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dl |
1.1 |
/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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package jsr166y; |
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jsr166 |
1.18 |
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dl |
1.1 |
import java.util.concurrent.*; |
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jsr166 |
1.18 |
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dl |
1.31 |
import java.util.Random; |
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jsr166 |
1.18 |
import java.util.Collection; |
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dl |
1.31 |
import java.util.concurrent.locks.LockSupport; |
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dl |
1.1 |
|
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/** |
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dl |
1.2 |
* A thread managed by a {@link ForkJoinPool}. This class is |
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* subclassable solely for the sake of adding functionality -- there |
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jsr166 |
1.25 |
* are no overridable methods dealing with scheduling or execution. |
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* However, you can override initialization and termination methods |
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* surrounding the main task processing loop. If you do create such a |
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* subclass, you will also need to supply a custom {@link |
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dl |
1.26 |
* ForkJoinPool.ForkJoinWorkerThreadFactory} to use it in a {@code |
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* ForkJoinPool}. |
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jsr166 |
1.6 |
* |
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jsr166 |
1.13 |
* @since 1.7 |
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* @author Doug Lea |
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dl |
1.1 |
*/ |
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public class ForkJoinWorkerThread extends Thread { |
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/* |
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dl |
1.31 |
* Overview: |
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dl |
1.1 |
* |
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dl |
1.31 |
* ForkJoinWorkerThreads are managed by ForkJoinPools and perform |
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* ForkJoinTasks. This class includes bookkeeping in support of |
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* worker activation, suspension, and lifecycle control described |
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* in more detail in the internal documentation of class |
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* ForkJoinPool. And as described further below, this class also |
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* includes special-cased support for some ForkJoinTask |
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* methods. But the main mechanics involve work-stealing: |
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* |
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* Work-stealing queues are special forms of Deques that support |
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* only three of the four possible end-operations -- push, pop, |
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* and deq (aka steal), under the further constraints that push |
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* and pop are called only from the owning thread, while deq may |
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* be called from other threads. (If you are unfamiliar with |
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* them, you probably want to read Herlihy and Shavit's book "The |
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* Art of Multiprocessor programming", chapter 16 describing these |
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* in more detail before proceeding.) The main work-stealing |
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* queue design is roughly similar to those in the papers "Dynamic |
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* Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005 |
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* (http://research.sun.com/scalable/pubs/index.html) and |
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* "Idempotent work stealing" by Michael, Saraswat, and Vechev, |
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* PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186). |
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* The main differences ultimately stem from gc requirements that |
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* we null out taken slots as soon as we can, to maintain as small |
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* a footprint as possible even in programs generating huge |
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* numbers of tasks. To accomplish this, we shift the CAS |
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* arbitrating pop vs deq (steal) from being on the indices |
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* ("base" and "sp") to the slots themselves (mainly via method |
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* "casSlotNull()"). So, both a successful pop and deq mainly |
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* entail a CAS of a slot from non-null to null. Because we rely |
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* on CASes of references, we do not need tag bits on base or sp. |
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* They are simple ints as used in any circular array-based queue |
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* (see for example ArrayDeque). Updates to the indices must |
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* still be ordered in a way that guarantees that sp == base means |
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* the queue is empty, but otherwise may err on the side of |
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* possibly making the queue appear nonempty when a push, pop, or |
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* deq have not fully committed. Note that this means that the deq |
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* operation, considered individually, is not wait-free. One thief |
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* cannot successfully continue until another in-progress one (or, |
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* if previously empty, a push) completes. However, in the |
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* aggregate, we ensure at least probabilistic non-blockingness. |
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* If an attempted steal fails, a thief always chooses a different |
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* random victim target to try next. So, in order for one thief to |
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* progress, it suffices for any in-progress deq or new push on |
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* any empty queue to complete. One reason this works well here is |
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* that apparently-nonempty often means soon-to-be-stealable, |
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* which gives threads a chance to set activation status if |
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* necessary before stealing. |
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dl |
1.1 |
* |
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dl |
1.23 |
* This approach also enables support for "async mode" where local |
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* task processing is in FIFO, not LIFO order; simply by using a |
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* version of deq rather than pop when locallyFifo is true (as set |
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* by the ForkJoinPool). This allows use in message-passing |
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* frameworks in which tasks are never joined. |
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* |
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dl |
1.35 |
* When a worker would otherwise be blocked waiting to join a |
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* task, it first tries a form of linear helping: Each worker |
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dl |
1.36 |
* records (in field currentSteal) the most recent task it stole |
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* from some other worker. Plus, it records (in field currentJoin) |
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* the task it is currently actively joining. Method joinTask uses |
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dl |
1.35 |
* these markers to try to find a worker to help (i.e., steal back |
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* a task from and execute it) that could hasten completion of the |
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* actively joined task. In essence, the joiner executes a task |
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* that would be on its own local deque had the to-be-joined task |
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* not been stolen. This may be seen as a conservative variant of |
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* the approach in Wagner & Calder "Leapfrogging: a portable |
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* technique for implementing efficient futures" SIGPLAN Notices, |
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* 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs |
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* in that: (1) We only maintain dependency links across workers |
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dl |
1.39 |
* upon steals, rather than use per-task bookkeeping. This may |
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* require a linear scan of workers array to locate stealers, but |
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* usually doesn't because stealers leave hints (that may become |
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* stale/wrong) of where to locate them. This isolates cost to |
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* when it is needed, rather than adding to per-task overhead. |
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* (2) It is "shallow", ignoring nesting and potentially cyclic |
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* mutual steals. (3) It is intentionally racy: field currentJoin |
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* is updated only while actively joining, which means that we |
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* miss links in the chain during long-lived tasks, GC stalls etc |
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* (which is OK since blocking in such cases is usually a good |
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* idea). (4) We bound the number of attempts to find work (see |
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* MAX_HELP_DEPTH) and fall back to suspending the worker and if |
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* necessary replacing it with a spare (see |
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dl |
1.42 |
* ForkJoinPool.awaitJoin). |
114 |
dl |
1.35 |
* |
115 |
dl |
1.36 |
* Efficient implementation of these algorithms currently relies |
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* on an uncomfortable amount of "Unsafe" mechanics. To maintain |
117 |
dl |
1.1 |
* correct orderings, reads and writes of variable base require |
118 |
dl |
1.31 |
* volatile ordering. Variable sp does not require volatile |
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* writes but still needs store-ordering, which we accomplish by |
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* pre-incrementing sp before filling the slot with an ordered |
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* store. (Pre-incrementing also enables backouts used in |
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dl |
1.36 |
* joinTask.) Because they are protected by volatile base reads, |
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* reads of the queue array and its slots by other threads do not |
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* need volatile load semantics, but writes (in push) require |
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* store order and CASes (in pop and deq) require (volatile) CAS |
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* semantics. (Michael, Saraswat, and Vechev's algorithm has |
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* similar properties, but without support for nulling slots.) |
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* Since these combinations aren't supported using ordinary |
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* volatiles, the only way to accomplish these efficiently is to |
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* use direct Unsafe calls. (Using external AtomicIntegers and |
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* AtomicReferenceArrays for the indices and array is |
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* significantly slower because of memory locality and indirection |
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* effects.) |
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jsr166 |
1.29 |
* |
135 |
dl |
1.28 |
* Further, performance on most platforms is very sensitive to |
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* placement and sizing of the (resizable) queue array. Even |
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* though these queues don't usually become all that big, the |
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* initial size must be large enough to counteract cache |
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dl |
1.1 |
* contention effects across multiple queues (especially in the |
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* presence of GC cardmarking). Also, to improve thread-locality, |
141 |
dl |
1.31 |
* queues are initialized after starting. All together, these |
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* low-level implementation choices produce as much as a factor of |
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* 4 performance improvement compared to naive implementations, |
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* and enable the processing of billions of tasks per second, |
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* sometimes at the expense of ugliness. |
146 |
dl |
1.1 |
*/ |
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/** |
149 |
dl |
1.31 |
* Generator for initial random seeds for random victim |
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* selection. This is used only to create initial seeds. Random |
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* steals use a cheaper xorshift generator per steal attempt. We |
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* expect only rare contention on seedGenerator, so just use a |
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* plain Random. |
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*/ |
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private static final Random seedGenerator = new Random(); |
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/** |
158 |
dl |
1.36 |
* The maximum stolen->joining link depth allowed in helpJoinTask. |
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* Depths for legitimate chains are unbounded, but we use a fixed |
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* constant to avoid (otherwise unchecked) cycles and bound |
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* staleness of traversal parameters at the expense of sometimes |
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* blocking when we could be helping. |
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*/ |
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private static final int MAX_HELP_DEPTH = 8; |
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/** |
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dl |
1.1 |
* Capacity of work-stealing queue array upon initialization. |
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dl |
1.34 |
* Must be a power of two. Initial size must be at least 4, but is |
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dl |
1.1 |
* padded to minimize cache effects. |
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*/ |
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private static final int INITIAL_QUEUE_CAPACITY = 1 << 13; |
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/** |
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* Maximum work-stealing queue array size. Must be less than or |
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dl |
1.5 |
* equal to 1 << 28 to ensure lack of index wraparound. (This |
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* is less than usual bounds, because we need leftshift by 3 |
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* to be in int range). |
178 |
dl |
1.1 |
*/ |
179 |
dl |
1.5 |
private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28; |
180 |
dl |
1.1 |
|
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/** |
182 |
jsr166 |
1.16 |
* The pool this thread works in. Accessed directly by ForkJoinTask. |
183 |
dl |
1.1 |
*/ |
184 |
dl |
1.5 |
final ForkJoinPool pool; |
185 |
dl |
1.1 |
|
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/** |
187 |
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* The work-stealing queue array. Size must be a power of two. |
188 |
dl |
1.31 |
* Initialized in onStart, to improve memory locality. |
189 |
dl |
1.1 |
*/ |
190 |
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private ForkJoinTask<?>[] queue; |
191 |
dl |
1.37 |
|
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dl |
1.1 |
/** |
193 |
dl |
1.31 |
* Index (mod queue.length) of least valid queue slot, which is |
194 |
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* always the next position to steal from if nonempty. |
195 |
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*/ |
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private volatile int base; |
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/** |
199 |
dl |
1.1 |
* Index (mod queue.length) of next queue slot to push to or pop |
200 |
dl |
1.31 |
* from. It is written only by owner thread, and accessed by other |
201 |
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* threads only after reading (volatile) base. Both sp and base |
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* are allowed to wrap around on overflow, but (sp - base) still |
203 |
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* estimates size. |
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*/ |
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private int sp; |
206 |
dl |
1.1 |
|
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/** |
208 |
dl |
1.36 |
* The index of most recent stealer, used as a hint to avoid |
209 |
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* traversal in method helpJoinTask. This is only a hint because a |
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* worker might have had multiple steals and this only holds one |
211 |
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* of them (usually the most current). Declared non-volatile, |
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* relying on other prevailing sync to keep reasonably current. |
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*/ |
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private int stealHint; |
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/** |
217 |
dl |
1.31 |
* Run state of this worker. In addition to the usual run levels, |
218 |
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* tracks if this worker is suspended as a spare, and if it was |
219 |
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* killed (trimmed) while suspended. However, "active" status is |
220 |
dl |
1.41 |
* maintained separately and modified only in conjunction with |
221 |
dl |
1.42 |
* CASes of the pool's runState (which are currently sadly |
222 |
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* manually inlined for performance.) Accessed directly by pool |
223 |
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* to simplify checks for normal (zero) status. |
224 |
dl |
1.1 |
*/ |
225 |
dl |
1.42 |
volatile int runState; |
226 |
dl |
1.31 |
|
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private static final int TERMINATING = 0x01; |
228 |
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private static final int TERMINATED = 0x02; |
229 |
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private static final int SUSPENDED = 0x04; // inactive spare |
230 |
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private static final int TRIMMED = 0x08; // killed while suspended |
231 |
dl |
1.1 |
|
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/** |
233 |
dl |
1.31 |
* Number of steals, transferred and reset in pool callbacks pool |
234 |
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* when idle Accessed directly by pool. |
235 |
dl |
1.1 |
*/ |
236 |
dl |
1.31 |
int stealCount; |
237 |
dl |
1.1 |
|
238 |
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/** |
239 |
dl |
1.5 |
* Seed for random number generator for choosing steal victims. |
240 |
dl |
1.31 |
* Uses Marsaglia xorshift. Must be initialized as nonzero. |
241 |
dl |
1.1 |
*/ |
242 |
dl |
1.5 |
private int seed; |
243 |
dl |
1.1 |
|
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/** |
245 |
dl |
1.31 |
* Activity status. When true, this worker is considered active. |
246 |
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* Accessed directly by pool. Must be false upon construction. |
247 |
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*/ |
248 |
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boolean active; |
249 |
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250 |
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/** |
251 |
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* True if use local fifo, not default lifo, for local polling. |
252 |
dl |
1.39 |
* Shadows value from ForkJoinPool. |
253 |
dl |
1.1 |
*/ |
254 |
dl |
1.35 |
private final boolean locallyFifo; |
255 |
dl |
1.37 |
|
256 |
dl |
1.1 |
/** |
257 |
dl |
1.5 |
* Index of this worker in pool array. Set once by pool before |
258 |
dl |
1.31 |
* running, and accessed directly by pool to locate this worker in |
259 |
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* its workers array. |
260 |
dl |
1.1 |
*/ |
261 |
dl |
1.5 |
int poolIndex; |
262 |
dl |
1.1 |
|
263 |
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/** |
264 |
dl |
1.31 |
* The last pool event waited for. Accessed only by pool in |
265 |
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* callback methods invoked within this thread. |
266 |
dl |
1.1 |
*/ |
267 |
dl |
1.31 |
int lastEventCount; |
268 |
dl |
1.1 |
|
269 |
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/** |
270 |
dl |
1.42 |
* Encoded index and event count of next event waiter. Accessed |
271 |
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* only by ForkJoinPool for managing event waiters. |
272 |
dl |
1.7 |
*/ |
273 |
dl |
1.31 |
volatile long nextWaiter; |
274 |
dl |
1.7 |
|
275 |
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/** |
276 |
dl |
1.42 |
* Number of times this thread suspended as spare. Accessed only |
277 |
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* by pool. |
278 |
dl |
1.40 |
*/ |
279 |
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int spareCount; |
280 |
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|
281 |
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/** |
282 |
dl |
1.42 |
* Encoded index and count of next spare waiter. Accessed only |
283 |
dl |
1.40 |
* by ForkJoinPool for managing spares. |
284 |
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*/ |
285 |
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volatile int nextSpare; |
286 |
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|
287 |
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/** |
288 |
dl |
1.36 |
* The task currently being joined, set only when actively trying |
289 |
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* to helpStealer. Written only by current thread, but read by |
290 |
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* others. |
291 |
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*/ |
292 |
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private volatile ForkJoinTask<?> currentJoin; |
293 |
dl |
1.37 |
|
294 |
dl |
1.36 |
/** |
295 |
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* The task most recently stolen from another worker (or |
296 |
dl |
1.42 |
* submission queue). Written only by current thread, but read by |
297 |
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* others. |
298 |
dl |
1.36 |
*/ |
299 |
dl |
1.42 |
private volatile ForkJoinTask<?> currentSteal; |
300 |
dl |
1.36 |
|
301 |
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/** |
302 |
dl |
1.1 |
* Creates a ForkJoinWorkerThread operating in the given pool. |
303 |
jsr166 |
1.11 |
* |
304 |
dl |
1.1 |
* @param pool the pool this thread works in |
305 |
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* @throws NullPointerException if pool is null |
306 |
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*/ |
307 |
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protected ForkJoinWorkerThread(ForkJoinPool pool) { |
308 |
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this.pool = pool; |
309 |
dl |
1.35 |
this.locallyFifo = pool.locallyFifo; |
310 |
dl |
1.40 |
setDaemon(true); |
311 |
dl |
1.31 |
// To avoid exposing construction details to subclasses, |
312 |
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// remaining initialization is in start() and onStart() |
313 |
dl |
1.1 |
} |
314 |
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|
315 |
dl |
1.31 |
/** |
316 |
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* Performs additional initialization and starts this thread |
317 |
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*/ |
318 |
dl |
1.35 |
final void start(int poolIndex, UncaughtExceptionHandler ueh) { |
319 |
dl |
1.31 |
this.poolIndex = poolIndex; |
320 |
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if (ueh != null) |
321 |
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setUncaughtExceptionHandler(ueh); |
322 |
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start(); |
323 |
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} |
324 |
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|
325 |
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// Public/protected methods |
326 |
dl |
1.2 |
|
327 |
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/** |
328 |
jsr166 |
1.11 |
* Returns the pool hosting this thread. |
329 |
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* |
330 |
dl |
1.2 |
* @return the pool |
331 |
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*/ |
332 |
dl |
1.4 |
public ForkJoinPool getPool() { |
333 |
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return pool; |
334 |
dl |
1.2 |
} |
335 |
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|
336 |
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/** |
337 |
dl |
1.4 |
* Returns the index number of this thread in its pool. The |
338 |
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* returned value ranges from zero to the maximum number of |
339 |
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* threads (minus one) that have ever been created in the pool. |
340 |
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* This method may be useful for applications that track status or |
341 |
dl |
1.5 |
* collect results per-worker rather than per-task. |
342 |
jsr166 |
1.11 |
* |
343 |
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* @return the index number |
344 |
dl |
1.2 |
*/ |
345 |
dl |
1.4 |
public int getPoolIndex() { |
346 |
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return poolIndex; |
347 |
dl |
1.2 |
} |
348 |
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|
349 |
dl |
1.7 |
/** |
350 |
dl |
1.31 |
* Initializes internal state after construction but before |
351 |
|
|
* processing any tasks. If you override this method, you must |
352 |
|
|
* invoke super.onStart() at the beginning of the method. |
353 |
|
|
* Initialization requires care: Most fields must have legal |
354 |
|
|
* default values, to ensure that attempted accesses from other |
355 |
|
|
* threads work correctly even before this thread starts |
356 |
|
|
* processing tasks. |
357 |
dl |
1.7 |
*/ |
358 |
dl |
1.31 |
protected void onStart() { |
359 |
|
|
int rs = seedGenerator.nextInt(); |
360 |
|
|
seed = rs == 0? 1 : rs; // seed must be nonzero |
361 |
dl |
1.5 |
|
362 |
dl |
1.35 |
// Allocate name string and arrays in this thread |
363 |
dl |
1.31 |
String pid = Integer.toString(pool.getPoolNumber()); |
364 |
|
|
String wid = Integer.toString(poolIndex); |
365 |
|
|
setName("ForkJoinPool-" + pid + "-worker-" + wid); |
366 |
dl |
1.5 |
|
367 |
dl |
1.31 |
queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY]; |
368 |
|
|
} |
369 |
dl |
1.5 |
|
370 |
|
|
/** |
371 |
dl |
1.31 |
* 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. |
374 |
jsr166 |
1.21 |
* |
375 |
dl |
1.31 |
* @param exception the exception causing this thread to abort due |
376 |
|
|
* to an unrecoverable error, or {@code null} if completed normally |
377 |
dl |
1.5 |
*/ |
378 |
dl |
1.31 |
protected void onTermination(Throwable exception) { |
379 |
|
|
try { |
380 |
dl |
1.41 |
ForkJoinPool p = pool; |
381 |
|
|
if (active) { |
382 |
|
|
int a; // inline p.tryDecrementActiveCount |
383 |
|
|
active = false; |
384 |
jsr166 |
1.43 |
do {} while (!UNSAFE.compareAndSwapInt |
385 |
|
|
(p, poolRunStateOffset, a = p.runState, a - 1)); |
386 |
dl |
1.41 |
} |
387 |
dl |
1.31 |
cancelTasks(); |
388 |
|
|
setTerminated(); |
389 |
dl |
1.41 |
p.workerTerminated(this); |
390 |
dl |
1.31 |
} catch (Throwable ex) { // Shouldn't ever happen |
391 |
|
|
if (exception == null) // but if so, at least rethrown |
392 |
|
|
exception = ex; |
393 |
|
|
} finally { |
394 |
|
|
if (exception != null) |
395 |
|
|
UNSAFE.throwException(exception); |
396 |
dl |
1.5 |
} |
397 |
|
|
} |
398 |
|
|
|
399 |
|
|
/** |
400 |
|
|
* This method is required to be public, but should never be |
401 |
|
|
* called explicitly. It performs the main run loop to execute |
402 |
|
|
* ForkJoinTasks. |
403 |
dl |
1.1 |
*/ |
404 |
dl |
1.5 |
public void run() { |
405 |
|
|
Throwable exception = null; |
406 |
|
|
try { |
407 |
|
|
onStart(); |
408 |
|
|
mainLoop(); |
409 |
|
|
} catch (Throwable ex) { |
410 |
|
|
exception = ex; |
411 |
|
|
} finally { |
412 |
|
|
onTermination(exception); |
413 |
|
|
} |
414 |
dl |
1.1 |
} |
415 |
|
|
|
416 |
dl |
1.31 |
// helpers for run() |
417 |
|
|
|
418 |
dl |
1.1 |
/** |
419 |
dl |
1.31 |
* Find and execute tasks and check status while running |
420 |
dl |
1.1 |
*/ |
421 |
dl |
1.5 |
private void mainLoop() { |
422 |
dl |
1.42 |
boolean ran = false; // true if ran a task on last step |
423 |
dl |
1.31 |
ForkJoinPool p = pool; |
424 |
|
|
for (;;) { |
425 |
dl |
1.42 |
p.preStep(this, ran); |
426 |
dl |
1.31 |
if (runState != 0) |
427 |
dl |
1.40 |
break; |
428 |
dl |
1.42 |
ran = tryExecSteal() || tryExecSubmission(); |
429 |
dl |
1.5 |
} |
430 |
dl |
1.1 |
} |
431 |
|
|
|
432 |
dl |
1.5 |
/** |
433 |
dl |
1.40 |
* Try to steal a task and execute it |
434 |
|
|
* |
435 |
|
|
* @return true if ran a task |
436 |
dl |
1.5 |
*/ |
437 |
dl |
1.40 |
private boolean tryExecSteal() { |
438 |
|
|
ForkJoinTask<?> t; |
439 |
dl |
1.42 |
if ((t = scan()) != null) { |
440 |
dl |
1.40 |
t.quietlyExec(); |
441 |
dl |
1.42 |
UNSAFE.putOrderedObject(this, currentStealOffset, null); |
442 |
dl |
1.40 |
if (sp != base) |
443 |
|
|
execLocalTasks(); |
444 |
|
|
return true; |
445 |
dl |
1.31 |
} |
446 |
dl |
1.40 |
return false; |
447 |
dl |
1.5 |
} |
448 |
dl |
1.1 |
|
449 |
|
|
/** |
450 |
dl |
1.40 |
* If a submission exists, try to activate and run it; |
451 |
dl |
1.5 |
* |
452 |
dl |
1.40 |
* @return true if ran a task |
453 |
dl |
1.1 |
*/ |
454 |
dl |
1.40 |
private boolean tryExecSubmission() { |
455 |
dl |
1.31 |
ForkJoinPool p = pool; |
456 |
|
|
while (p.hasQueuedSubmissions()) { |
457 |
dl |
1.41 |
ForkJoinTask<?> t; int a; |
458 |
dl |
1.42 |
if (active || // inline p.tryIncrementActiveCount |
459 |
dl |
1.41 |
(active = UNSAFE.compareAndSwapInt(p, poolRunStateOffset, |
460 |
|
|
a = p.runState, a + 1))) { |
461 |
dl |
1.40 |
if ((t = p.pollSubmission()) != null) { |
462 |
dl |
1.42 |
UNSAFE.putOrderedObject(this, currentStealOffset, t); |
463 |
dl |
1.40 |
t.quietlyExec(); |
464 |
dl |
1.42 |
UNSAFE.putOrderedObject(this, currentStealOffset, null); |
465 |
dl |
1.40 |
if (sp != base) |
466 |
|
|
execLocalTasks(); |
467 |
|
|
return true; |
468 |
dl |
1.36 |
} |
469 |
dl |
1.5 |
} |
470 |
|
|
} |
471 |
dl |
1.40 |
return false; |
472 |
|
|
} |
473 |
|
|
|
474 |
|
|
/** |
475 |
|
|
* Runs local tasks until queue is empty or shut down. Call only |
476 |
|
|
* while active. |
477 |
|
|
*/ |
478 |
|
|
private void execLocalTasks() { |
479 |
|
|
while (runState == 0) { |
480 |
|
|
ForkJoinTask<?> t = locallyFifo? locallyDeqTask() : popTask(); |
481 |
|
|
if (t != null) |
482 |
|
|
t.quietlyExec(); |
483 |
|
|
else if (sp == base) |
484 |
|
|
break; |
485 |
|
|
} |
486 |
dl |
1.1 |
} |
487 |
|
|
|
488 |
dl |
1.31 |
/* |
489 |
|
|
* Intrinsics-based atomic writes for queue slots. These are |
490 |
|
|
* basically the same as methods in AtomicObjectArray, but |
491 |
|
|
* specialized for (1) ForkJoinTask elements (2) requirement that |
492 |
|
|
* nullness and bounds checks have already been performed by |
493 |
|
|
* callers and (3) effective offsets are known not to overflow |
494 |
|
|
* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't |
495 |
|
|
* need corresponding version for reads: plain array reads are OK |
496 |
|
|
* because they protected by other volatile reads and are |
497 |
|
|
* confirmed by CASes. |
498 |
|
|
* |
499 |
|
|
* Most uses don't actually call these methods, but instead contain |
500 |
|
|
* inlined forms that enable more predictable optimization. We |
501 |
|
|
* don't define the version of write used in pushTask at all, but |
502 |
|
|
* instead inline there a store-fenced array slot write. |
503 |
dl |
1.1 |
*/ |
504 |
|
|
|
505 |
|
|
/** |
506 |
dl |
1.31 |
* CASes slot i of array q from t to null. Caller must ensure q is |
507 |
|
|
* non-null and index is in range. |
508 |
dl |
1.1 |
*/ |
509 |
dl |
1.31 |
private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i, |
510 |
|
|
ForkJoinTask<?> t) { |
511 |
|
|
return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null); |
512 |
dl |
1.1 |
} |
513 |
|
|
|
514 |
dl |
1.5 |
/** |
515 |
dl |
1.31 |
* Performs a volatile write of the given task at given slot of |
516 |
|
|
* array q. Caller must ensure q is non-null and index is in |
517 |
|
|
* range. This method is used only during resets and backouts. |
518 |
dl |
1.5 |
*/ |
519 |
dl |
1.31 |
private static final void writeSlot(ForkJoinTask<?>[] q, int i, |
520 |
|
|
ForkJoinTask<?> t) { |
521 |
|
|
UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t); |
522 |
dl |
1.5 |
} |
523 |
|
|
|
524 |
dl |
1.31 |
// queue methods |
525 |
dl |
1.1 |
|
526 |
|
|
/** |
527 |
dl |
1.31 |
* Pushes a task. Call only from this thread. |
528 |
jsr166 |
1.11 |
* |
529 |
jsr166 |
1.10 |
* @param t the task. Caller must ensure non-null. |
530 |
dl |
1.1 |
*/ |
531 |
|
|
final void pushTask(ForkJoinTask<?> t) { |
532 |
|
|
ForkJoinTask<?>[] q = queue; |
533 |
dl |
1.31 |
int mask = q.length - 1; // implicit assert q != null |
534 |
dl |
1.34 |
int s = sp++; // ok to increment sp before slot write |
535 |
|
|
UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t); |
536 |
|
|
if ((s -= base) == 0) |
537 |
|
|
pool.signalWork(); // was empty |
538 |
|
|
else if (s == mask) |
539 |
|
|
growQueue(); // is full |
540 |
dl |
1.1 |
} |
541 |
|
|
|
542 |
|
|
/** |
543 |
|
|
* Tries to take a task from the base of the queue, failing if |
544 |
dl |
1.31 |
* empty or contended. Note: Specializations of this code appear |
545 |
dl |
1.35 |
* in locallyDeqTask and elsewhere. |
546 |
jsr166 |
1.11 |
* |
547 |
|
|
* @return a task, or null if none or contended |
548 |
dl |
1.1 |
*/ |
549 |
dl |
1.7 |
final ForkJoinTask<?> deqTask() { |
550 |
dl |
1.5 |
ForkJoinTask<?> t; |
551 |
dl |
1.1 |
ForkJoinTask<?>[] q; |
552 |
dl |
1.31 |
int b, i; |
553 |
dl |
1.40 |
if (sp != (b = base) && |
554 |
dl |
1.1 |
(q = queue) != null && // must read q after b |
555 |
dl |
1.35 |
(t = q[i = (q.length - 1) & b]) != null && base == b && |
556 |
dl |
1.31 |
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) { |
557 |
dl |
1.1 |
base = b + 1; |
558 |
|
|
return t; |
559 |
|
|
} |
560 |
|
|
return null; |
561 |
|
|
} |
562 |
|
|
|
563 |
|
|
/** |
564 |
dl |
1.31 |
* Tries to take a task from the base of own queue. Assumes active |
565 |
|
|
* status. Called only by current thread. |
566 |
dl |
1.23 |
* |
567 |
|
|
* @return a task, or null if none |
568 |
|
|
*/ |
569 |
|
|
final ForkJoinTask<?> locallyDeqTask() { |
570 |
dl |
1.31 |
ForkJoinTask<?>[] q = queue; |
571 |
|
|
if (q != null) { |
572 |
|
|
ForkJoinTask<?> t; |
573 |
|
|
int b, i; |
574 |
|
|
while (sp != (b = base)) { |
575 |
dl |
1.35 |
if ((t = q[i = (q.length - 1) & b]) != null && base == b && |
576 |
dl |
1.31 |
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, |
577 |
|
|
t, null)) { |
578 |
dl |
1.23 |
base = b + 1; |
579 |
|
|
return t; |
580 |
|
|
} |
581 |
|
|
} |
582 |
|
|
} |
583 |
|
|
return null; |
584 |
|
|
} |
585 |
|
|
|
586 |
|
|
/** |
587 |
dl |
1.31 |
* Returns a popped task, or null if empty. Assumes active status. |
588 |
dl |
1.37 |
* Called only by current thread. |
589 |
dl |
1.1 |
*/ |
590 |
dl |
1.40 |
private ForkJoinTask<?> popTask() { |
591 |
|
|
ForkJoinTask<?>[] q = queue; |
592 |
|
|
if (q != null) { |
593 |
|
|
int s; |
594 |
|
|
while ((s = sp) != base) { |
595 |
|
|
int i = (q.length - 1) & --s; |
596 |
|
|
long u = (i << qShift) + qBase; // raw offset |
597 |
|
|
ForkJoinTask<?> t = q[i]; |
598 |
|
|
if (t == null) // lost to stealer |
599 |
|
|
break; |
600 |
|
|
if (UNSAFE.compareAndSwapObject(q, u, t, null)) { |
601 |
|
|
sp = s; // putOrderedInt may encourage more timely write |
602 |
|
|
// UNSAFE.putOrderedInt(this, spOffset, s); |
603 |
|
|
return t; |
604 |
|
|
} |
605 |
dl |
1.5 |
} |
606 |
dl |
1.1 |
} |
607 |
|
|
return null; |
608 |
|
|
} |
609 |
|
|
|
610 |
|
|
/** |
611 |
dl |
1.33 |
* Specialized version of popTask to pop only if topmost element |
612 |
|
|
* is the given task. Called only by current thread while |
613 |
|
|
* active. |
614 |
jsr166 |
1.11 |
* |
615 |
|
|
* @param t the task. Caller must ensure non-null. |
616 |
dl |
1.1 |
*/ |
617 |
|
|
final boolean unpushTask(ForkJoinTask<?> t) { |
618 |
dl |
1.31 |
int s; |
619 |
dl |
1.40 |
ForkJoinTask<?>[] q = queue; |
620 |
|
|
if ((s = sp) != base && q != null && |
621 |
dl |
1.33 |
UNSAFE.compareAndSwapObject |
622 |
|
|
(q, (((q.length - 1) & --s) << qShift) + qBase, t, null)) { |
623 |
dl |
1.42 |
sp = s; // putOrderedInt may encourage more timely write |
624 |
dl |
1.40 |
// UNSAFE.putOrderedInt(this, spOffset, s); |
625 |
dl |
1.1 |
return true; |
626 |
|
|
} |
627 |
|
|
return false; |
628 |
|
|
} |
629 |
|
|
|
630 |
|
|
/** |
631 |
dl |
1.23 |
* Returns next task or null if empty or contended |
632 |
dl |
1.1 |
*/ |
633 |
dl |
1.2 |
final ForkJoinTask<?> peekTask() { |
634 |
dl |
1.1 |
ForkJoinTask<?>[] q = queue; |
635 |
dl |
1.7 |
if (q == null) |
636 |
|
|
return null; |
637 |
|
|
int mask = q.length - 1; |
638 |
jsr166 |
1.15 |
int i = locallyFifo ? base : (sp - 1); |
639 |
dl |
1.7 |
return q[i & mask]; |
640 |
dl |
1.1 |
} |
641 |
|
|
|
642 |
|
|
/** |
643 |
|
|
* Doubles queue array size. Transfers elements by emulating |
644 |
|
|
* steals (deqs) from old array and placing, oldest first, into |
645 |
|
|
* new array. |
646 |
|
|
*/ |
647 |
|
|
private void growQueue() { |
648 |
|
|
ForkJoinTask<?>[] oldQ = queue; |
649 |
|
|
int oldSize = oldQ.length; |
650 |
|
|
int newSize = oldSize << 1; |
651 |
|
|
if (newSize > MAXIMUM_QUEUE_CAPACITY) |
652 |
|
|
throw new RejectedExecutionException("Queue capacity exceeded"); |
653 |
|
|
ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize]; |
654 |
|
|
|
655 |
|
|
int b = base; |
656 |
|
|
int bf = b + oldSize; |
657 |
|
|
int oldMask = oldSize - 1; |
658 |
|
|
int newMask = newSize - 1; |
659 |
|
|
do { |
660 |
|
|
int oldIndex = b & oldMask; |
661 |
|
|
ForkJoinTask<?> t = oldQ[oldIndex]; |
662 |
|
|
if (t != null && !casSlotNull(oldQ, oldIndex, t)) |
663 |
|
|
t = null; |
664 |
dl |
1.31 |
writeSlot(newQ, b & newMask, t); |
665 |
dl |
1.1 |
} while (++b != bf); |
666 |
dl |
1.5 |
pool.signalWork(); |
667 |
dl |
1.1 |
} |
668 |
|
|
|
669 |
|
|
/** |
670 |
dl |
1.31 |
* Computes next value for random victim probe in scan(). Scans |
671 |
|
|
* don't require a very high quality generator, but also not a |
672 |
|
|
* crummy one. Marsaglia xor-shift is cheap and works well enough. |
673 |
|
|
* Note: This is manually inlined in scan() |
674 |
|
|
*/ |
675 |
|
|
private static final int xorShift(int r) { |
676 |
|
|
r ^= r << 13; |
677 |
|
|
r ^= r >>> 17; |
678 |
|
|
return r ^ (r << 5); |
679 |
|
|
} |
680 |
|
|
|
681 |
|
|
/** |
682 |
dl |
1.5 |
* Tries to steal a task from another worker. Starts at a random |
683 |
|
|
* index of workers array, and probes workers until finding one |
684 |
|
|
* with non-empty queue or finding that all are empty. It |
685 |
|
|
* randomly selects the first n probes. If these are empty, it |
686 |
dl |
1.31 |
* resorts to a circular sweep, which is necessary to accurately |
687 |
|
|
* set active status. (The circular sweep uses steps of |
688 |
|
|
* approximately half the array size plus 1, to avoid bias |
689 |
|
|
* stemming from leftmost packing of the array in ForkJoinPool.) |
690 |
dl |
1.1 |
* |
691 |
|
|
* This method must be both fast and quiet -- usually avoiding |
692 |
|
|
* memory accesses that could disrupt cache sharing etc other than |
693 |
dl |
1.31 |
* those needed to check for and take tasks (or to activate if not |
694 |
|
|
* already active). This accounts for, among other things, |
695 |
|
|
* updating random seed in place without storing it until exit. |
696 |
dl |
1.1 |
* |
697 |
|
|
* @return a task, or null if none found |
698 |
|
|
*/ |
699 |
dl |
1.5 |
private ForkJoinTask<?> scan() { |
700 |
dl |
1.31 |
ForkJoinPool p = pool; |
701 |
dl |
1.33 |
ForkJoinWorkerThread[] ws; // worker array |
702 |
|
|
int n; // upper bound of #workers |
703 |
|
|
if ((ws = p.workers) != null && (n = ws.length) > 1) { |
704 |
|
|
boolean canSteal = active; // shadow active status |
705 |
|
|
int r = seed; // extract seed once |
706 |
|
|
int mask = n - 1; |
707 |
|
|
int j = -n; // loop counter |
708 |
|
|
int k = r; // worker index, random if j < 0 |
709 |
|
|
for (;;) { |
710 |
|
|
ForkJoinWorkerThread v = ws[k & mask]; |
711 |
|
|
r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift |
712 |
dl |
1.42 |
ForkJoinTask<?>[] q; ForkJoinTask<?> t; int b, a; |
713 |
|
|
if (v != null && (b = v.base) != v.sp && |
714 |
|
|
(q = v.queue) != null) { |
715 |
|
|
int i = (q.length - 1) & b; |
716 |
|
|
long u = (i << qShift) + qBase; // raw offset |
717 |
|
|
int pid = poolIndex; |
718 |
|
|
if ((t = q[i]) != null) { |
719 |
|
|
if (!canSteal && // inline p.tryIncrementActiveCount |
720 |
|
|
UNSAFE.compareAndSwapInt(p, poolRunStateOffset, |
721 |
|
|
a = p.runState, a + 1)) |
722 |
|
|
canSteal = active = true; |
723 |
|
|
if (canSteal && v.base == b++ && |
724 |
dl |
1.40 |
UNSAFE.compareAndSwapObject(q, u, t, null)) { |
725 |
dl |
1.42 |
v.base = b; |
726 |
dl |
1.40 |
v.stealHint = pid; |
727 |
dl |
1.42 |
UNSAFE.putOrderedObject(this, |
728 |
|
|
currentStealOffset, t); |
729 |
dl |
1.40 |
seed = r; |
730 |
|
|
++stealCount; |
731 |
|
|
return t; |
732 |
dl |
1.35 |
} |
733 |
dl |
1.1 |
} |
734 |
dl |
1.33 |
j = -n; |
735 |
|
|
k = r; // restart on contention |
736 |
dl |
1.1 |
} |
737 |
dl |
1.33 |
else if (++j <= 0) |
738 |
|
|
k = r; |
739 |
|
|
else if (j <= n) |
740 |
|
|
k += (n >>> 1) | 1; |
741 |
|
|
else |
742 |
|
|
break; |
743 |
dl |
1.1 |
} |
744 |
dl |
1.31 |
} |
745 |
|
|
return null; |
746 |
dl |
1.1 |
} |
747 |
|
|
|
748 |
dl |
1.31 |
// Run State management |
749 |
|
|
|
750 |
|
|
// status check methods used mainly by ForkJoinPool |
751 |
dl |
1.40 |
final boolean isRunning() { return runState == 0; } |
752 |
dl |
1.31 |
final boolean isTerminating() { return (runState & TERMINATING) != 0; } |
753 |
|
|
final boolean isTerminated() { return (runState & TERMINATED) != 0; } |
754 |
|
|
final boolean isSuspended() { return (runState & SUSPENDED) != 0; } |
755 |
|
|
final boolean isTrimmed() { return (runState & TRIMMED) != 0; } |
756 |
|
|
|
757 |
dl |
1.1 |
/** |
758 |
dl |
1.41 |
* Sets state to TERMINATING. Does NOT unpark or interrupt |
759 |
dl |
1.42 |
* to wake up if currently blocked. Callers must do so if desired. |
760 |
dl |
1.31 |
*/ |
761 |
dl |
1.41 |
final void shutdown() { |
762 |
dl |
1.31 |
for (;;) { |
763 |
|
|
int s = runState; |
764 |
dl |
1.40 |
if ((s & (TERMINATING|TERMINATED)) != 0) |
765 |
|
|
break; |
766 |
dl |
1.31 |
if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended |
767 |
|
|
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
768 |
|
|
(s & ~SUSPENDED) | |
769 |
dl |
1.40 |
(TRIMMED|TERMINATING))) |
770 |
dl |
1.31 |
break; |
771 |
|
|
} |
772 |
|
|
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
773 |
|
|
s | TERMINATING)) |
774 |
|
|
break; |
775 |
|
|
} |
776 |
|
|
} |
777 |
|
|
|
778 |
|
|
/** |
779 |
dl |
1.40 |
* Sets state to TERMINATED. Called only by onTermination() |
780 |
dl |
1.31 |
*/ |
781 |
|
|
private void setTerminated() { |
782 |
|
|
int s; |
783 |
|
|
do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset, |
784 |
|
|
s = runState, |
785 |
|
|
s | (TERMINATING|TERMINATED))); |
786 |
|
|
} |
787 |
|
|
|
788 |
|
|
/** |
789 |
dl |
1.41 |
* If suspended, tries to set status to unsuspended. |
790 |
dl |
1.42 |
* Does NOT wake up if blocked. |
791 |
jsr166 |
1.11 |
* |
792 |
dl |
1.31 |
* @return true if successful |
793 |
dl |
1.7 |
*/ |
794 |
dl |
1.40 |
final boolean tryUnsuspend() { |
795 |
|
|
int s; |
796 |
|
|
while (((s = runState) & SUSPENDED) != 0) { |
797 |
|
|
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
798 |
|
|
s & ~SUSPENDED)) |
799 |
|
|
return true; |
800 |
dl |
1.36 |
} |
801 |
dl |
1.35 |
return false; |
802 |
dl |
1.7 |
} |
803 |
|
|
|
804 |
|
|
/** |
805 |
dl |
1.40 |
* Sets suspended status and blocks as spare until resumed |
806 |
|
|
* or shutdown. |
807 |
dl |
1.1 |
*/ |
808 |
dl |
1.41 |
final void suspendAsSpare() { |
809 |
dl |
1.40 |
for (;;) { // set suspended unless terminating |
810 |
dl |
1.31 |
int s = runState; |
811 |
|
|
if ((s & TERMINATING) != 0) { // must kill |
812 |
|
|
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
813 |
|
|
s | (TRIMMED | TERMINATING))) |
814 |
dl |
1.41 |
return; |
815 |
dl |
1.31 |
} |
816 |
|
|
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
817 |
|
|
s | SUSPENDED)) |
818 |
|
|
break; |
819 |
|
|
} |
820 |
dl |
1.40 |
ForkJoinPool p = pool; |
821 |
|
|
p.pushSpare(this); |
822 |
dl |
1.31 |
while ((runState & SUSPENDED) != 0) { |
823 |
dl |
1.41 |
if (p.tryAccumulateStealCount(this)) { |
824 |
|
|
interrupted(); // clear/ignore interrupts |
825 |
dl |
1.40 |
if ((runState & SUSPENDED) == 0) |
826 |
|
|
break; |
827 |
dl |
1.42 |
LockSupport.park(this); |
828 |
dl |
1.31 |
} |
829 |
dl |
1.1 |
} |
830 |
dl |
1.31 |
} |
831 |
|
|
|
832 |
|
|
// Misc support methods for ForkJoinPool |
833 |
|
|
|
834 |
|
|
/** |
835 |
|
|
* Returns an estimate of the number of tasks in the queue. Also |
836 |
|
|
* used by ForkJoinTask. |
837 |
|
|
*/ |
838 |
|
|
final int getQueueSize() { |
839 |
dl |
1.40 |
int n; // external calls must read base first |
840 |
|
|
return (n = -base + sp) <= 0 ? 0 : n; |
841 |
dl |
1.1 |
} |
842 |
dl |
1.5 |
|
843 |
dl |
1.31 |
/** |
844 |
dl |
1.5 |
* Removes and cancels all tasks in queue. Can be called from any |
845 |
|
|
* thread. |
846 |
dl |
1.1 |
*/ |
847 |
dl |
1.5 |
final void cancelTasks() { |
848 |
dl |
1.40 |
ForkJoinTask<?> cj = currentJoin; // try to cancel ongoing tasks |
849 |
dl |
1.36 |
if (cj != null) { |
850 |
|
|
currentJoin = null; |
851 |
|
|
cj.cancelIgnoringExceptions(); |
852 |
dl |
1.40 |
try { |
853 |
|
|
this.interrupt(); // awaken wait |
854 |
|
|
} catch (SecurityException ignore) { |
855 |
|
|
} |
856 |
dl |
1.36 |
} |
857 |
|
|
ForkJoinTask<?> cs = currentSteal; |
858 |
|
|
if (cs != null) { |
859 |
|
|
currentSteal = null; |
860 |
|
|
cs.cancelIgnoringExceptions(); |
861 |
|
|
} |
862 |
dl |
1.31 |
while (base != sp) { |
863 |
|
|
ForkJoinTask<?> t = deqTask(); |
864 |
|
|
if (t != null) |
865 |
|
|
t.cancelIgnoringExceptions(); |
866 |
|
|
} |
867 |
dl |
1.1 |
} |
868 |
|
|
|
869 |
|
|
/** |
870 |
jsr166 |
1.11 |
* Drains tasks to given collection c. |
871 |
|
|
* |
872 |
dl |
1.7 |
* @return the number of tasks drained |
873 |
|
|
*/ |
874 |
dl |
1.22 |
final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) { |
875 |
dl |
1.7 |
int n = 0; |
876 |
dl |
1.31 |
while (base != sp) { |
877 |
|
|
ForkJoinTask<?> t = deqTask(); |
878 |
|
|
if (t != null) { |
879 |
|
|
c.add(t); |
880 |
|
|
++n; |
881 |
|
|
} |
882 |
dl |
1.7 |
} |
883 |
|
|
return n; |
884 |
|
|
} |
885 |
|
|
|
886 |
dl |
1.31 |
// Support methods for ForkJoinTask |
887 |
|
|
|
888 |
dl |
1.7 |
/** |
889 |
dl |
1.36 |
* Gets and removes a local task. |
890 |
|
|
* |
891 |
|
|
* @return a task, if available |
892 |
|
|
*/ |
893 |
|
|
final ForkJoinTask<?> pollLocalTask() { |
894 |
dl |
1.41 |
ForkJoinPool p = pool; |
895 |
dl |
1.36 |
while (sp != base) { |
896 |
dl |
1.41 |
int a; // inline p.tryIncrementActiveCount |
897 |
|
|
if (active || |
898 |
|
|
(active = UNSAFE.compareAndSwapInt(p, poolRunStateOffset, |
899 |
|
|
a = p.runState, a + 1))) |
900 |
dl |
1.36 |
return locallyFifo? locallyDeqTask() : popTask(); |
901 |
|
|
} |
902 |
|
|
return null; |
903 |
|
|
} |
904 |
|
|
|
905 |
|
|
/** |
906 |
|
|
* Gets and removes a local or stolen task. |
907 |
|
|
* |
908 |
|
|
* @return a task, if available |
909 |
|
|
*/ |
910 |
|
|
final ForkJoinTask<?> pollTask() { |
911 |
dl |
1.39 |
ForkJoinTask<?> t = pollLocalTask(); |
912 |
|
|
if (t == null) { |
913 |
|
|
t = scan(); |
914 |
dl |
1.42 |
// cannot retain/track/help steal |
915 |
|
|
UNSAFE.putOrderedObject(this, currentStealOffset, null); |
916 |
dl |
1.39 |
} |
917 |
|
|
return t; |
918 |
dl |
1.36 |
} |
919 |
|
|
|
920 |
|
|
/** |
921 |
dl |
1.35 |
* Possibly runs some tasks and/or blocks, until task is done. |
922 |
dl |
1.36 |
* |
923 |
|
|
* @param joinMe the task to join |
924 |
|
|
*/ |
925 |
dl |
1.40 |
final void joinTask(ForkJoinTask<?> joinMe) { |
926 |
|
|
// currentJoin only written by this thread; only need ordered store |
927 |
dl |
1.36 |
ForkJoinTask<?> prevJoin = currentJoin; |
928 |
dl |
1.39 |
UNSAFE.putOrderedObject(this, currentJoinOffset, joinMe); |
929 |
dl |
1.40 |
if (sp != base) |
930 |
|
|
localHelpJoinTask(joinMe); |
931 |
|
|
if (joinMe.status >= 0) |
932 |
|
|
pool.awaitJoin(joinMe, this); |
933 |
dl |
1.39 |
UNSAFE.putOrderedObject(this, currentJoinOffset, prevJoin); |
934 |
dl |
1.36 |
} |
935 |
|
|
|
936 |
|
|
/** |
937 |
|
|
* Run tasks in local queue until given task is done. |
938 |
dl |
1.35 |
* |
939 |
|
|
* @param joinMe the task to join |
940 |
|
|
*/ |
941 |
dl |
1.40 |
private void localHelpJoinTask(ForkJoinTask<?> joinMe) { |
942 |
|
|
int s; |
943 |
dl |
1.36 |
ForkJoinTask<?>[] q; |
944 |
dl |
1.40 |
while (joinMe.status >= 0 && (s = sp) != base && (q = queue) != null) { |
945 |
dl |
1.35 |
int i = (q.length - 1) & --s; |
946 |
|
|
long u = (i << qShift) + qBase; // raw offset |
947 |
dl |
1.40 |
ForkJoinTask<?> t = q[i]; |
948 |
|
|
if (t == null) // lost to a stealer |
949 |
|
|
break; |
950 |
|
|
if (UNSAFE.compareAndSwapObject(q, u, t, null)) { |
951 |
dl |
1.35 |
/* |
952 |
dl |
1.36 |
* This recheck (and similarly in helpJoinTask) |
953 |
dl |
1.35 |
* handles cases where joinMe is independently |
954 |
|
|
* cancelled or forced even though there is other work |
955 |
|
|
* available. Back out of the pop by putting t back |
956 |
dl |
1.36 |
* into slot before we commit by writing sp. |
957 |
dl |
1.35 |
*/ |
958 |
dl |
1.40 |
if (joinMe.status < 0) { |
959 |
dl |
1.35 |
UNSAFE.putObjectVolatile(q, u, t); |
960 |
|
|
break; |
961 |
|
|
} |
962 |
|
|
sp = s; |
963 |
dl |
1.40 |
// UNSAFE.putOrderedInt(this, spOffset, s); |
964 |
|
|
t.quietlyExec(); |
965 |
dl |
1.35 |
} |
966 |
|
|
} |
967 |
|
|
} |
968 |
|
|
|
969 |
|
|
/** |
970 |
dl |
1.42 |
* Unless terminating, tries to locate and help perform tasks for |
971 |
|
|
* a stealer of the given task, or in turn one of its stealers. |
972 |
|
|
* Traces currentSteal->currentJoin links looking for a thread |
973 |
|
|
* working on a descendant of the given task and with a non-empty |
974 |
|
|
* queue to steal back and execute tasks from. |
975 |
dl |
1.40 |
* |
976 |
dl |
1.42 |
* The implementation is very branchy to cope with potential |
977 |
dl |
1.40 |
* inconsistencies or loops encountering chains that are stale, |
978 |
|
|
* unknown, or of length greater than MAX_HELP_DEPTH links. All |
979 |
|
|
* of these cases are dealt with by just returning back to the |
980 |
|
|
* caller, who is expected to retry if other join mechanisms also |
981 |
|
|
* don't work out. |
982 |
dl |
1.35 |
* |
983 |
|
|
* @param joinMe the task to join |
984 |
|
|
*/ |
985 |
dl |
1.40 |
final void helpJoinTask(ForkJoinTask<?> joinMe) { |
986 |
dl |
1.42 |
ForkJoinWorkerThread[] ws; |
987 |
|
|
int n; |
988 |
|
|
if (joinMe.status < 0) // already done |
989 |
|
|
return; |
990 |
|
|
if ((runState & TERMINATING) != 0) { // cancel if shutting down |
991 |
|
|
joinMe.cancelIgnoringExceptions(); |
992 |
|
|
return; |
993 |
|
|
} |
994 |
|
|
if ((ws = pool.workers) == null || (n = ws.length) <= 1) |
995 |
|
|
return; // need at least 2 workers |
996 |
|
|
|
997 |
|
|
ForkJoinTask<?> task = joinMe; // base of chain |
998 |
|
|
ForkJoinWorkerThread thread = this; // thread with stolen task |
999 |
|
|
for (int d = 0; d < MAX_HELP_DEPTH; ++d) { // chain length |
1000 |
|
|
// Try to find v, the stealer of task, by first using hint |
1001 |
|
|
ForkJoinWorkerThread v = ws[thread.stealHint & (n - 1)]; |
1002 |
|
|
if (v == null || v.currentSteal != task) { |
1003 |
|
|
for (int j = 0; ; ++j) { // search array |
1004 |
|
|
if (j < n) { |
1005 |
|
|
ForkJoinTask<?> vs; |
1006 |
|
|
if ((v = ws[j]) != null && |
1007 |
|
|
(vs = v.currentSteal) != null) { |
1008 |
|
|
if (joinMe.status < 0 || task.status < 0) |
1009 |
|
|
return; // stale or done |
1010 |
|
|
if (vs == task) { |
1011 |
|
|
thread.stealHint = j; |
1012 |
|
|
break; // save hint for next time |
1013 |
dl |
1.40 |
} |
1014 |
dl |
1.36 |
} |
1015 |
dl |
1.35 |
} |
1016 |
dl |
1.42 |
else |
1017 |
|
|
return; // no stealer |
1018 |
dl |
1.35 |
} |
1019 |
dl |
1.42 |
} |
1020 |
|
|
for (;;) { // Try to help v, using specialized form of deqTask |
1021 |
|
|
if (joinMe.status < 0) |
1022 |
|
|
return; |
1023 |
|
|
int b = v.base; |
1024 |
|
|
ForkJoinTask<?>[] q = v.queue; |
1025 |
|
|
if (b == v.sp || q == null) |
1026 |
|
|
break; |
1027 |
|
|
int i = (q.length - 1) & b; |
1028 |
|
|
long u = (i << qShift) + qBase; |
1029 |
|
|
ForkJoinTask<?> t = q[i]; |
1030 |
|
|
int pid = poolIndex; |
1031 |
|
|
ForkJoinTask<?> ps = currentSteal; |
1032 |
|
|
if (task.status < 0) |
1033 |
|
|
return; // stale or done |
1034 |
|
|
if (t != null && v.base == b++ && |
1035 |
|
|
UNSAFE.compareAndSwapObject(q, u, t, null)) { |
1036 |
|
|
if (joinMe.status < 0) { |
1037 |
|
|
UNSAFE.putObjectVolatile(q, u, t); |
1038 |
|
|
return; // back out on cancel |
1039 |
dl |
1.35 |
} |
1040 |
dl |
1.42 |
v.base = b; |
1041 |
|
|
v.stealHint = pid; |
1042 |
|
|
UNSAFE.putOrderedObject(this, currentStealOffset, t); |
1043 |
|
|
t.quietlyExec(); |
1044 |
|
|
UNSAFE.putOrderedObject(this, currentStealOffset, ps); |
1045 |
dl |
1.35 |
} |
1046 |
|
|
} |
1047 |
dl |
1.42 |
// Try to descend to find v's stealer |
1048 |
|
|
ForkJoinTask<?> next = v.currentJoin; |
1049 |
|
|
if (task.status < 0 || next == null || next == task || |
1050 |
|
|
joinMe.status < 0) |
1051 |
|
|
return; |
1052 |
|
|
task = next; |
1053 |
|
|
thread = v; |
1054 |
dl |
1.35 |
} |
1055 |
|
|
} |
1056 |
|
|
|
1057 |
|
|
/** |
1058 |
dl |
1.31 |
* Returns an estimate of the number of tasks, offset by a |
1059 |
|
|
* function of number of idle workers. |
1060 |
|
|
* |
1061 |
|
|
* This method provides a cheap heuristic guide for task |
1062 |
|
|
* partitioning when programmers, frameworks, tools, or languages |
1063 |
|
|
* have little or no idea about task granularity. In essence by |
1064 |
|
|
* offering this method, we ask users only about tradeoffs in |
1065 |
|
|
* overhead vs expected throughput and its variance, rather than |
1066 |
|
|
* how finely to partition tasks. |
1067 |
|
|
* |
1068 |
|
|
* In a steady state strict (tree-structured) computation, each |
1069 |
|
|
* thread makes available for stealing enough tasks for other |
1070 |
|
|
* threads to remain active. Inductively, if all threads play by |
1071 |
|
|
* the same rules, each thread should make available only a |
1072 |
|
|
* constant number of tasks. |
1073 |
|
|
* |
1074 |
|
|
* The minimum useful constant is just 1. But using a value of 1 |
1075 |
|
|
* would require immediate replenishment upon each steal to |
1076 |
|
|
* maintain enough tasks, which is infeasible. Further, |
1077 |
|
|
* partitionings/granularities of offered tasks should minimize |
1078 |
|
|
* steal rates, which in general means that threads nearer the top |
1079 |
|
|
* of computation tree should generate more than those nearer the |
1080 |
|
|
* bottom. In perfect steady state, each thread is at |
1081 |
|
|
* approximately the same level of computation tree. However, |
1082 |
|
|
* producing extra tasks amortizes the uncertainty of progress and |
1083 |
|
|
* diffusion assumptions. |
1084 |
|
|
* |
1085 |
|
|
* So, users will want to use values larger, but not much larger |
1086 |
|
|
* than 1 to both smooth over transient shortages and hedge |
1087 |
|
|
* against uneven progress; as traded off against the cost of |
1088 |
|
|
* extra task overhead. We leave the user to pick a threshold |
1089 |
|
|
* value to compare with the results of this call to guide |
1090 |
|
|
* decisions, but recommend values such as 3. |
1091 |
|
|
* |
1092 |
|
|
* When all threads are active, it is on average OK to estimate |
1093 |
|
|
* surplus strictly locally. In steady-state, if one thread is |
1094 |
|
|
* maintaining say 2 surplus tasks, then so are others. So we can |
1095 |
|
|
* just use estimated queue length (although note that (sp - base) |
1096 |
|
|
* can be an overestimate because of stealers lagging increments |
1097 |
|
|
* of base). However, this strategy alone leads to serious |
1098 |
|
|
* mis-estimates in some non-steady-state conditions (ramp-up, |
1099 |
|
|
* ramp-down, other stalls). We can detect many of these by |
1100 |
|
|
* further considering the number of "idle" threads, that are |
1101 |
|
|
* known to have zero queued tasks, so compensate by a factor of |
1102 |
|
|
* (#idle/#active) threads. |
1103 |
dl |
1.1 |
*/ |
1104 |
dl |
1.31 |
final int getEstimatedSurplusTaskCount() { |
1105 |
|
|
return sp - base - pool.idlePerActive(); |
1106 |
dl |
1.5 |
} |
1107 |
|
|
|
1108 |
|
|
/** |
1109 |
jsr166 |
1.16 |
* Runs tasks until {@code pool.isQuiescent()}. |
1110 |
dl |
1.1 |
*/ |
1111 |
dl |
1.5 |
final void helpQuiescePool() { |
1112 |
dl |
1.42 |
ForkJoinTask<?> ps = currentSteal; // to restore below |
1113 |
dl |
1.5 |
for (;;) { |
1114 |
dl |
1.31 |
ForkJoinTask<?> t = pollLocalTask(); |
1115 |
dl |
1.42 |
if (t != null || (t = scan()) != null) |
1116 |
dl |
1.40 |
t.quietlyExec(); |
1117 |
dl |
1.31 |
else { |
1118 |
|
|
ForkJoinPool p = pool; |
1119 |
dl |
1.41 |
int a; // to inline CASes |
1120 |
dl |
1.31 |
if (active) { |
1121 |
dl |
1.41 |
if (!UNSAFE.compareAndSwapInt |
1122 |
|
|
(p, poolRunStateOffset, a = p.runState, a - 1)) |
1123 |
dl |
1.40 |
continue; // retry later |
1124 |
dl |
1.31 |
active = false; // inactivate |
1125 |
dl |
1.42 |
UNSAFE.putOrderedObject(this, currentStealOffset, ps); |
1126 |
dl |
1.31 |
} |
1127 |
|
|
if (p.isQuiescent()) { |
1128 |
|
|
active = true; // re-activate |
1129 |
jsr166 |
1.43 |
do {} while (!UNSAFE.compareAndSwapInt |
1130 |
|
|
(p, poolRunStateOffset, a = p.runState, a+1)); |
1131 |
dl |
1.31 |
return; |
1132 |
|
|
} |
1133 |
|
|
} |
1134 |
dl |
1.5 |
} |
1135 |
dl |
1.1 |
} |
1136 |
|
|
|
1137 |
jsr166 |
1.20 |
// Unsafe mechanics |
1138 |
|
|
|
1139 |
|
|
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1140 |
dl |
1.40 |
private static final long spOffset = |
1141 |
|
|
objectFieldOffset("sp", ForkJoinWorkerThread.class); |
1142 |
jsr166 |
1.20 |
private static final long runStateOffset = |
1143 |
|
|
objectFieldOffset("runState", ForkJoinWorkerThread.class); |
1144 |
dl |
1.39 |
private static final long currentJoinOffset = |
1145 |
|
|
objectFieldOffset("currentJoin", ForkJoinWorkerThread.class); |
1146 |
dl |
1.40 |
private static final long currentStealOffset = |
1147 |
|
|
objectFieldOffset("currentSteal", ForkJoinWorkerThread.class); |
1148 |
dl |
1.31 |
private static final long qBase = |
1149 |
|
|
UNSAFE.arrayBaseOffset(ForkJoinTask[].class); |
1150 |
dl |
1.41 |
private static final long poolRunStateOffset = // to inline CAS |
1151 |
|
|
objectFieldOffset("runState", ForkJoinPool.class); |
1152 |
dl |
1.40 |
|
1153 |
jsr166 |
1.20 |
private static final int qShift; |
1154 |
|
|
|
1155 |
|
|
static { |
1156 |
|
|
int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class); |
1157 |
|
|
if ((s & (s-1)) != 0) |
1158 |
|
|
throw new Error("data type scale not a power of two"); |
1159 |
|
|
qShift = 31 - Integer.numberOfLeadingZeros(s); |
1160 |
|
|
} |
1161 |
|
|
|
1162 |
|
|
private static long objectFieldOffset(String field, Class<?> klazz) { |
1163 |
|
|
try { |
1164 |
|
|
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1165 |
|
|
} catch (NoSuchFieldException e) { |
1166 |
|
|
// Convert Exception to corresponding Error |
1167 |
|
|
NoSuchFieldError error = new NoSuchFieldError(field); |
1168 |
|
|
error.initCause(e); |
1169 |
|
|
throw error; |
1170 |
|
|
} |
1171 |
|
|
} |
1172 |
|
|
|
1173 |
|
|
/** |
1174 |
|
|
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1175 |
|
|
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1176 |
|
|
* into a jdk. |
1177 |
|
|
* |
1178 |
|
|
* @return a sun.misc.Unsafe |
1179 |
|
|
*/ |
1180 |
jsr166 |
1.17 |
private static sun.misc.Unsafe getUnsafe() { |
1181 |
jsr166 |
1.6 |
try { |
1182 |
jsr166 |
1.17 |
return sun.misc.Unsafe.getUnsafe(); |
1183 |
jsr166 |
1.6 |
} catch (SecurityException se) { |
1184 |
|
|
try { |
1185 |
|
|
return java.security.AccessController.doPrivileged |
1186 |
jsr166 |
1.20 |
(new java.security |
1187 |
|
|
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1188 |
jsr166 |
1.17 |
public sun.misc.Unsafe run() throws Exception { |
1189 |
jsr166 |
1.20 |
java.lang.reflect.Field f = sun.misc |
1190 |
|
|
.Unsafe.class.getDeclaredField("theUnsafe"); |
1191 |
|
|
f.setAccessible(true); |
1192 |
|
|
return (sun.misc.Unsafe) f.get(null); |
1193 |
jsr166 |
1.6 |
}}); |
1194 |
|
|
} catch (java.security.PrivilegedActionException e) { |
1195 |
jsr166 |
1.17 |
throw new RuntimeException("Could not initialize intrinsics", |
1196 |
|
|
e.getCause()); |
1197 |
jsr166 |
1.6 |
} |
1198 |
|
|
} |
1199 |
|
|
} |
1200 |
dl |
1.1 |
} |