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Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.21 by jsr166, Mon Jul 27 20:57:44 2009 UTC vs.
Revision 1.37 by dl, Fri Jul 23 14:09:17 2010 UTC

#	Line 8 | Line 8 | package jsr166y;
8
9		import java.util.concurrent.*;
10
11	+	import java.util.Random;
12		import java.util.Collection;
13	+	import java.util.concurrent.locks.LockSupport;
14
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	<	* methods surrounding the main task processing loop.  If you do
21	<	* create such a subclass, you will also need to supply a custom
22	<	* ForkJoinWorkerThreadFactory to use it in a ForkJoinPool.
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	<	* 1. Work-Stealing: Work-stealing queues are special forms of
33	<	* Deques that support only three of the four possible
34	<	* end-operations -- push, pop, and deq (aka steal), and only do
35	<	* so under the constraints that push and pop are called only from
36	<	* the owning thread, while deq may be called from other threads.
37	<	* (If you are unfamiliar with them, you probably want to read
38	<	* Herlihy and Shavit's book "The Art of Multiprocessor
39	<	* programming", chapter 16 describing these in more detail before
40	<	* proceeding.)  The main work-stealing queue design is roughly
41	<	* similar to "Dynamic Circular Work-Stealing Deque" by David
42	<	* Chase and Yossi Lev, SPAA 2005
43	<	* (http://research.sun.com/scalable/pubs/index.html).  The main
44	<	* difference ultimately stems from gc requirements that we null
45	<	* out taken slots as soon as we can, to maintain as small a
46	<	* footprint as possible even in programs generating huge numbers
47	<	* of tasks. To accomplish this, we shift the CAS arbitrating pop
48	<	* vs deq (steal) from being on the indices ("base" and "sp") to
49	<	* the slots themselves (mainly via method "casSlotNull()"). So,
50	<	* both a successful pop and deq mainly entail CAS'ing a non-null
51	<	* slot to null.  Because we rely on CASes of references, we do
52	<	* not need tag bits on base or sp.  They are simple ints as used
53	<	* in any circular array-based queue (see for example ArrayDeque).
54	<	* Updates to the indices must still be ordered in a way that
55	<	* guarantees that (sp - base) > 0 means the queue is empty, but
56	<	* otherwise may err on the side of possibly making the queue
57	<	* appear nonempty when a push, pop, or deq have not fully
58	<	* committed. Note that this means that the deq operation,
59	<	* considered individually, is not wait-free. One thief cannot
60	<	* successfully continue until another in-progress one (or, if
61	<	* previously empty, a push) completes.  However, in the
62	<	* aggregate, we ensure at least probabilistic non-blockingness. If
63	<	* an attempted steal fails, a thief always chooses a different
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	>	* 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	>	* When a worker would otherwise be blocked waiting to join a
87	>	* task, it first tries a form of linear helping: Each worker
88	>	* records (in field currentSteal) the most recent task it stole
89	>	* from some other worker. Plus, it records (in field currentJoin)
90	>	* the task it is currently actively joining. Method joinTask uses
91	>	* these markers to try to find a worker to help (i.e., steal back
92	>	* a task from and execute it) that could hasten completion of the
93	>	* actively joined task. In essence, the joiner executes a task
94	>	* that would be on its own local deque had the to-be-joined task
95	>	* not been stolen. This may be seen as a conservative variant of
96	>	* the approach in Wagner & Calder "Leapfrogging: a portable
97	>	* technique for implementing efficient futures" SIGPLAN Notices,
98	>	* 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
99	>	* in that: (1) We only maintain dependency links across workers
100	>	* upon steals, rather than maintain per-task bookkeeping.  This
101	>	* may require a linear scan of workers array to locate stealers,
102	>	* but usually doesn't because stealers leave hints (that may
103	>	* become stale/wrong) of where to locate the kathem. This
104	>	* isolates cost to when it is needed, rather than adding to
105	>	* per-task overhead.  (2) It is "shallow", ignoring nesting and
106	>	* potentially cyclic mutual steals.  (3) It is intentionally
107	>	* racy: field currentJoin is updated only while actively joining,
108	>	* which means that we could miss links in the chain during
109	>	* long-lived tasks, GC stalls etc.  (4) We bound the number of
110	>	* attempts to find work (see MAX_HELP_DEPTH) and fall back to
111	>	* suspending the worker and if necessary replacing it with a
112	>	* spare (see ForkJoinPool.tryAwaitJoin).
113		*
114	<	* Efficient implementation of this approach currently relies on
115	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
114	>	* Efficient implementation of these algorithms currently relies
115	>	* on an uncomfortable amount of "Unsafe" mechanics. To maintain
116		* correct orderings, reads and writes of variable base require
117	<	* volatile ordering.  Variable sp does not require volatile write
118	<	* but needs cheaper store-ordering on writes.  Because they are
119	<	* protected by volatile base reads, reads of the queue array and
120	<	* its slots do not need volatile load semantics, but writes (in
121	<	* push) require store order and CASes (in pop and deq) require
122	<	* (volatile) CAS semantics. Since these combinations aren't
123	<	* supported using ordinary volatiles, the only way to accomplish
124	<	* these efficiently is to use direct Unsafe calls. (Using external
125	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
126	<	* array is significantly slower because of memory locality and
127	<	* indirection effects.) Further, performance on most platforms is
128	<	* very sensitive to placement and sizing of the (resizable) queue
129	<	* array.  Even though these queues don't usually become all that
130	<	* big, the initial size must be large enough to counteract cache
117	>	* volatile ordering.  Variable sp does not require volatile
118	>	* writes but still needs store-ordering, which we accomplish by
119	>	* pre-incrementing sp before filling the slot with an ordered
120	>	* store.  (Pre-incrementing also enables backouts used in
121	>	* joinTask.)  Because they are protected by volatile base reads,
122	>	* reads of the queue array and its slots by other threads do not
123	>	* need volatile load semantics, but writes (in push) require
124	>	* store order and CASes (in pop and deq) require (volatile) CAS
125	>	* semantics.  (Michael, Saraswat, and Vechev's algorithm has
126	>	* similar properties, but without support for nulling slots.)
127	>	* Since these combinations aren't supported using ordinary
128	>	* volatiles, the only way to accomplish these efficiently is to
129	>	* use direct Unsafe calls. (Using external AtomicIntegers and
130	>	* AtomicReferenceArrays for the indices and array is
131	>	* significantly slower because of memory locality and indirection
132	>	* effects.)
133	>	*
134	>	* Further, performance on most platforms is very sensitive to
135	>	* placement and sizing of the (resizable) queue array.  Even
136	>	* though these queues don't usually become all that big, the
137	>	* initial size must be large enough to counteract cache
138		* contention effects across multiple queues (especially in the
139		* presence of GC cardmarking). Also, to improve thread-locality,
140	<	* queues are currently initialized immediately after the thread
141	<	* gets the initial signal to start processing tasks.  However,
142	<	* all queue-related methods except pushTask are written in a way
143	<	* that allows them to instead be lazily allocated and/or disposed
144	<	* of when empty. All together, these low-level implementation
92	<	* choices produce as much as a factor of 4 performance
93	<	* improvement compared to naive implementations, and enable the
94	<	* processing of billions of tasks per second, sometimes at the
95	<	* expense of ugliness.
96	<	*
97	<	* 2. Run control: The primary run control is based on a global
98	<	* counter (activeCount) held by the pool. It uses an algorithm
99	<	* similar to that in Herlihy and Shavit section 17.6 to cause
100	<	* threads to eventually block when all threads declare they are
101	<	* inactive. (See variable "scans".)  For this to work, threads
102	<	* must be declared active when executing tasks, and before
103	<	* stealing a task. They must be inactive before blocking on the
104	<	* Pool Barrier (awaiting a new submission or other Pool
105	<	* event). In between, there is some free play which we take
106	<	* advantage of to avoid contention and rapid flickering of the
107	<	* global activeCount: If inactive, we activate only if a victim
108	<	* queue appears to be nonempty (see above).  Similarly, a thread
109	<	* tries to inactivate only after a full scan of other threads.
110	<	* The net effect is that contention on activeCount is rarely a
111	<	* measurable performance issue. (There are also a few other cases
112	<	* where we scan for work rather than retry/block upon
113	<	* contention.)
114	<	*
115	<	* 3. Selection control. We maintain policy of always choosing to
116	<	* run local tasks rather than stealing, and always trying to
117	<	* steal tasks before trying to run a new submission. All steals
118	<	* are currently performed in randomly-chosen deq-order. It may be
119	<	* worthwhile to bias these with locality / anti-locality
120	<	* information, but doing this well probably requires more
121	<	* lower-level information from JVMs than currently provided.
140	>	* queues are initialized after starting.  All together, these
141	>	* low-level implementation choices produce as much as a factor of
142	>	* 4 performance improvement compared to naive implementations,
143	>	* and enable the processing of billions of tasks per second,
144	>	* sometimes at the expense of ugliness.
145		*/
146
147		/**
148	+	* Generator for initial random seeds for random victim
149	+	* selection. This is used only to create initial seeds. Random
150	+	* steals use a cheaper xorshift generator per steal attempt. We
151	+	* expect only rare contention on seedGenerator, so just use a
152	+	* plain Random.
153	+	*/
154	+	private static final Random seedGenerator = new Random();
155	+
156	+	/**
157	+	* The timeout value for suspending spares. Spare workers that
158	+	* remain unsignalled for more than this time may be trimmed
159	+	* (killed and removed from pool).  Since our goal is to avoid
160	+	* long-term thread buildup, the exact value of timeout does not
161	+	* matter too much so long as it avoids most false-alarm timeouts
162	+	* under GC stalls or momentarily high system load.
163	+	*/
164	+	private static final long SPARE_KEEPALIVE_NANOS =
165	+	5L * 1000L * 1000L * 1000L; // 5 secs
166	+
167	+	/**
168	+	* The maximum stolen->joining link depth allowed in helpJoinTask.
169	+	* Depths for legitimate chains are unbounded, but we use a fixed
170	+	* constant to avoid (otherwise unchecked) cycles and bound
171	+	* staleness of traversal parameters at the expense of sometimes
172	+	* blocking when we could be helping.
173	+	*/
174	+	private static final int MAX_HELP_DEPTH = 8;
175	+
176	+	/**
177		* Capacity of work-stealing queue array upon initialization.
178	<	* Must be a power of two. Initial size must be at least 2, but is
178	>	* Must be a power of two. Initial size must be at least 4, but is
179		* padded to minimize cache effects.
180		*/
181		private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
#	Line 143 | Line 195 | public class ForkJoinWorkerThread extend
195
196		/**
197		* The work-stealing queue array. Size must be a power of two.
198	<	* Initialized when thread starts, to improve memory locality.
198	>	* Initialized in onStart, to improve memory locality.
199		*/
200		private ForkJoinTask<?>[] queue;
201
202		/**
151	–	* Index (mod queue.length) of next queue slot to push to or pop
152	–	* from. It is written only by owner thread, via ordered store.
153	–	* Both sp and base are allowed to wrap around on overflow, but
154	–	* (sp - base) still estimates size.
155	–	*/
156	–	private volatile int sp;
157	–
158	–	/**
203		* Index (mod queue.length) of least valid queue slot, which is
204		* always the next position to steal from if nonempty.
205		*/
206		private volatile int base;
207
208		/**
209	<	* Activity status. When true, this worker is considered active.
210	<	* Must be false upon construction. It must be true when executing
211	<	* tasks, and BEFORE stealing a task. It must be false before
212	<	* calling pool.sync.
209	>	* Index (mod queue.length) of next queue slot to push to or pop
210	>	* from. It is written only by owner thread, and accessed by other
211	>	* threads only after reading (volatile) base.  Both sp and base
212	>	* are allowed to wrap around on overflow, but (sp - base) still
213	>	* estimates size.
214	>	*/
215	>	private int sp;
216	>
217	>	/**
218	>	* The index of most recent stealer, used as a hint to avoid
219	>	* traversal in method helpJoinTask. This is only a hint because a
220	>	* worker might have had multiple steals and this only holds one
221	>	* of them (usually the most current). Declared non-volatile,
222	>	* relying on other prevailing sync to keep reasonably current.
223		*/
224	<	private boolean active;
224	>	private int stealHint;
225
226		/**
227	<	* Run state of this worker. Supports simple versions of the usual
228	<	* shutdown/shutdownNow control.
227	>	* Run state of this worker. In addition to the usual run levels,
228	>	* tracks if this worker is suspended as a spare, and if it was
229	>	* killed (trimmed) while suspended. However, "active" status is
230	>	* maintained separately.
231		*/
232		private volatile int runState;
233
234	+	private static final int TERMINATING = 0x01;
235	+	private static final int TERMINATED  = 0x02;
236	+	private static final int SUSPENDED   = 0x04; // inactive spare
237	+	private static final int TRIMMED     = 0x08; // killed while suspended
238	+
239	+	/**
240	+	* Number of LockSupport.park calls to block this thread for
241	+	* suspension or event waits. Used for internal instrumention;
242	+	* currently not exported but included because volatile write upon
243	+	* park also provides a workaround for a JVM bug.
244	+	*/
245	+	volatile int parkCount;
246	+
247	+	/**
248	+	* Number of steals, transferred and reset in pool callbacks pool
249	+	* when idle Accessed directly by pool.
250	+	*/
251	+	int stealCount;
252	+
253		/**
254		* Seed for random number generator for choosing steal victims.
255	<	* Uses Marsaglia xorshift. Must be nonzero upon initialization.
255	>	* Uses Marsaglia xorshift. Must be initialized as nonzero.
256		*/
257		private int seed;
258
259	+
260	+	/**
261	+	* Activity status. When true, this worker is considered active.
262	+	* Accessed directly by pool.  Must be false upon construction.
263	+	*/
264	+	boolean active;
265	+
266		/**
267	<	* Number of steals, transferred to pool when idle
267	>	* True if use local fifo, not default lifo, for local polling.
268	>	* Shadows value from ForkJoinPool, which resets it if changed
269	>	* pool-wide.
270		*/
271	<	private int stealCount;
271	>	private final boolean locallyFifo;
272
273		/**
274		* Index of this worker in pool array. Set once by pool before
275	<	* running, and accessed directly by pool during cleanup etc.
275	>	* running, and accessed directly by pool to locate this worker in
276	>	* its workers array.
277		*/
278		int poolIndex;
279
280		/**
281	<	* The last barrier event waited for. Accessed in pool callback
282	<	* methods, but only by current thread.
281	>	* The last pool event waited for. Accessed only by pool in
282	>	* callback methods invoked within this thread.
283		*/
284	<	long lastEventCount;
284	>	int lastEventCount;
285
286		/**
287	<	* True if use local fifo, not default lifo, for local polling
287	>	* Encoded index and event count of next event waiter. Used only
288	>	* by ForkJoinPool for managing event waiters.
289		*/
290	<	private boolean locallyFifo;
290	>	volatile long nextWaiter;
291	>
292	>	/**
293	>	* The task currently being joined, set only when actively trying
294	>	* to helpStealer. Written only by current thread, but read by
295	>	* others.
296	>	*/
297	>	private volatile ForkJoinTask<?> currentJoin;
298	>
299	>	/**
300	>	* The task most recently stolen from another worker (or
301	>	* submission queue).  Not volatile because always read/written in
302	>	* presence of related volatiles in those cases where it matters.
303	>	*/
304	>	private ForkJoinTask<?> currentSteal;
305
306		/**
307		* Creates a ForkJoinWorkerThread operating in the given pool.
#	Line 210 | Line 310 | public class ForkJoinWorkerThread extend
310		* @throws NullPointerException if pool is null
311		*/
312		protected ForkJoinWorkerThread(ForkJoinPool pool) {
213	–	if (pool == null) throw new NullPointerException();
313		this.pool = pool;
314	<	// Note: poolIndex is set by pool during construction
315	<	// Remaining initialization is deferred to onStart
314	>	this.locallyFifo = pool.locallyFifo;
315	>	// To avoid exposing construction details to subclasses,
316	>	// remaining initialization is in start() and onStart()
317		}
318
319	<	// Public access methods
319	>	/**
320	>	* Performs additional initialization and starts this thread
321	>	*/
322	>	final void start(int poolIndex, UncaughtExceptionHandler ueh) {
323	>	this.poolIndex = poolIndex;
324	>	if (ueh != null)
325	>	setUncaughtExceptionHandler(ueh);
326	>	setDaemon(true);
327	>	start();
328	>	}
329	>
330	>	// Public/protected methods
331
332		/**
333		* Returns the pool hosting this thread.
#	Line 241 | Line 352 | public class ForkJoinWorkerThread extend
352		}
353
354		/**
355	<	* Establishes local first-in-first-out scheduling mode for forked
356	<	* tasks that are never joined.
357	<	*
358	<	* @param async if true, use locally FIFO scheduling
355	>	* Initializes internal state after construction but before
356	>	* processing any tasks. If you override this method, you must
357	>	* invoke super.onStart() at the beginning of the method.
358	>	* Initialization requires care: Most fields must have legal
359	>	* default values, to ensure that attempted accesses from other
360	>	* threads work correctly even before this thread starts
361	>	* processing tasks.
362		*/
363	<	void setAsyncMode(boolean async) {
364	<	locallyFifo = async;
365	<	}
252	<
253	<	// Runstate management
254	<
255	<	// Runstate values. Order matters
256	<	private static final int RUNNING     = 0;
257	<	private static final int SHUTDOWN    = 1;
258	<	private static final int TERMINATING = 2;
259	<	private static final int TERMINATED  = 3;
260	<
261	<	final boolean isShutdown()    { return runState >= SHUTDOWN;  }
262	<	final boolean isTerminating() { return runState >= TERMINATING;  }
263	<	final boolean isTerminated()  { return runState == TERMINATED; }
264	<	final boolean shutdown()      { return transitionRunStateTo(SHUTDOWN); }
265	<	final boolean shutdownNow()   { return transitionRunStateTo(TERMINATING); }
363	>	protected void onStart() {
364	>	int rs = seedGenerator.nextInt();
365	>	seed = rs == 0? 1 : rs; // seed must be nonzero
366
367	<	/**
368	<	* Transitions to at least the given state.
369	<	*
370	<	* @return {@code true} if not already at least at given state
271	<	*/
272	<	private boolean transitionRunStateTo(int state) {
273	<	for (;;) {
274	<	int s = runState;
275	<	if (s >= state)
276	<	return false;
277	<	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, state))
278	<	return true;
279	<	}
280	<	}
367	>	// Allocate name string and arrays in this thread
368	>	String pid = Integer.toString(pool.getPoolNumber());
369	>	String wid = Integer.toString(poolIndex);
370	>	setName("ForkJoinPool-" + pid + "-worker-" + wid);
371
372	<	/**
283	<	* Tries to set status to active; fails on contention.
284	<	*/
285	<	private boolean tryActivate() {
286	<	if (!active) {
287	<	if (!pool.tryIncrementActiveCount())
288	<	return false;
289	<	active = true;
290	<	}
291	<	return true;
372	>	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
373		}
374
375		/**
376	<	* Tries to set status to inactive; fails on contention.
376	>	* Performs cleanup associated with termination of this worker
377	>	* thread.  If you override this method, you must invoke
378	>	* {@code super.onTermination} at the end of the overridden method.
379	>	*
380	>	* @param exception the exception causing this thread to abort due
381	>	* to an unrecoverable error, or {@code null} if completed normally
382		*/
383	<	private boolean tryInactivate() {
384	<	if (active) {
385	<	if (!pool.tryDecrementActiveCount())
386	<	return false;
387	<	active = false;
383	>	protected void onTermination(Throwable exception) {
384	>	try {
385	>	cancelTasks();
386	>	setTerminated();
387	>	pool.workerTerminated(this);
388	>	} catch (Throwable ex) {        // Shouldn't ever happen
389	>	if (exception == null)      // but if so, at least rethrown
390	>	exception = ex;
391	>	} finally {
392	>	if (exception != null)
393	>	UNSAFE.throwException(exception);
394		}
303	–	return true;
304	–	}
305	–
306	–	/**
307	–	* Computes next value for random victim probe.  Scans don't
308	–	* require a very high quality generator, but also not a crummy
309	–	* one.  Marsaglia xor-shift is cheap and works well.
310	–	*/
311	–	private static int xorShift(int r) {
312	–	r ^= r << 1;
313	–	r ^= r >>> 3;
314	–	r ^= r << 10;
315	–	return r;
395		}
396
318	–	// Lifecycle methods
319	–
397		/**
398		* This method is required to be public, but should never be
399		* called explicitly. It performs the main run loop to execute
#	Line 326 | Line 403 | public class ForkJoinWorkerThread extend
403		Throwable exception = null;
404		try {
405		onStart();
329	–	pool.sync(this); // await first pool event
406		mainLoop();
407		} catch (Throwable ex) {
408		exception = ex;
#	Line 335 | Line 411 | public class ForkJoinWorkerThread extend
411		}
412		}
413
414	+	// helpers for run()
415	+
416		/**
417	<	* Executes tasks until shut down.
417	>	* Find and execute tasks and check status while running
418		*/
419		private void mainLoop() {
420	<	while (!isShutdown()) {
421	<	ForkJoinTask<?> t = pollTask();
422	<	if (t != null || (t = pollSubmission()) != null)
423	<	t.quietlyExec();
424	<	else if (tryInactivate())
425	<	pool.sync(this);
420	>	int emptyScans = 0; // consecutive times failed to find work
421	>	ForkJoinPool p = pool;
422	>	for (;;) {
423	>	p.preStep(this, emptyScans);
424	>	if (runState != 0)
425	>	return;
426	>	ForkJoinTask<?> t; // try to get and run stolen or submitted task
427	>	if ((t = scan()) != null || (t = pollSubmission()) != null) {
428	>	t.tryExec();
429	>	if (base != sp)
430	>	runLocalTasks();
431	>	currentSteal = null;
432	>	emptyScans = 0;
433	>	}
434	>	else
435	>	++emptyScans;
436		}
437		}
438
439		/**
440	<	* Initializes internal state after construction but before
441	<	* processing any tasks. If you override this method, you must
354	<	* invoke super.onStart() at the beginning of the method.
355	<	* Initialization requires care: Most fields must have legal
356	<	* default values, to ensure that attempted accesses from other
357	<	* threads work correctly even before this thread starts
358	<	* processing tasks.
440	>	* Runs local tasks until queue is empty or shut down.  Call only
441	>	* while active.
442		*/
443	<	protected void onStart() {
444	<	// Allocate while starting to improve chances of thread-local
445	<	// isolation
446	<	queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
447	<	// Initial value of seed need not be especially random but
448	<	// should differ across workers and must be nonzero
449	<	int p = poolIndex + 1;
450	<	seed = p + (p << 8) + (p << 16) + (p << 24); // spread bits
443	>	private void runLocalTasks() {
444	>	while (runState == 0) {
445	>	ForkJoinTask<?> t = locallyFifo? locallyDeqTask() : popTask();
446	>	if (t != null)
447	>	t.tryExec();
448	>	else if (base == sp)
449	>	break;
450	>	}
451		}
452
453		/**
454	<	* Performs cleanup associated with termination of this worker
372	<	* thread.  If you override this method, you must invoke
373	<	* {@code super.onTermination} at the end of the overridden method.
454	>	* If a submission exists, try to activate and take it
455		*
456	<	* @param exception the exception causing this thread to abort due
376	<	* to an unrecoverable error, or {@code null} if completed normally
456	>	* @return a task, if available
457		*/
458	<	protected void onTermination(Throwable exception) {
459	<	// Execute remaining local tasks unless aborting or terminating
460	<	while (exception == null &&  !pool.isTerminating() && base != sp) {
461	<	try {
462	<	ForkJoinTask<?> t = popTask();
463	<	if (t != null)
464	<	t.quietlyExec();
465	<	} catch (Throwable ex) {
466	<	exception = ex;
458	>	private ForkJoinTask<?> pollSubmission() {
459	>	ForkJoinPool p = pool;
460	>	while (p.hasQueuedSubmissions()) {
461	>	if (active || (active = p.tryIncrementActiveCount())) {
462	>	ForkJoinTask<?> t = p.pollSubmission();
463	>	if (t != null) {
464	>	currentSteal = t;
465	>	return t;
466	>	}
467	>	return scan(); // if missed, rescan
468		}
469		}
470	<	// Cancel other tasks, transition status, notify pool, and
390	<	// propagate exception to uncaught exception handler
391	<	try {
392	<	do {} while (!tryInactivate()); // ensure inactive
393	<	cancelTasks();
394	<	runState = TERMINATED;
395	<	pool.workerTerminated(this);
396	<	} catch (Throwable ex) {        // Shouldn't ever happen
397	<	if (exception == null)      // but if so, at least rethrown
398	<	exception = ex;
399	<	} finally {
400	<	if (exception != null)
401	<	ForkJoinTask.rethrowException(exception);
402	<	}
470	>	return null;
471		}
472
473	<	// Intrinsics-based support for queue operations.
474	<
475	<	/**
476	<	* Adds in store-order the given task at given slot of q to null.
477	<	* Caller must ensure q is non-null and index is in range.
473	>	/*
474	>	* Intrinsics-based atomic writes for queue slots. These are
475	>	* basically the same as methods in AtomicObjectArray, but
476	>	* specialized for (1) ForkJoinTask elements (2) requirement that
477	>	* nullness and bounds checks have already been performed by
478	>	* callers and (3) effective offsets are known not to overflow
479	>	* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
480	>	* need corresponding version for reads: plain array reads are OK
481	>	* because they protected by other volatile reads and are
482	>	* confirmed by CASes.
483	>	*
484	>	* Most uses don't actually call these methods, but instead contain
485	>	* inlined forms that enable more predictable optimization.  We
486	>	* don't define the version of write used in pushTask at all, but
487	>	* instead inline there a store-fenced array slot write.
488		*/
411	–	private static void setSlot(ForkJoinTask<?>[] q, int i,
412	–	ForkJoinTask<?> t) {
413	–	UNSAFE.putOrderedObject(q, (i << qShift) + qBase, t);
414	–	}
489
490		/**
491	<	* CAS given slot of q to null. Caller must ensure q is non-null
492	<	* and index is in range.
491	>	* CASes slot i of array q from t to null. Caller must ensure q is
492	>	* non-null and index is in range.
493		*/
494	<	private static boolean casSlotNull(ForkJoinTask<?>[] q, int i,
495	<	ForkJoinTask<?> t) {
494	>	private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
495	>	ForkJoinTask<?> t) {
496		return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
497		}
498
499		/**
500	<	* Sets sp in store-order.
500	>	* Performs a volatile write of the given task at given slot of
501	>	* array q.  Caller must ensure q is non-null and index is in
502	>	* range. This method is used only during resets and backouts.
503		*/
504	<	private void storeSp(int s) {
505	<	UNSAFE.putOrderedInt(this, spOffset, s);
504	>	private static final void writeSlot(ForkJoinTask<?>[] q, int i,
505	>	ForkJoinTask<?> t) {
506	>	UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t);
507		}
508
509	<	// Main queue methods
509	>	// queue methods
510
511		/**
512	<	* Pushes a task. Called only by current thread.
512	>	* Pushes a task. Call only from this thread.
513		*
514		* @param t the task. Caller must ensure non-null.
515		*/
516		final void pushTask(ForkJoinTask<?> t) {
517		ForkJoinTask<?>[] q = queue;
518	<	int mask = q.length - 1;
519	<	int s = sp;
520	<	setSlot(q, s & mask, t);
521	<	storeSp(++s);
522	<	if ((s -= base) == 1)
523	<	pool.signalWork();
524	<	else if (s >= mask)
448	<	growQueue();
518	>	int mask = q.length - 1; // implicit assert q != null
519	>	int s = sp++;            // ok to increment sp before slot write
520	>	UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t);
521	>	if ((s -= base) == 0)
522	>	pool.signalWork();   // was empty
523	>	else if (s == mask)
524	>	growQueue();         // is full
525		}
526
527		/**
528		* Tries to take a task from the base of the queue, failing if
529	<	* either empty or contended.
529	>	* empty or contended. Note: Specializations of this code appear
530	>	* in locallyDeqTask and elsewhere.
531		*
532		* @return a task, or null if none or contended
533		*/
534		final ForkJoinTask<?> deqTask() {
535		ForkJoinTask<?> t;
536		ForkJoinTask<?>[] q;
537	<	int i;
538	<	int b;
462	<	if (sp != (b = base) &&
537	>	int b, i;
538	>	if ((b = base) != sp &&
539		(q = queue) != null && // must read q after b
540	<	(t = q[i = (q.length - 1) & b]) != null &&
541	<	casSlotNull(q, i, t)) {
540	>	(t = q[i = (q.length - 1) & b]) != null && base == b &&
541	>	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
542		base = b + 1;
543		return t;
544		}
#	Line 470 | Line 546 | public class ForkJoinWorkerThread extend
546		}
547
548		/**
549	<	* Returns a popped task, or null if empty. Ensures active status
550	<	* if non-null. Called only by current thread.
549	>	* Tries to take a task from the base of own queue. Assumes active
550	>	* status.  Called only by current thread.
551	>	*
552	>	* @return a task, or null if none
553	>	*/
554	>	final ForkJoinTask<?> locallyDeqTask() {
555	>	ForkJoinTask<?>[] q = queue;
556	>	if (q != null) {
557	>	ForkJoinTask<?> t;
558	>	int b, i;
559	>	while (sp != (b = base)) {
560	>	if ((t = q[i = (q.length - 1) & b]) != null && base == b &&
561	>	UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase,
562	>	t, null)) {
563	>	base = b + 1;
564	>	return t;
565	>	}
566	>	}
567	>	}
568	>	return null;
569	>	}
570	>
571	>	/**
572	>	* Returns a popped task, or null if empty. Assumes active status.
573	>	* Called only by current thread.
574		*/
575		final ForkJoinTask<?> popTask() {
576	<	int s = sp;
577	<	while (s != base) {
578	<	if (tryActivate()) {
579	<	ForkJoinTask<?>[] q = queue;
580	<	int mask = q.length - 1;
581	<	int i = (s - 1) & mask;
582	<	ForkJoinTask<?> t = q[i];
583	<	if (t == null || !casSlotNull(q, i, t))
485	<	break;
486	<	storeSp(s - 1);
576	>	int s;
577	>	ForkJoinTask<?>[] q;
578	>	if (base != (s = sp) && (q = queue) != null) {
579	>	int i = (q.length - 1) & --s;
580	>	ForkJoinTask<?> t = q[i];
581	>	if (t != null && UNSAFE.compareAndSwapObject
582	>	(q, (i << qShift) + qBase, t, null)) {
583	>	sp = s;
584		return t;
585		}
586		}
#	Line 491 | Line 588 | public class ForkJoinWorkerThread extend
588		}
589
590		/**
591	<	* Specialized version of popTask to pop only if
592	<	* topmost element is the given task. Called only
593	<	* by current thread while active.
591	>	* Specialized version of popTask to pop only if topmost element
592	>	* is the given task. Called only by current thread while
593	>	* active.
594		*
595		* @param t the task. Caller must ensure non-null.
596		*/
597		final boolean unpushTask(ForkJoinTask<?> t) {
598	<	ForkJoinTask<?>[] q = queue;
599	<	int mask = q.length - 1;
600	<	int s = sp - 1;
601	<	if (casSlotNull(q, s & mask, t)) {
602	<	storeSp(s);
598	>	int s;
599	>	ForkJoinTask<?>[] q;
600	>	if (base != (s = sp) && (q = queue) != null &&
601	>	UNSAFE.compareAndSwapObject
602	>	(q, (((q.length - 1) & --s) << qShift) + qBase, t, null)) {
603	>	sp = s;
604		return true;
605		}
606		return false;
607		}
608
609		/**
610	<	* Returns next task.
610	>	* Returns next task or null if empty or contended
611		*/
612		final ForkJoinTask<?> peekTask() {
613		ForkJoinTask<?>[] q = queue;
#	Line 542 | Line 640 | public class ForkJoinWorkerThread extend
640		ForkJoinTask<?> t = oldQ[oldIndex];
641		if (t != null && !casSlotNull(oldQ, oldIndex, t))
642		t = null;
643	<	setSlot(newQ, b & newMask, t);
643	>	writeSlot(newQ, b & newMask, t);
644		} while (++b != bf);
645		pool.signalWork();
646		}
647
648		/**
649	+	* Computes next value for random victim probe in scan().  Scans
650	+	* don't require a very high quality generator, but also not a
651	+	* crummy one.  Marsaglia xor-shift is cheap and works well enough.
652	+	* Note: This is manually inlined in scan()
653	+	*/
654	+	private static final int xorShift(int r) {
655	+	r ^= r << 13;
656	+	r ^= r >>> 17;
657	+	return r ^ (r << 5);
658	+	}
659	+
660	+	/**
661		* Tries to steal a task from another worker. Starts at a random
662		* index of workers array, and probes workers until finding one
663		* with non-empty queue or finding that all are empty.  It
664		* randomly selects the first n probes. If these are empty, it
665	<	* resorts to a full circular traversal, which is necessary to
666	<	* accurately set active status by caller. Also restarts if pool
667	<	* events occurred since last scan, which forces refresh of
668	<	* workers array, in case barrier was associated with resize.
665	>	* resorts to a circular sweep, which is necessary to accurately
666	>	* set active status. (The circular sweep uses steps of
667	>	* approximately half the array size plus 1, to avoid bias
668	>	* stemming from leftmost packing of the array in ForkJoinPool.)
669		*
670		* This method must be both fast and quiet -- usually avoiding
671		* memory accesses that could disrupt cache sharing etc other than
672	<	* those needed to check for and take tasks. This accounts for,
673	<	* among other things, updating random seed in place without
674	<	* storing it until exit.
672	>	* those needed to check for and take tasks (or to activate if not
673	>	* already active). This accounts for, among other things,
674	>	* updating random seed in place without storing it until exit.
675		*
676		* @return a task, or null if none found
677		*/
678		private ForkJoinTask<?> scan() {
679	<	ForkJoinTask<?> t = null;
680	<	int r = seed;                    // extract once to keep scan quiet
681	<	ForkJoinWorkerThread[] ws;       // refreshed on outer loop
682	<	int mask;                        // must be power 2 minus 1 and > 0
683	<	outer:do {
684	<	if ((ws = pool.workers) != null && (mask = ws.length - 1) > 0) {
685	<	int idx = r;
686	<	int probes = ~mask;      // use random index while negative
687	<	for (;;) {
688	<	r = xorShift(r);     // update random seed
689	<	ForkJoinWorkerThread v = ws[mask & idx];
690	<	if (v == null || v.sp == v.base) {
691	<	if (probes <= mask)
692	<	idx = (probes++ < 0) ? r : (idx + 1);
693	<	else
694	<	break;
679	>	ForkJoinPool p = pool;
680	>	ForkJoinWorkerThread[] ws;        // worker array
681	>	int n;                            // upper bound of #workers
682	>	if ((ws = p.workers) != null && (n = ws.length) > 1) {
683	>	boolean canSteal = active;    // shadow active status
684	>	int r = seed;                 // extract seed once
685	>	int mask = n - 1;
686	>	int j = -n;                   // loop counter
687	>	int k = r;                    // worker index, random if j < 0
688	>	for (;;) {
689	>	ForkJoinWorkerThread v = ws[k & mask];
690	>	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift
691	>	if (v != null && v.base != v.sp) {
692	>	if (canSteal ||       // ensure active status
693	>	(canSteal = active = p.tryIncrementActiveCount())) {
694	>	int b = v.base;   // inline specialized deqTask
695	>	ForkJoinTask<?>[] q;
696	>	if (b != v.sp && (q = v.queue) != null) {
697	>	ForkJoinTask<?> t;
698	>	int i = (q.length - 1) & b;
699	>	long u = (i << qShift) + qBase; // raw offset
700	>	if ((t = q[i]) != null && v.base == b &&
701	>	UNSAFE.compareAndSwapObject(q, u, t, null)) {
702	>	currentSteal = t;
703	>	v.stealHint = poolIndex;
704	>	v.base = b + 1;
705	>	seed = r;
706	>	++stealCount;
707	>	return t;
708	>	}
709	>	}
710		}
711	<	else if (!tryActivate() || (t = v.deqTask()) == null)
712	<	continue outer;  // restart on contention
588	<	else
589	<	break outer;
711	>	j = -n;
712	>	k = r;                // restart on contention
713		}
714	+	else if (++j <= 0)
715	+	k = r;
716	+	else if (j <= n)
717	+	k += (n >>> 1) | 1;
718	+	else
719	+	break;
720		}
721	<	} while (pool.hasNewSyncEvent(this)); // retry on pool events
722	<	seed = r;
594	<	return t;
721	>	}
722	>	return null;
723		}
724
725	+	// Run State management
726	+
727	+	// status check methods used mainly by ForkJoinPool
728	+	final boolean isTerminating() { return (runState & TERMINATING) != 0; }
729	+	final boolean isTerminated()  { return (runState & TERMINATED) != 0; }
730	+	final boolean isSuspended()   { return (runState & SUSPENDED) != 0; }
731	+	final boolean isTrimmed()     { return (runState & TRIMMED) != 0; }
732	+
733		/**
734	<	* Gets and removes a local or stolen task.
599	<	*
600	<	* @return a task, if available
734	>	* Sets state to TERMINATING, also resuming if suspended.
735		*/
736	<	final ForkJoinTask<?> pollTask() {
737	<	ForkJoinTask<?> t = locallyFifo ? deqTask() : popTask();
738	<	if (t == null && (t = scan()) != null)
739	<	++stealCount;
740	<	return t;
736	>	final void shutdown() {
737	>	for (;;) {
738	>	int s = runState;
739	>	if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended
740	>	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
741	>	(s & ~SUSPENDED) |
742	>	(TRIMMED|TERMINATING))) {
743	>	LockSupport.unpark(this);
744	>	break;
745	>	}
746	>	}
747	>	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
748	>	s | TERMINATING))
749	>	break;
750	>	}
751		}
752
753		/**
754	<	* Gets a local task.
754	>	* Sets state to TERMINATED. Called only by this thread.
755	>	*/
756	>	private void setTerminated() {
757	>	int s;
758	>	do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset,
759	>	s = runState,
760	>	s | (TERMINATING|TERMINATED)));
761	>	}
762	>
763	>	/**
764	>	* Instrumented version of park used by ForkJoinPool.awaitEvent
765	>	*/
766	>	final void doPark() {
767	>	++parkCount;
768	>	LockSupport.park(this);
769	>	}
770	>
771	>	/**
772	>	* If suspended, tries to set status to unsuspended and unparks.
773		*
774	<	* @return a task, if available
774	>	* @return true if successful
775		*/
776	<	final ForkJoinTask<?> pollLocalTask() {
777	<	return locallyFifo ? deqTask() : popTask();
776	>	final boolean tryResumeSpare() {
777	>	int s = runState;
778	>	if ((s & SUSPENDED) != 0 &&
779	>	UNSAFE.compareAndSwapInt(this, runStateOffset, s,
780	>	s & ~SUSPENDED)) {
781	>	LockSupport.unpark(this);
782	>	return true;
783	>	}
784	>	return false;
785		}
786
787		/**
788	<	* Returns a pool submission, if one exists, activating first.
788	>	* Sets suspended status and blocks as spare until resumed,
789	>	* shutdown, or timed out.
790		*
791	<	* @return a submission, if available
791	>	* @return false if trimmed
792		*/
793	<	private ForkJoinTask<?> pollSubmission() {
794	<	ForkJoinPool p = pool;
795	<	while (p.hasQueuedSubmissions()) {
796	<	ForkJoinTask<?> t;
797	<	if (tryActivate() && (t = p.pollSubmission()) != null)
798	<	return t;
793	>	final boolean suspendAsSpare() {
794	>	for (;;) {               // set suspended unless terminating
795	>	int s = runState;
796	>	if ((s & TERMINATING) != 0) { // must kill
797	>	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
798	>	s | (TRIMMED | TERMINATING)))
799	>	return false;
800	>	}
801	>	else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
802	>	s | SUSPENDED))
803	>	break;
804		}
805	<	return null;
805	>	boolean timed;
806	>	long nanos;
807	>	long startTime;
808	>	if (poolIndex < pool.parallelism) {
809	>	timed = false;
810	>	nanos = 0L;
811	>	startTime = 0L;
812	>	}
813	>	else {
814	>	timed = true;
815	>	nanos = SPARE_KEEPALIVE_NANOS;
816	>	startTime = System.nanoTime();
817	>	}
818	>	pool.accumulateStealCount(this);
819	>	lastEventCount = 0;      // reset upon resume
820	>	interrupted();           // clear/ignore interrupts
821	>	while ((runState & SUSPENDED) != 0) {
822	>	++parkCount;
823	>	if (!timed)
824	>	LockSupport.park(this);
825	>	else if ((nanos -= (System.nanoTime() - startTime)) > 0)
826	>	LockSupport.parkNanos(this, nanos);
827	>	else { // try to trim on timeout
828	>	int s = runState;
829	>	if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
830	>	(s & ~SUSPENDED) |
831	>	(TRIMMED|TERMINATING)))
832	>	return false;
833	>	}
834	>	}
835	>	return true;
836		}
837
838	<	// Methods accessed only by Pool
838	>	// Misc support methods for ForkJoinPool
839	>
840	>	/**
841	>	* Returns an estimate of the number of tasks in the queue.  Also
842	>	* used by ForkJoinTask.
843	>	*/
844	>	final int getQueueSize() {
845	>	return -base + sp;
846	>	}
847
848		/**
849		* Removes and cancels all tasks in queue.  Can be called from any
850		* thread.
851		*/
852		final void cancelTasks() {
853	<	ForkJoinTask<?> t;
854	<	while (base != sp && (t = deqTask()) != null)
855	<	t.cancelIgnoringExceptions();
853	>	ForkJoinTask<?> cj = currentJoin; // try to kill live tasks
854	>	if (cj != null) {
855	>	currentJoin = null;
856	>	cj.cancelIgnoringExceptions();
857	>	}
858	>	ForkJoinTask<?> cs = currentSteal;
859	>	if (cs != null) {
860	>	currentSteal = null;
861	>	cs.cancelIgnoringExceptions();
862	>	}
863	>	while (base != sp) {
864	>	ForkJoinTask<?> t = deqTask();
865	>	if (t != null)
866	>	t.cancelIgnoringExceptions();
867	>	}
868		}
869
870		/**
#	Line 647 | Line 872 | public class ForkJoinWorkerThread extend
872		*
873		* @return the number of tasks drained
874		*/
875	<	final int drainTasksTo(Collection<ForkJoinTask<?>> c) {
875	>	final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
876		int n = 0;
877	<	ForkJoinTask<?> t;
878	<	while (base != sp && (t = deqTask()) != null) {
879	<	c.add(t);
880	<	++n;
877	>	while (base != sp) {
878	>	ForkJoinTask<?> t = deqTask();
879	>	if (t != null) {
880	>	c.add(t);
881	>	++n;
882	>	}
883		}
884		return n;
885		}
886
887	+	// Support methods for ForkJoinTask
888	+
889		/**
890	<	* Gets and clears steal count for accumulation by pool.  Called
891	<	* only when known to be idle (in pool.sync and termination).
890	>	* Gets and removes a local task.
891	>	*
892	>	* @return a task, if available
893		*/
894	<	final int getAndClearStealCount() {
895	<	int sc = stealCount;
896	<	stealCount = 0;
897	<	return sc;
894	>	final ForkJoinTask<?> pollLocalTask() {
895	>	while (sp != base) {
896	>	if (active || (active = pool.tryIncrementActiveCount()))
897	>	return locallyFifo? locallyDeqTask() : popTask();
898	>	}
899	>	return null;
900	>	}
901	>
902	>	/**
903	>	* Gets and removes a local or stolen task.
904	>	*
905	>	* @return a task, if available
906	>	*/
907	>	final ForkJoinTask<?> pollTask() {
908	>	ForkJoinTask<?> t;
909	>	return (t = pollLocalTask()) != null ? t : scan();
910	>	}
911	>
912	>	/**
913	>	* Possibly runs some tasks and/or blocks, until task is done.
914	>	* The main body is basically a big spinloop, alternating between
915	>	* calls to helpJoinTask and pool.tryAwaitJoin with increased
916	>	* patience parameters until either the task is done without
917	>	* waiting, or we have, if necessary, created or resumed a
918	>	* replacement for this thread while it blocks.
919	>	*
920	>	* @param joinMe the task to join
921	>	* @return task status on exit
922	>	*/
923	>	final int joinTask(ForkJoinTask<?> joinMe) {
924	>	int stat;
925	>	ForkJoinTask<?> prevJoin = currentJoin;
926	>	currentJoin = joinMe;
927	>	if ((stat = joinMe.status) >= 0 &&
928	>	(sp == base || (stat = localHelpJoinTask(joinMe)) >= 0)) {
929	>	ForkJoinPool p = pool;
930	>	int helpRetries = 2;     // initial patience values
931	>	int awaitRetries = -1;   // -1 is sentinel for replace-check only
932	>	do {
933	>	helpJoinTask(joinMe, helpRetries);
934	>	if ((stat = joinMe.status) < 0)
935	>	break;
936	>	boolean busy = p.tryAwaitJoin(joinMe, awaitRetries);
937	>	if ((stat = joinMe.status) < 0)
938	>	break;
939	>	if (awaitRetries == -1)
940	>	awaitRetries = 0;
941	>	else if (busy)
942	>	++awaitRetries;
943	>	if (helpRetries < p.parallelism)
944	>	helpRetries <<= 1;
945	>	Thread.yield(); // tame unbounded loop
946	>	} while (joinMe.status >= 0);
947	>	}
948	>	currentJoin = prevJoin;
949	>	return stat;
950		}
951
952		/**
953	<	* Returns {@code true} if at least one worker in the given array
672	<	* appears to have at least one queued task.
953	>	* Run tasks in local queue until given task is done.
954		*
955	<	* @param ws array of workers
955	>	* @param joinMe the task to join
956	>	* @return task status on exit
957		*/
958	<	static boolean hasQueuedTasks(ForkJoinWorkerThread[] ws) {
959	<	if (ws != null) {
960	<	int len = ws.length;
961	<	for (int j = 0; j < 2; ++j) { // need two passes for clean sweep
962	<	for (int i = 0; i < len; ++i) {
963	<	ForkJoinWorkerThread w = ws[i];
964	<	if (w != null && w.sp != w.base)
965	<	return true;
958	>	private int localHelpJoinTask(ForkJoinTask<?> joinMe) {
959	>	int stat, s;
960	>	ForkJoinTask<?>[] q;
961	>	while ((stat = joinMe.status) >= 0 &&
962	>	base != (s = sp) && (q = queue) != null) {
963	>	ForkJoinTask<?> t;
964	>	int i = (q.length - 1) & --s;
965	>	long u = (i << qShift) + qBase; // raw offset
966	>	if ((t = q[i]) != null &&
967	>	UNSAFE.compareAndSwapObject(q, u, t, null)) {
968	>	/*
969	>	* This recheck (and similarly in helpJoinTask)
970	>	* handles cases where joinMe is independently
971	>	* cancelled or forced even though there is other work
972	>	* available. Back out of the pop by putting t back
973	>	* into slot before we commit by writing sp.
974	>	*/
975	>	if ((stat = joinMe.status) < 0) {
976	>	UNSAFE.putObjectVolatile(q, u, t);
977	>	break;
978		}
979	+	sp = s;
980	+	t.tryExec();
981		}
982		}
983	<	return false;
983	>	return stat;
984		}
985
690	–	// Support methods for ForkJoinTask
691	–
986		/**
987	<	* Returns an estimate of the number of tasks in the queue.
987	>	* Tries to locate and help perform tasks for a stealer of the
988	>	* given task, or in turn one of its stealers.  Traces
989	>	* currentSteal->currentJoin links looking for a thread working on
990	>	* a descendant of the given task and with a non-empty queue to
991	>	* steal back and execute tasks from. Restarts search upon
992	>	* encountering chains that are stale, unknown, or of length
993	>	* greater than MAX_HELP_DEPTH links, to avoid unbounded cycles.
994	>	*
995	>	* The implementation is very branchy to cope with the restart
996	>	* cases.  Returns void, not task status (which must be reread by
997	>	* caller anyway) to slightly simplify control paths.
998	>	*
999	>	* @param joinMe the task to join
1000		*/
1001	<	final int getQueueSize() {
1002	<	// suppress momentarily negative values
1003	<	return Math.max(0, sp - base);
1001	>	final void helpJoinTask(ForkJoinTask<?> joinMe, int retries) {
1002	>	ForkJoinWorkerThread[] ws = pool.workers;
1003	>	int n;
1004	>	if (ws == null || (n = ws.length) <= 1)
1005	>	return;                   // need at least 2 workers
1006	>
1007	>	restart:while (joinMe.status >= 0 && --retries >= 0) {
1008	>	ForkJoinTask<?> task = joinMe;        // base of chain
1009	>	ForkJoinWorkerThread thread = this;   // thread with stolen task
1010	>	for (int depth = 0; depth < MAX_HELP_DEPTH; ++depth) {
1011	>	// Try to find v, the stealer of task, by first using hint
1012	>	ForkJoinWorkerThread v = ws[thread.stealHint & (n - 1)];
1013	>	if (v == null || v.currentSteal != task) {
1014	>	for (int j = 0; ; ++j) {      // search array
1015	>	if (task.status < 0 || j == n)
1016	>	continue restart;     // stale or no stealer
1017	>	if ((v = ws[j]) != null && v.currentSteal == task) {
1018	>	thread.stealHint = j; // save for next time
1019	>	break;
1020	>	}
1021	>	}
1022	>	}
1023	>	// Try to help v, using specialized form of deqTask
1024	>	int b;
1025	>	ForkJoinTask<?>[] q;
1026	>	while ((b = v.base) != v.sp && (q = v.queue) != null) {
1027	>	int i = (q.length - 1) & b;
1028	>	long u = (i << qShift) + qBase;
1029	>	ForkJoinTask<?> t = q[i];
1030	>	if (task.status < 0)          // stale
1031	>	continue restart;
1032	>	if (v.base == b) {            // recheck after reading t
1033	>	if (t == null)            // producer stalled
1034	>	continue restart;     // retry via restart
1035	>	if (UNSAFE.compareAndSwapObject(q, u, t, null)) {
1036	>	if (joinMe.status < 0) {
1037	>	UNSAFE.putObjectVolatile(q, u, t);
1038	>	return;           // back out on cancel
1039	>	}
1040	>	ForkJoinTask<?> prevSteal = currentSteal;
1041	>	currentSteal = t;
1042	>	v.stealHint = poolIndex;
1043	>	v.base = b + 1;
1044	>	t.tryExec();
1045	>	currentSteal = prevSteal;
1046	>	}
1047	>	}
1048	>	if (joinMe.status < 0)
1049	>	return;
1050	>	}
1051	>	// Try to descend to find v's stealer
1052	>	ForkJoinTask<?> next = v.currentJoin;
1053	>	if (next == null || task.status < 0)
1054	>	continue restart;             // no descendent or stale
1055	>	if (joinMe.status < 0)
1056	>	return;
1057	>	task = next;
1058	>	thread = v;
1059	>	}
1060	>	}
1061		}
1062
1063		/**
1064		* Returns an estimate of the number of tasks, offset by a
1065		* function of number of idle workers.
1066	+	*
1067	+	* This method provides a cheap heuristic guide for task
1068	+	* partitioning when programmers, frameworks, tools, or languages
1069	+	* have little or no idea about task granularity.  In essence by
1070	+	* offering this method, we ask users only about tradeoffs in
1071	+	* overhead vs expected throughput and its variance, rather than
1072	+	* how finely to partition tasks.
1073	+	*
1074	+	* In a steady state strict (tree-structured) computation, each
1075	+	* thread makes available for stealing enough tasks for other
1076	+	* threads to remain active. Inductively, if all threads play by
1077	+	* the same rules, each thread should make available only a
1078	+	* constant number of tasks.
1079	+	*
1080	+	* The minimum useful constant is just 1. But using a value of 1
1081	+	* would require immediate replenishment upon each steal to
1082	+	* maintain enough tasks, which is infeasible.  Further,
1083	+	* partitionings/granularities of offered tasks should minimize
1084	+	* steal rates, which in general means that threads nearer the top
1085	+	* of computation tree should generate more than those nearer the
1086	+	* bottom. In perfect steady state, each thread is at
1087	+	* approximately the same level of computation tree. However,
1088	+	* producing extra tasks amortizes the uncertainty of progress and
1089	+	* diffusion assumptions.
1090	+	*
1091	+	* So, users will want to use values larger, but not much larger
1092	+	* than 1 to both smooth over transient shortages and hedge
1093	+	* against uneven progress; as traded off against the cost of
1094	+	* extra task overhead. We leave the user to pick a threshold
1095	+	* value to compare with the results of this call to guide
1096	+	* decisions, but recommend values such as 3.
1097	+	*
1098	+	* When all threads are active, it is on average OK to estimate
1099	+	* surplus strictly locally. In steady-state, if one thread is
1100	+	* maintaining say 2 surplus tasks, then so are others. So we can
1101	+	* just use estimated queue length (although note that (sp - base)
1102	+	* can be an overestimate because of stealers lagging increments
1103	+	* of base).  However, this strategy alone leads to serious
1104	+	* mis-estimates in some non-steady-state conditions (ramp-up,
1105	+	* ramp-down, other stalls). We can detect many of these by
1106	+	* further considering the number of "idle" threads, that are
1107	+	* known to have zero queued tasks, so compensate by a factor of
1108	+	* (#idle/#active) threads.
1109		*/
1110		final int getEstimatedSurplusTaskCount() {
1111	<	// The halving approximates weighting idle vs non-idle workers
706	<	return (sp - base) - (pool.getIdleThreadCount() >>> 1);
707	<	}
708	<
709	<	/**
710	<	* Scans, returning early if joinMe done.
711	<	*/
712	<	final ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
713	<	ForkJoinTask<?> t = pollTask();
714	<	if (t != null && joinMe.status < 0 && sp == base) {
715	<	pushTask(t); // unsteal if done and this task would be stealable
716	<	t = null;
717	<	}
718	<	return t;
1111	>	return sp - base - pool.idlePerActive();
1112		}
1113
1114		/**
#	Line 723 | Line 1116 | public class ForkJoinWorkerThread extend
1116		*/
1117		final void helpQuiescePool() {
1118		for (;;) {
1119	<	ForkJoinTask<?> t = pollTask();
1120	<	if (t != null)
1121	<	t.quietlyExec();
1122	<	else if (tryInactivate() && pool.isQuiescent())
1123	<	break;
1119	>	ForkJoinTask<?> t = pollLocalTask();
1120	>	if (t != null || (t = scan()) != null) {
1121	>	t.tryExec();
1122	>	currentSteal = null;
1123	>	}
1124	>	else {
1125	>	ForkJoinPool p = pool;
1126	>	if (active) {
1127	>	active = false; // inactivate
1128	>	do {} while (!p.tryDecrementActiveCount());
1129	>	}
1130	>	if (p.isQuiescent()) {
1131	>	active = true; // re-activate
1132	>	do {} while (!p.tryIncrementActiveCount());
1133	>	return;
1134	>	}
1135	>	}
1136		}
732	–	do {} while (!tryActivate()); // re-activate on exit
1137		}
1138
1139		// Unsafe mechanics
1140
1141		private static final sun.misc.Unsafe UNSAFE = getUnsafe();
738	–	private static final long spOffset =
739	–	objectFieldOffset("sp", ForkJoinWorkerThread.class);
1142		private static final long runStateOffset =
1143		objectFieldOffset("runState", ForkJoinWorkerThread.class);
1144	<	private static final long qBase;
1144	>	private static final long qBase =
1145	>	UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
1146	>	private static final long threadStatusOffset =
1147	>	objectFieldOffset("threadStatus", Thread.class);
1148		private static final int qShift;
1149
1150		static {
746	–	qBase = UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
1151		int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
1152		if ((s & (s-1)) != 0)
1153		throw new Error("data type scale not a power of two");

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