by
848	>	* further considering the number of "idle" threads, that are
849	>	* known to have zero queued tasks, so compensate by a factor of
850	>	* (#idle/#active) threads.
851		*/
852	<	private void step() {
853	<	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	<	}
852	>	final int getEstimatedSurplusTaskCount() {
853	>	return sp - base - pool.idlePerActive();
854		}
855
856	<	// scanning for and stealing tasks
856	>	/**
857	>	* Gets and removes a local task.
858	>	*
859	>	* @return a task, if available
860	>	*/
861	>	final ForkJoinTask<?> pollLocalTask() {
862	>	while (base != sp) {
863	>	if (active || (active = pool.tryIncrementActiveCount()))
864	>	return locallyFifo? locallyDeqTask() : popTask();
865	>	}
866	>	return null;
867	>	}
868
869		/**
870	<	* 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.
870	>	* Gets and removes a local or stolen task.
871		*
872	<	* This is currently unused, and manually inlined
872	>	* @return a task, if available
873		*/
874	<	private static int xorShift(int r) {
875	<	r ^= r << 1;
876	<	r ^= r >>> 3;
576	<	r ^= r << 10;
577	<	return r;
874	>	final ForkJoinTask<?> pollTask() {
875	>	ForkJoinTask<?> t;
876	>	return (t = pollLocalTask()) != null ? t : scan();
877		}
878
879		/**
880	<	* 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.
880	>	* Returns a stolen task, if available, unless joinMe is done
881		*
882	<	* This method must be both fast and quiet -- usually avoiding
883	<	* memory accesses that could disrupt cache sharing etc other than
884	<	* those needed to check for and take tasks. This accounts for,
885	<	* among other things, updating random seed in place without
886	<	* storing it until exit. (Note that we only need to store it if
887	<	* we found a task; otherwise it doesn't matter if we start at the
888	<	* same place next time.)
882	>	* This method is intrinsically nonmodular. To maintain the
883	>	* property that tasks are never stolen if the awaited task is
884	>	* ready, we must interleave mechanics of scan with status
885	>	* checks. We rely here on the commit points of deq that allow us
886	>	* to cancel a steal even after CASing slot to null, but before
887	>	* adjusting base index: If, after the CAS, we see that joinMe is
888	>	* ready, we can back out by placing the task back into the slot,
889	>	* without adjusting index. The scan loop is otherwise the same as
890	>	* in scan.
891		*
892	<	* @param joinMe if non null; exit early if done
893	<	* @param checkSubmissions true if OK to take submissions
894	<	* @return a task, or null if none found
895	<	*/
896	<	private ForkJoinTask<?> scan(ForkJoinTask<?> joinMe,
897	<	boolean checkSubmissions) {
892	>	* The outer loop cannot be allowed to run forever, because it
893	>	* could lead to a form of deadlock if all threads are executing
894	>	* this method. However, we must also be patient before giving up,
895	>	* to cope with GC stalls, transient high loads, etc. The loop
896	>	* terminates (causing caller to possibly block this thread and
897	>	* create a replacement) only after #workers clean sweeps during
898	>	* which all running threads are active.
899	>	*/
900	>	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
901	>	int sweeps = 0;
902	>	int r = seed;
903		ForkJoinPool p = pool;
904	<	if (p == null)                    // Never null, but avoids
905	<	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;
904	>	p.releaseWaiters(); // help other threads progress
905	>	while (joinMe.status >= 0) {
906		ForkJoinWorkerThread[] ws = p.workers;
907	<	if (ws != null && (mask = ws.length - 1) > 0) {
908	<	int probes = -mask;       // use random index while negative
909	<	int idx = r;
910	<	for (;;) {
911	<	ForkJoinWorkerThread v;
912	<	// inlined xorshift to update seed
913	<	r ^= r << 1;  r ^= r >>> 3; r ^= r << 10;
914	<	if ((v = ws[mask & idx]) != null && v.sp != v.base) {
915	<	ForkJoinTask<?> t;
916	<	activate();
917	<	if ((joinMe == null || joinMe.status >= 0) &&
918	<	(t = v.deqTask()) != null) {
919	<	randomVictimSeed = r;
920	<	++stealCount;
921	<	return t;
907	>	int n = ws.length;
908	>	int k = r;
909	>	for (int j = -n; j < n; ++j) {
910	>	ForkJoinWorkerThread v = ws[k & (n - 1)];
911	>	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
912	>	if (v != null) {
913	>	int b = v.base;
914	>	ForkJoinTask<?>[] q;
915	>	if (b != v.sp && (q = v.queue) != null) {
916	>	int i = (q.length - 1) & b;
917	>	ForkJoinTask<?> t = q[i];
918	>	if (t != null) {
919	>	if (joinMe.status < 0)
920	>	return null;
921	>	if (UNSAFE.compareAndSwapObject
922	>	(q, (i << qShift) + qBase, t, null)) {
923	>	if (joinMe.status < 0) {
924	>	writeSlot(q, i, t); // back out
925	>	return null;
926	>	}
927	>	v.base = b + 1;
928	>	seed = r;
929	>	++stealCount;
930	>	return t;
931	>	}
932		}
933	<	continue restart; // restart on contention
933	>	sweeps = 0; // ensure rescan on contention
934		}
631	–	if ((probes >> 1) <= mask) // n-1 random then circular
632	–	idx = (probes++ < 0)? r : (idx + 1);
633	–	else
634	–	break;
935		}
936	+	k = j >= 0? k + ((n >>> 1) | 1) : r;
937	+	if ((j & 7) == 0 && joinMe.status < 0) // periodically recheck
938	+	return null;
939		}
940	<	if (checkSubmissions && p.hasQueuedSubmissions()) {
941	<	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	<	}
940	>	if ((sweeps = p.inactiveCount() == 0 ? sweeps + 1 : 0) > n)
941	>	return null;
942		}
943		return null;
944		}
945
946		/**
947	<	* 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
947	>	* Runs tasks until {@code pool.isQuiescent()}.
948		*/
949		final void helpQuiescePool() {
950		for (;;) {
951	<	ForkJoinTask<?> t = getLocalOrStolenTask();
952	<	if (t != null) {
953	<	activate();
710	<	t.quietlyExec();
711	<	}
951	>	ForkJoinTask<?> t = pollLocalTask();
952	>	if (t != null || (t = scan()) != null)
953	>	t.tryExec();
954		else {
955	<	inactivate();
956	<	if (pool.isQuiescent()) {
957	<	activate(); // re-activate on exit
958	<	break;
955	>	ForkJoinPool p = pool;
956	>	if (active) {
957	>	active = false; // inactivate
958	>	do {} while (!p.tryDecrementActiveCount());
959	>	}
960	>	if (p.isQuiescent()) {
961	>	active = true; // re-activate
962	>	do {} while (!p.tryIncrementActiveCount());
963	>	return;
964		}
965		}
966		}
967		}
968
969	<	/**
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	<	}
969	>	// Unsafe mechanics
970
971	<	/**
972	<	* Returns an estimate of the number of tasks, offset by a
973	<	* function of number of idle workers.
974	<	*/
975	<	final int getEstimatedSurplusTaskCount() {
976	<	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);
971	>	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
972	>	private static final long runStateOffset =
973	>	objectFieldOffset("runState", ForkJoinWorkerThread.class);
974	>	private static final long qBase =
975	>	UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
976	>	private static final int qShift;
977
978	<	for (;;) {
979	<	int val = bits % n;
980	<	if (bits - val + (n-1) >= 0)
981	<	return val;
982	<	bits = nextJURandom(31);
765	<	}
766	<	}
767	<
768	<	private final long nextJURandomLong() {
769	<	return ((long)(nextJURandom(32)) << 32) + nextJURandom(32);
978	>	static {
979	>	int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
980	>	if ((s & (s-1)) != 0)
981	>	throw new Error("data type scale not a power of two");
982	>	qShift = 31 - Integer.numberOfLeadingZeros(s);
983		}
984
985	<	private final long nextJURandomLong(long n) {
986	<	if (n <= 0)
987	<	throw new IllegalArgumentException("n must be positive");
988	<	long offset = 0;
989	<	while (n >= Integer.MAX_VALUE) { // randomly pick half range
990	<	int bits = nextJURandom(2); // 2nd bit for odd vs even split
991	<	long half = n >>> 1;
992	<	long nextn = ((bits & 2) == 0)? half : n - half;
780	<	if ((bits & 1) == 0)
781	<	offset += n - nextn;
782	<	n = nextn;
985	>	private static long objectFieldOffset(String field, Class<?> klazz) {
986	>	try {
987	>	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
988	>	} catch (NoSuchFieldException e) {
989	>	// Convert Exception to corresponding Error
990	>	NoSuchFieldError error = new NoSuchFieldError(field);
991	>	error.initCause(e);
992	>	throw error;
993		}
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);
994		}
995
996		/**
997	<	* Returns a random integer using a per-worker random
998	<	* number generator with the same properties as
999	<	* {@link java.util.Random#nextInt(int)}
1000	<	* @param n the bound on the random number to be returned.  Must be
1001	<	*        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
997	>	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
998	>	* Replace with a simple call to Unsafe.getUnsafe when integrating
999	>	* into a jdk.
1000	>	*
1001	>	* @return a sun.misc.Unsafe
1002		*/
1003	<	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 {
1003	>	private static sun.misc.Unsafe getUnsafe() {
1004		try {
1005	<	if (ForkJoinWorkerThread.class.getClassLoader() != null) {
1006	<	Field f = Unsafe.class.getDeclaredField("theUnsafe");
1007	<	f.setAccessible(true);
1008	<	_unsafe = (Unsafe)f.get(null);
1009	<	}
1010	<	else
1011	<	_unsafe = Unsafe.getUnsafe();
1012	<	baseOffset = _unsafe.objectFieldOffset
1013	<	(ForkJoinWorkerThread.class.getDeclaredField("base"));
1014	<	spOffset = _unsafe.objectFieldOffset
1015	<	(ForkJoinWorkerThread.class.getDeclaredField("sp"));
1016	<	runStateOffset = _unsafe.objectFieldOffset
1017	<	(ForkJoinWorkerThread.class.getDeclaredField("runState"));
1018	<	qBase = _unsafe.arrayBaseOffset(ForkJoinTask[].class);
1019	<	int s = _unsafe.arrayIndexScale(ForkJoinTask[].class);
1020	<	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);
1005	>	return sun.misc.Unsafe.getUnsafe();
1006	>	} catch (SecurityException se) {
1007	>	try {
1008	>	return java.security.AccessController.doPrivileged
1009	>	(new java.security
1010	>	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1011	>	public sun.misc.Unsafe run() throws Exception {
1012	>	java.lang.reflect.Field f = sun.misc
1013	>	.Unsafe.class.getDeclaredField("theUnsafe");
1014	>	f.setAccessible(true);
1015	>	return (sun.misc.Unsafe) f.get(null);
1016	>	}});
1017	>	} catch (java.security.PrivilegedActionException e) {
1018	>	throw new RuntimeException("Could not initialize intrinsics",
1019	>	e.getCause());
1020	>	}
1021		}
1022		}
1023		}

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