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/* |
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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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|
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package jsr166y; |
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|
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import java.util.concurrent.*; |
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|
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import java.util.Random; |
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import java.util.Collection; |
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import java.util.concurrent.locks.LockSupport; |
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|
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/** |
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* 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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* 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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* ForkJoinPool.ForkJoinWorkerThreadFactory} to use it in a {@code |
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* ForkJoinPool}. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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*/ |
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public class ForkJoinWorkerThread extends Thread { |
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/* |
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* Overview: |
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* |
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* 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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* |
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* 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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* 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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* 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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* 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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* 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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* ForkJoinPool.awaitJoin). |
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* |
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* Efficient implementation of these algorithms currently relies |
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* on an uncomfortable amount of "Unsafe" mechanics. To maintain |
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* correct orderings, reads and writes of variable base require |
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* 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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* 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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* |
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* 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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* contention effects across multiple queues (especially in the |
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* presence of GC cardmarking). Also, to improve thread-locality, |
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* 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. |
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*/ |
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|
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/** |
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* 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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|
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/** |
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* 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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/** |
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* Capacity of work-stealing queue array upon initialization. |
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* Must be a power of two. Initial size must be at least 4, but is |
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* 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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/** |
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* Maximum work-stealing queue array size. Must be less than or |
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* equal to 1 << (31 - width of array entry) to ensure lack of |
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* index wraparound. The value is set in the static block |
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* at the end of this file after obtaining width. |
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*/ |
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private static final int MAXIMUM_QUEUE_CAPACITY; |
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|
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/** |
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* The pool this thread works in. Accessed directly by ForkJoinTask. |
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*/ |
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final ForkJoinPool pool; |
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|
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/** |
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* The work-stealing queue array. Size must be a power of two. |
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* Initialized in onStart, to improve memory locality. |
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*/ |
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private ForkJoinTask<?>[] queue; |
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|
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/** |
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* Index (mod queue.length) of least valid queue slot, which is |
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* always the next position to steal from if nonempty. |
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*/ |
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private volatile int base; |
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|
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/** |
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* Index (mod queue.length) of next queue slot to push to or pop |
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* from. It is written only by owner thread, and accessed by other |
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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 |
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* estimates size. |
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*/ |
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private int sp; |
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|
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/** |
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* The index of most recent stealer, used as a hint to avoid |
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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 |
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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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|
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/** |
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* Run state of this worker. In addition to the usual run levels, |
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* tracks if this worker is suspended as a spare, and if it was |
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* killed (trimmed) while suspended. However, "active" status is |
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* maintained separately and modified only in conjunction with |
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* CASes of the pool's runState (which are currently sadly |
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* manually inlined for performance.) Accessed directly by pool |
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* to simplify checks for normal (zero) status. |
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*/ |
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volatile int runState; |
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|
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private static final int TERMINATING = 0x01; |
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private static final int TERMINATED = 0x02; |
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private static final int SUSPENDED = 0x04; // inactive spare |
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private static final int TRIMMED = 0x08; // killed while suspended |
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|
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/** |
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* Number of steals. Directly accessed (and reset) by |
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* pool.tryAccumulateStealCount when idle. |
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*/ |
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int stealCount; |
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|
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/** |
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* Seed for random number generator for choosing steal victims. |
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* Uses Marsaglia xorshift. Must be initialized as nonzero. |
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*/ |
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private int seed; |
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|
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/** |
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* Activity status. When true, this worker is considered active. |
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* Accessed directly by pool. Must be false upon construction. |
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*/ |
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boolean active; |
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|
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/** |
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* True if use local fifo, not default lifo, for local polling. |
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* Shadows value from ForkJoinPool. |
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*/ |
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private final boolean locallyFifo; |
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|
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/** |
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* Index of this worker in pool array. Set once by pool before |
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* running, and accessed directly by pool to locate this worker in |
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* its workers array. |
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*/ |
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int poolIndex; |
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|
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/** |
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* The last pool event waited for. Accessed only by pool in |
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* callback methods invoked within this thread. |
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*/ |
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int lastEventCount; |
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|
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/** |
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* Encoded index and event count of next event waiter. Accessed |
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* only by ForkJoinPool for managing event waiters. |
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*/ |
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volatile long nextWaiter; |
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|
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/** |
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* Number of times this thread suspended as spare. Accessed only |
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* by pool. |
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*/ |
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int spareCount; |
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|
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/** |
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* Encoded index and count of next spare waiter. Accessed only |
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* by ForkJoinPool for managing spares. |
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*/ |
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volatile int nextSpare; |
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|
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/** |
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* The task currently being joined, set only when actively trying |
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* to help other stealers in helpJoinTask. Written only by this |
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* thread, but read by others. |
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*/ |
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private volatile ForkJoinTask<?> currentJoin; |
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|
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/** |
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* The task most recently stolen from another worker (or |
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* submission queue). Written only by this thread, but read by |
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* others. |
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*/ |
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private volatile ForkJoinTask<?> currentSteal; |
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|
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/** |
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* Creates a ForkJoinWorkerThread operating in the given pool. |
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* |
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* @param pool the pool this thread works in |
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* @throws NullPointerException if pool is null |
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*/ |
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protected ForkJoinWorkerThread(ForkJoinPool pool) { |
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this.pool = pool; |
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this.locallyFifo = pool.locallyFifo; |
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setDaemon(true); |
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// To avoid exposing construction details to subclasses, |
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// remaining initialization is in start() and onStart() |
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} |
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|
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/** |
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* Performs additional initialization and starts this thread. |
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*/ |
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final void start(int poolIndex, UncaughtExceptionHandler ueh) { |
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this.poolIndex = poolIndex; |
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if (ueh != null) |
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setUncaughtExceptionHandler(ueh); |
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start(); |
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} |
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|
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// Public/protected methods |
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|
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/** |
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* Returns the pool hosting this thread. |
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* |
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* @return the pool |
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*/ |
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public ForkJoinPool getPool() { |
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return pool; |
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} |
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|
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/** |
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* Returns the index number of this thread in its pool. The |
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* returned value ranges from zero to the maximum number of |
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* threads (minus one) that have ever been created in the pool. |
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* This method may be useful for applications that track status or |
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* collect results per-worker rather than per-task. |
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* |
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* @return the index number |
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*/ |
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public int getPoolIndex() { |
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return poolIndex; |
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} |
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|
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/** |
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* Initializes internal state after construction but before |
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* processing any tasks. If you override this method, you must |
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* invoke @code{super.onStart()} at the beginning of the method. |
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* Initialization requires care: Most fields must have legal |
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* default values, to ensure that attempted accesses from other |
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* threads work correctly even before this thread starts |
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* processing tasks. |
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*/ |
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protected void onStart() { |
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int rs = seedGenerator.nextInt(); |
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seed = rs == 0? 1 : rs; // seed must be nonzero |
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|
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// Allocate name string and arrays in this thread |
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String pid = Integer.toString(pool.getPoolNumber()); |
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String wid = Integer.toString(poolIndex); |
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setName("ForkJoinPool-" + pid + "-worker-" + wid); |
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|
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queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY]; |
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} |
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|
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/** |
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* Performs cleanup associated with termination of this worker |
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* thread. If you override this method, you must invoke |
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* {@code super.onTermination} at the end of the overridden method. |
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* |
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* @param exception the exception causing this thread to abort due |
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* to an unrecoverable error, or {@code null} if completed normally |
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*/ |
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protected void onTermination(Throwable exception) { |
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try { |
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ForkJoinPool p = pool; |
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if (active) { |
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int a; // inline p.tryDecrementActiveCount |
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active = false; |
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do {} while (!UNSAFE.compareAndSwapInt |
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(p, poolRunStateOffset, a = p.runState, a - 1)); |
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} |
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cancelTasks(); |
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setTerminated(); |
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p.workerTerminated(this); |
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} catch (Throwable ex) { // Shouldn't ever happen |
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if (exception == null) // but if so, at least rethrown |
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exception = ex; |
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} finally { |
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if (exception != null) |
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UNSAFE.throwException(exception); |
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} |
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} |
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|
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/** |
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* This method is required to be public, but should never be |
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* called explicitly. It performs the main run loop to execute |
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* ForkJoinTasks. |
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*/ |
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public void run() { |
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Throwable exception = null; |
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try { |
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onStart(); |
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mainLoop(); |
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} catch (Throwable ex) { |
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exception = ex; |
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} finally { |
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onTermination(exception); |
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} |
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} |
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|
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// helpers for run() |
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|
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/** |
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* Finds and executes tasks, and checks status while running. |
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*/ |
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private void mainLoop() { |
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boolean ran = false; // true if ran a task on last step |
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ForkJoinPool p = pool; |
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for (;;) { |
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p.preStep(this, ran); |
426 |
if (runState != 0) |
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break; |
428 |
ran = tryExecSteal() || tryExecSubmission(); |
429 |
} |
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} |
431 |
|
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/** |
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* Tries to steal a task and execute it. |
434 |
* |
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* @return true if ran a task |
436 |
*/ |
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private boolean tryExecSteal() { |
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ForkJoinTask<?> t; |
439 |
if ((t = scan()) != null) { |
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t.quietlyExec(); |
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UNSAFE.putOrderedObject(this, currentStealOffset, null); |
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if (sp != base) |
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execLocalTasks(); |
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return true; |
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} |
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return false; |
447 |
} |
448 |
|
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/** |
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* If a submission exists, try to activate and run it. |
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* |
452 |
* @return true if ran a task |
453 |
*/ |
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private boolean tryExecSubmission() { |
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ForkJoinPool p = pool; |
456 |
// This loop is needed in case attempt to activate fails, in |
457 |
// which case we only retry if there still appears to be a |
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// submission. |
459 |
while (p.hasQueuedSubmissions()) { |
460 |
ForkJoinTask<?> t; int a; |
461 |
if (active || // inline p.tryIncrementActiveCount |
462 |
(active = UNSAFE.compareAndSwapInt(p, poolRunStateOffset, |
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a = p.runState, a + 1))) { |
464 |
if ((t = p.pollSubmission()) != null) { |
465 |
UNSAFE.putOrderedObject(this, currentStealOffset, t); |
466 |
t.quietlyExec(); |
467 |
UNSAFE.putOrderedObject(this, currentStealOffset, null); |
468 |
if (sp != base) |
469 |
execLocalTasks(); |
470 |
return true; |
471 |
} |
472 |
} |
473 |
} |
474 |
return false; |
475 |
} |
476 |
|
477 |
/** |
478 |
* Runs local tasks until queue is empty or shut down. Call only |
479 |
* while active. |
480 |
*/ |
481 |
private void execLocalTasks() { |
482 |
while (runState == 0) { |
483 |
ForkJoinTask<?> t = locallyFifo ? locallyDeqTask() : popTask(); |
484 |
if (t != null) |
485 |
t.quietlyExec(); |
486 |
else if (sp == base) |
487 |
break; |
488 |
} |
489 |
} |
490 |
|
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/* |
492 |
* Intrinsics-based atomic writes for queue slots. These are |
493 |
* basically the same as methods in AtomicReferenceArray, but |
494 |
* specialized for (1) ForkJoinTask elements (2) requirement that |
495 |
* nullness and bounds checks have already been performed by |
496 |
* callers and (3) effective offsets are known not to overflow |
497 |
* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't |
498 |
* need corresponding version for reads: plain array reads are OK |
499 |
* because they are protected by other volatile reads and are |
500 |
* confirmed by CASes. |
501 |
* |
502 |
* Most uses don't actually call these methods, but instead contain |
503 |
* inlined forms that enable more predictable optimization. We |
504 |
* don't define the version of write used in pushTask at all, but |
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* instead inline there a store-fenced array slot write. |
506 |
*/ |
507 |
|
508 |
/** |
509 |
* CASes slot i of array q from t to null. Caller must ensure q is |
510 |
* non-null and index is in range. |
511 |
*/ |
512 |
private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i, |
513 |
ForkJoinTask<?> t) { |
514 |
return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null); |
515 |
} |
516 |
|
517 |
/** |
518 |
* Performs a volatile write of the given task at given slot of |
519 |
* array q. Caller must ensure q is non-null and index is in |
520 |
* range. This method is used only during resets and backouts. |
521 |
*/ |
522 |
private static final void writeSlot(ForkJoinTask<?>[] q, int i, |
523 |
ForkJoinTask<?> t) { |
524 |
UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t); |
525 |
} |
526 |
|
527 |
// queue methods |
528 |
|
529 |
/** |
530 |
* Pushes a task. Call only from this thread. |
531 |
* |
532 |
* @param t the task. Caller must ensure non-null. |
533 |
*/ |
534 |
final void pushTask(ForkJoinTask<?> t) { |
535 |
ForkJoinTask<?>[] q = queue; |
536 |
int mask = q.length - 1; // implicit assert q != null |
537 |
int s = sp++; // ok to increment sp before slot write |
538 |
UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t); |
539 |
if ((s -= base) == 0) |
540 |
pool.signalWork(); // was empty |
541 |
else if (s == mask) |
542 |
growQueue(); // is full |
543 |
} |
544 |
|
545 |
/** |
546 |
* Tries to take a task from the base of the queue, failing if |
547 |
* empty or contended. Note: Specializations of this code appear |
548 |
* in locallyDeqTask and elsewhere. |
549 |
* |
550 |
* @return a task, or null if none or contended |
551 |
*/ |
552 |
final ForkJoinTask<?> deqTask() { |
553 |
ForkJoinTask<?> t; |
554 |
ForkJoinTask<?>[] q; |
555 |
int b, i; |
556 |
if (sp != (b = base) && |
557 |
(q = queue) != null && // must read q after b |
558 |
(t = q[i = (q.length - 1) & b]) != null && base == b && |
559 |
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) { |
560 |
base = b + 1; |
561 |
return t; |
562 |
} |
563 |
return null; |
564 |
} |
565 |
|
566 |
/** |
567 |
* Tries to take a task from the base of own queue. Assumes active |
568 |
* status. Called only by this thread. |
569 |
* |
570 |
* @return a task, or null if none |
571 |
*/ |
572 |
final ForkJoinTask<?> locallyDeqTask() { |
573 |
ForkJoinTask<?>[] q = queue; |
574 |
if (q != null) { |
575 |
ForkJoinTask<?> t; |
576 |
int b, i; |
577 |
while (sp != (b = base)) { |
578 |
if ((t = q[i = (q.length - 1) & b]) != null && base == b && |
579 |
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, |
580 |
t, null)) { |
581 |
base = b + 1; |
582 |
return t; |
583 |
} |
584 |
} |
585 |
} |
586 |
return null; |
587 |
} |
588 |
|
589 |
/** |
590 |
* Returns a popped task, or null if empty. Assumes active status. |
591 |
* Called only by this thread. |
592 |
*/ |
593 |
private ForkJoinTask<?> popTask() { |
594 |
ForkJoinTask<?>[] q = queue; |
595 |
if (q != null) { |
596 |
int s; |
597 |
while ((s = sp) != base) { |
598 |
int i = (q.length - 1) & --s; |
599 |
long u = (i << qShift) + qBase; // raw offset |
600 |
ForkJoinTask<?> t = q[i]; |
601 |
if (t == null) // lost to stealer |
602 |
break; |
603 |
if (UNSAFE.compareAndSwapObject(q, u, t, null)) { |
604 |
sp = s; // putOrderedInt may encourage more timely write |
605 |
// UNSAFE.putOrderedInt(this, spOffset, s); |
606 |
return t; |
607 |
} |
608 |
} |
609 |
} |
610 |
return null; |
611 |
} |
612 |
|
613 |
/** |
614 |
* Specialized version of popTask to pop only if topmost element |
615 |
* is the given task. Called only by this thread while active. |
616 |
* |
617 |
* @param t the task. Caller must ensure non-null. |
618 |
*/ |
619 |
final boolean unpushTask(ForkJoinTask<?> t) { |
620 |
int s; |
621 |
ForkJoinTask<?>[] q = queue; |
622 |
if ((s = sp) != base && q != null && |
623 |
UNSAFE.compareAndSwapObject |
624 |
(q, (((q.length - 1) & --s) << qShift) + qBase, t, null)) { |
625 |
sp = s; // putOrderedInt may encourage more timely write |
626 |
// UNSAFE.putOrderedInt(this, spOffset, s); |
627 |
return true; |
628 |
} |
629 |
return false; |
630 |
} |
631 |
|
632 |
/** |
633 |
* Returns next task, or null if empty or contended. |
634 |
*/ |
635 |
final ForkJoinTask<?> peekTask() { |
636 |
ForkJoinTask<?>[] q = queue; |
637 |
if (q == null) |
638 |
return null; |
639 |
int mask = q.length - 1; |
640 |
int i = locallyFifo ? base : (sp - 1); |
641 |
return q[i & mask]; |
642 |
} |
643 |
|
644 |
/** |
645 |
* Doubles queue array size. Transfers elements by emulating |
646 |
* steals (deqs) from old array and placing, oldest first, into |
647 |
* new array. |
648 |
*/ |
649 |
private void growQueue() { |
650 |
ForkJoinTask<?>[] oldQ = queue; |
651 |
int oldSize = oldQ.length; |
652 |
int newSize = oldSize << 1; |
653 |
if (newSize > MAXIMUM_QUEUE_CAPACITY) |
654 |
throw new RejectedExecutionException("Queue capacity exceeded"); |
655 |
ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize]; |
656 |
|
657 |
int b = base; |
658 |
int bf = b + oldSize; |
659 |
int oldMask = oldSize - 1; |
660 |
int newMask = newSize - 1; |
661 |
do { |
662 |
int oldIndex = b & oldMask; |
663 |
ForkJoinTask<?> t = oldQ[oldIndex]; |
664 |
if (t != null && !casSlotNull(oldQ, oldIndex, t)) |
665 |
t = null; |
666 |
writeSlot(newQ, b & newMask, t); |
667 |
} while (++b != bf); |
668 |
pool.signalWork(); |
669 |
} |
670 |
|
671 |
/** |
672 |
* Computes next value for random victim probe in scan(). Scans |
673 |
* don't require a very high quality generator, but also not a |
674 |
* crummy one. Marsaglia xor-shift is cheap and works well enough. |
675 |
* Note: This is manually inlined in scan(). |
676 |
*/ |
677 |
private static final int xorShift(int r) { |
678 |
r ^= r << 13; |
679 |
r ^= r >>> 17; |
680 |
return r ^ (r << 5); |
681 |
} |
682 |
|
683 |
/** |
684 |
* Tries to steal a task from another worker. Starts at a random |
685 |
* index of workers array, and probes workers until finding one |
686 |
* with non-empty queue or finding that all are empty. It |
687 |
* randomly selects the first n probes. If these are empty, it |
688 |
* resorts to a circular sweep, which is necessary to accurately |
689 |
* set active status. (The circular sweep uses steps of |
690 |
* approximately half the array size plus 1, to avoid bias |
691 |
* stemming from leftmost packing of the array in ForkJoinPool.) |
692 |
* |
693 |
* This method must be both fast and quiet -- usually avoiding |
694 |
* memory accesses that could disrupt cache sharing etc other than |
695 |
* those needed to check for and take tasks (or to activate if not |
696 |
* already active). This accounts for, among other things, |
697 |
* updating random seed in place without storing it until exit. |
698 |
* |
699 |
* @return a task, or null if none found |
700 |
*/ |
701 |
private ForkJoinTask<?> scan() { |
702 |
ForkJoinPool p = pool; |
703 |
ForkJoinWorkerThread[] ws; // worker array |
704 |
int n; // upper bound of #workers |
705 |
if ((ws = p.workers) != null && (n = ws.length) > 1) { |
706 |
boolean canSteal = active; // shadow active status |
707 |
int r = seed; // extract seed once |
708 |
int mask = n - 1; |
709 |
int j = -n; // loop counter |
710 |
int k = r; // worker index, random if j < 0 |
711 |
for (;;) { |
712 |
ForkJoinWorkerThread v = ws[k & mask]; |
713 |
r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift |
714 |
ForkJoinTask<?>[] q; ForkJoinTask<?> t; int b, a; |
715 |
if (v != null && (b = v.base) != v.sp && |
716 |
(q = v.queue) != null) { |
717 |
int i = (q.length - 1) & b; |
718 |
long u = (i << qShift) + qBase; // raw offset |
719 |
int pid = poolIndex; |
720 |
if ((t = q[i]) != null) { |
721 |
if (!canSteal && // inline p.tryIncrementActiveCount |
722 |
UNSAFE.compareAndSwapInt(p, poolRunStateOffset, |
723 |
a = p.runState, a + 1)) |
724 |
canSteal = active = true; |
725 |
if (canSteal && v.base == b++ && |
726 |
UNSAFE.compareAndSwapObject(q, u, t, null)) { |
727 |
v.base = b; |
728 |
v.stealHint = pid; |
729 |
UNSAFE.putOrderedObject(this, |
730 |
currentStealOffset, t); |
731 |
seed = r; |
732 |
++stealCount; |
733 |
return t; |
734 |
} |
735 |
} |
736 |
j = -n; |
737 |
k = r; // restart on contention |
738 |
} |
739 |
else if (++j <= 0) |
740 |
k = r; |
741 |
else if (j <= n) |
742 |
k += (n >>> 1) | 1; |
743 |
else |
744 |
break; |
745 |
} |
746 |
} |
747 |
return null; |
748 |
} |
749 |
|
750 |
// Run State management |
751 |
|
752 |
// status check methods used mainly by ForkJoinPool |
753 |
final boolean isRunning() { return runState == 0; } |
754 |
final boolean isTerminating() { return (runState & TERMINATING) != 0; } |
755 |
final boolean isTerminated() { return (runState & TERMINATED) != 0; } |
756 |
final boolean isSuspended() { return (runState & SUSPENDED) != 0; } |
757 |
final boolean isTrimmed() { return (runState & TRIMMED) != 0; } |
758 |
|
759 |
/** |
760 |
* Sets state to TERMINATING. Does NOT unpark or interrupt |
761 |
* to wake up if currently blocked. Callers must do so if desired. |
762 |
*/ |
763 |
final void shutdown() { |
764 |
for (;;) { |
765 |
int s = runState; |
766 |
if ((s & (TERMINATING|TERMINATED)) != 0) |
767 |
break; |
768 |
if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended |
769 |
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
770 |
(s & ~SUSPENDED) | |
771 |
(TRIMMED|TERMINATING))) |
772 |
break; |
773 |
} |
774 |
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
775 |
s | TERMINATING)) |
776 |
break; |
777 |
} |
778 |
} |
779 |
|
780 |
/** |
781 |
* Sets state to TERMINATED. Called only by onTermination(). |
782 |
*/ |
783 |
private void setTerminated() { |
784 |
int s; |
785 |
do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset, |
786 |
s = runState, |
787 |
s | (TERMINATING|TERMINATED))); |
788 |
} |
789 |
|
790 |
/** |
791 |
* If suspended, tries to set status to unsuspended. |
792 |
* Does NOT wake up if blocked. |
793 |
* |
794 |
* @return true if successful |
795 |
*/ |
796 |
final boolean tryUnsuspend() { |
797 |
int s; |
798 |
while (((s = runState) & SUSPENDED) != 0) { |
799 |
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
800 |
s & ~SUSPENDED)) |
801 |
return true; |
802 |
} |
803 |
return false; |
804 |
} |
805 |
|
806 |
/** |
807 |
* Sets suspended status and blocks as spare until resumed |
808 |
* or shutdown. |
809 |
*/ |
810 |
final void suspendAsSpare() { |
811 |
for (;;) { // set suspended unless terminating |
812 |
int s = runState; |
813 |
if ((s & TERMINATING) != 0) { // must kill |
814 |
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
815 |
s | (TRIMMED | TERMINATING))) |
816 |
return; |
817 |
} |
818 |
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
819 |
s | SUSPENDED)) |
820 |
break; |
821 |
} |
822 |
ForkJoinPool p = pool; |
823 |
p.pushSpare(this); |
824 |
while ((runState & SUSPENDED) != 0) { |
825 |
if (p.tryAccumulateStealCount(this)) { |
826 |
interrupted(); // clear/ignore interrupts |
827 |
if ((runState & SUSPENDED) == 0) |
828 |
break; |
829 |
LockSupport.park(this); |
830 |
} |
831 |
} |
832 |
} |
833 |
|
834 |
// Misc support methods for ForkJoinPool |
835 |
|
836 |
/** |
837 |
* Returns an estimate of the number of tasks in the queue. Also |
838 |
* used by ForkJoinTask. |
839 |
*/ |
840 |
final int getQueueSize() { |
841 |
int n; // external calls must read base first |
842 |
return (n = -base + sp) <= 0 ? 0 : n; |
843 |
} |
844 |
|
845 |
/** |
846 |
* Removes and cancels all tasks in queue. Can be called from any |
847 |
* thread. |
848 |
*/ |
849 |
final void cancelTasks() { |
850 |
ForkJoinTask<?> cj = currentJoin; // try to cancel ongoing tasks |
851 |
if (cj != null) { |
852 |
currentJoin = null; |
853 |
cj.cancelIgnoringExceptions(); |
854 |
try { |
855 |
this.interrupt(); // awaken wait |
856 |
} catch (SecurityException ignore) { |
857 |
} |
858 |
} |
859 |
ForkJoinTask<?> cs = currentSteal; |
860 |
if (cs != null) { |
861 |
currentSteal = null; |
862 |
cs.cancelIgnoringExceptions(); |
863 |
} |
864 |
while (base != sp) { |
865 |
ForkJoinTask<?> t = deqTask(); |
866 |
if (t != null) |
867 |
t.cancelIgnoringExceptions(); |
868 |
} |
869 |
} |
870 |
|
871 |
/** |
872 |
* Drains tasks to given collection c. |
873 |
* |
874 |
* @return the number of tasks drained |
875 |
*/ |
876 |
final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) { |
877 |
int n = 0; |
878 |
while (base != sp) { |
879 |
ForkJoinTask<?> t = deqTask(); |
880 |
if (t != null) { |
881 |
c.add(t); |
882 |
++n; |
883 |
} |
884 |
} |
885 |
return n; |
886 |
} |
887 |
|
888 |
// Support methods for ForkJoinTask |
889 |
|
890 |
/** |
891 |
* Gets and removes a local task. |
892 |
* |
893 |
* @return a task, if available |
894 |
*/ |
895 |
final ForkJoinTask<?> pollLocalTask() { |
896 |
ForkJoinPool p = pool; |
897 |
while (sp != base) { |
898 |
int a; // inline p.tryIncrementActiveCount |
899 |
if (active || |
900 |
(active = UNSAFE.compareAndSwapInt(p, poolRunStateOffset, |
901 |
a = p.runState, a + 1))) |
902 |
return locallyFifo ? locallyDeqTask() : popTask(); |
903 |
} |
904 |
return null; |
905 |
} |
906 |
|
907 |
/** |
908 |
* Gets and removes a local or stolen task. |
909 |
* |
910 |
* @return a task, if available |
911 |
*/ |
912 |
final ForkJoinTask<?> pollTask() { |
913 |
ForkJoinTask<?> t = pollLocalTask(); |
914 |
if (t == null) { |
915 |
t = scan(); |
916 |
// cannot retain/track/help steal |
917 |
UNSAFE.putOrderedObject(this, currentStealOffset, null); |
918 |
} |
919 |
return t; |
920 |
} |
921 |
|
922 |
/** |
923 |
* Possibly runs some tasks and/or blocks, until task is done. |
924 |
* |
925 |
* @param joinMe the task to join |
926 |
*/ |
927 |
final void joinTask(ForkJoinTask<?> joinMe) { |
928 |
// currentJoin only written by this thread; only need ordered store |
929 |
ForkJoinTask<?> prevJoin = currentJoin; |
930 |
UNSAFE.putOrderedObject(this, currentJoinOffset, joinMe); |
931 |
if (sp != base) |
932 |
localHelpJoinTask(joinMe); |
933 |
if (joinMe.status >= 0) |
934 |
pool.awaitJoin(joinMe, this); |
935 |
UNSAFE.putOrderedObject(this, currentJoinOffset, prevJoin); |
936 |
} |
937 |
|
938 |
/** |
939 |
* Run tasks in local queue until given task is done. |
940 |
* |
941 |
* @param joinMe the task to join |
942 |
*/ |
943 |
private void localHelpJoinTask(ForkJoinTask<?> joinMe) { |
944 |
int s; |
945 |
ForkJoinTask<?>[] q; |
946 |
while (joinMe.status >= 0 && (s = sp) != base && (q = queue) != null) { |
947 |
int i = (q.length - 1) & --s; |
948 |
long u = (i << qShift) + qBase; // raw offset |
949 |
ForkJoinTask<?> t = q[i]; |
950 |
if (t == null) // lost to a stealer |
951 |
break; |
952 |
if (UNSAFE.compareAndSwapObject(q, u, t, null)) { |
953 |
/* |
954 |
* This recheck (and similarly in helpJoinTask) |
955 |
* handles cases where joinMe is independently |
956 |
* cancelled or forced even though there is other work |
957 |
* available. Back out of the pop by putting t back |
958 |
* into slot before we commit by writing sp. |
959 |
*/ |
960 |
if (joinMe.status < 0) { |
961 |
UNSAFE.putObjectVolatile(q, u, t); |
962 |
break; |
963 |
} |
964 |
sp = s; |
965 |
// UNSAFE.putOrderedInt(this, spOffset, s); |
966 |
t.quietlyExec(); |
967 |
} |
968 |
} |
969 |
} |
970 |
|
971 |
/** |
972 |
* Unless terminating, tries to locate and help perform tasks for |
973 |
* a stealer of the given task, or in turn one of its stealers. |
974 |
* Traces currentSteal->currentJoin links looking for a thread |
975 |
* working on a descendant of the given task and with a non-empty |
976 |
* queue to steal back and execute tasks from. |
977 |
* |
978 |
* The implementation is very branchy to cope with potential |
979 |
* inconsistencies or loops encountering chains that are stale, |
980 |
* unknown, or of length greater than MAX_HELP_DEPTH links. All |
981 |
* of these cases are dealt with by just returning back to the |
982 |
* caller, who is expected to retry if other join mechanisms also |
983 |
* don't work out. |
984 |
* |
985 |
* @param joinMe the task to join |
986 |
*/ |
987 |
final void helpJoinTask(ForkJoinTask<?> joinMe) { |
988 |
ForkJoinWorkerThread[] ws; |
989 |
int n; |
990 |
if (joinMe.status < 0) // already done |
991 |
return; |
992 |
if ((runState & TERMINATING) != 0) { // cancel if shutting down |
993 |
joinMe.cancelIgnoringExceptions(); |
994 |
return; |
995 |
} |
996 |
if ((ws = pool.workers) == null || (n = ws.length) <= 1) |
997 |
return; // need at least 2 workers |
998 |
|
999 |
ForkJoinTask<?> task = joinMe; // base of chain |
1000 |
ForkJoinWorkerThread thread = this; // thread with stolen task |
1001 |
for (int d = 0; d < MAX_HELP_DEPTH; ++d) { // chain length |
1002 |
// Try to find v, the stealer of task, by first using hint |
1003 |
ForkJoinWorkerThread v = ws[thread.stealHint & (n - 1)]; |
1004 |
if (v == null || v.currentSteal != task) { |
1005 |
for (int j = 0; ; ++j) { // search array |
1006 |
if (j < n) { |
1007 |
ForkJoinTask<?> vs; |
1008 |
if ((v = ws[j]) != null && |
1009 |
(vs = v.currentSteal) != null) { |
1010 |
if (joinMe.status < 0 || task.status < 0) |
1011 |
return; // stale or done |
1012 |
if (vs == task) { |
1013 |
thread.stealHint = j; |
1014 |
break; // save hint for next time |
1015 |
} |
1016 |
} |
1017 |
} |
1018 |
else |
1019 |
return; // no stealer |
1020 |
} |
1021 |
} |
1022 |
for (;;) { // Try to help v, using specialized form of deqTask |
1023 |
if (joinMe.status < 0) |
1024 |
return; |
1025 |
int b = v.base; |
1026 |
ForkJoinTask<?>[] q = v.queue; |
1027 |
if (b == v.sp || q == null) |
1028 |
break; |
1029 |
int i = (q.length - 1) & b; |
1030 |
long u = (i << qShift) + qBase; |
1031 |
ForkJoinTask<?> t = q[i]; |
1032 |
int pid = poolIndex; |
1033 |
ForkJoinTask<?> ps = currentSteal; |
1034 |
if (task.status < 0) |
1035 |
return; // stale or done |
1036 |
if (t != null && v.base == b++ && |
1037 |
UNSAFE.compareAndSwapObject(q, u, t, null)) { |
1038 |
if (joinMe.status < 0) { |
1039 |
UNSAFE.putObjectVolatile(q, u, t); |
1040 |
return; // back out on cancel |
1041 |
} |
1042 |
v.base = b; |
1043 |
v.stealHint = pid; |
1044 |
UNSAFE.putOrderedObject(this, currentStealOffset, t); |
1045 |
t.quietlyExec(); |
1046 |
UNSAFE.putOrderedObject(this, currentStealOffset, ps); |
1047 |
} |
1048 |
} |
1049 |
// Try to descend to find v's stealer |
1050 |
ForkJoinTask<?> next = v.currentJoin; |
1051 |
if (task.status < 0 || next == null || next == task || |
1052 |
joinMe.status < 0) |
1053 |
return; |
1054 |
task = next; |
1055 |
thread = v; |
1056 |
} |
1057 |
} |
1058 |
|
1059 |
/** |
1060 |
* Implements ForJoinTask.getSurplusQueuedTaskCount(). |
1061 |
* Returns an estimate of the number of tasks, offset by a |
1062 |
* function of number of idle workers. |
1063 |
* |
1064 |
* This method provides a cheap heuristic guide for task |
1065 |
* partitioning when programmers, frameworks, tools, or languages |
1066 |
* have little or no idea about task granularity. In essence by |
1067 |
* offering this method, we ask users only about tradeoffs in |
1068 |
* overhead vs expected throughput and its variance, rather than |
1069 |
* how finely to partition tasks. |
1070 |
* |
1071 |
* In a steady state strict (tree-structured) computation, each |
1072 |
* thread makes available for stealing enough tasks for other |
1073 |
* threads to remain active. Inductively, if all threads play by |
1074 |
* the same rules, each thread should make available only a |
1075 |
* constant number of tasks. |
1076 |
* |
1077 |
* The minimum useful constant is just 1. But using a value of 1 |
1078 |
* would require immediate replenishment upon each steal to |
1079 |
* maintain enough tasks, which is infeasible. Further, |
1080 |
* partitionings/granularities of offered tasks should minimize |
1081 |
* steal rates, which in general means that threads nearer the top |
1082 |
* of computation tree should generate more than those nearer the |
1083 |
* bottom. In perfect steady state, each thread is at |
1084 |
* approximately the same level of computation tree. However, |
1085 |
* producing extra tasks amortizes the uncertainty of progress and |
1086 |
* diffusion assumptions. |
1087 |
* |
1088 |
* So, users will want to use values larger, but not much larger |
1089 |
* than 1 to both smooth over transient shortages and hedge |
1090 |
* against uneven progress; as traded off against the cost of |
1091 |
* extra task overhead. We leave the user to pick a threshold |
1092 |
* value to compare with the results of this call to guide |
1093 |
* decisions, but recommend values such as 3. |
1094 |
* |
1095 |
* When all threads are active, it is on average OK to estimate |
1096 |
* surplus strictly locally. In steady-state, if one thread is |
1097 |
* maintaining say 2 surplus tasks, then so are others. So we can |
1098 |
* just use estimated queue length (although note that (sp - base) |
1099 |
* can be an overestimate because of stealers lagging increments |
1100 |
* of base). However, this strategy alone leads to serious |
1101 |
* mis-estimates in some non-steady-state conditions (ramp-up, |
1102 |
* ramp-down, other stalls). We can detect many of these by |
1103 |
* further considering the number of "idle" threads, that are |
1104 |
* known to have zero queued tasks, so compensate by a factor of |
1105 |
* (#idle/#active) threads. |
1106 |
*/ |
1107 |
final int getEstimatedSurplusTaskCount() { |
1108 |
return sp - base - pool.idlePerActive(); |
1109 |
} |
1110 |
|
1111 |
/** |
1112 |
* Runs tasks until {@code pool.isQuiescent()}. |
1113 |
*/ |
1114 |
final void helpQuiescePool() { |
1115 |
ForkJoinTask<?> ps = currentSteal; // to restore below |
1116 |
for (;;) { |
1117 |
ForkJoinTask<?> t = pollLocalTask(); |
1118 |
if (t != null || (t = scan()) != null) |
1119 |
t.quietlyExec(); |
1120 |
else { |
1121 |
ForkJoinPool p = pool; |
1122 |
int a; // to inline CASes |
1123 |
if (active) { |
1124 |
if (!UNSAFE.compareAndSwapInt |
1125 |
(p, poolRunStateOffset, a = p.runState, a - 1)) |
1126 |
continue; // retry later |
1127 |
active = false; // inactivate |
1128 |
UNSAFE.putOrderedObject(this, currentStealOffset, ps); |
1129 |
} |
1130 |
if (p.isQuiescent()) { |
1131 |
active = true; // re-activate |
1132 |
do {} while (!UNSAFE.compareAndSwapInt |
1133 |
(p, poolRunStateOffset, a = p.runState, a+1)); |
1134 |
return; |
1135 |
} |
1136 |
} |
1137 |
} |
1138 |
} |
1139 |
|
1140 |
// Unsafe mechanics |
1141 |
|
1142 |
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1143 |
private static final long spOffset = |
1144 |
objectFieldOffset("sp", ForkJoinWorkerThread.class); |
1145 |
private static final long runStateOffset = |
1146 |
objectFieldOffset("runState", ForkJoinWorkerThread.class); |
1147 |
private static final long currentJoinOffset = |
1148 |
objectFieldOffset("currentJoin", ForkJoinWorkerThread.class); |
1149 |
private static final long currentStealOffset = |
1150 |
objectFieldOffset("currentSteal", ForkJoinWorkerThread.class); |
1151 |
private static final long qBase = |
1152 |
UNSAFE.arrayBaseOffset(ForkJoinTask[].class); |
1153 |
private static final long poolRunStateOffset = // to inline CAS |
1154 |
objectFieldOffset("runState", ForkJoinPool.class); |
1155 |
|
1156 |
private static final int qShift; |
1157 |
|
1158 |
static { |
1159 |
int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class); |
1160 |
if ((s & (s-1)) != 0) |
1161 |
throw new Error("data type scale not a power of two"); |
1162 |
qShift = 31 - Integer.numberOfLeadingZeros(s); |
1163 |
MAXIMUM_QUEUE_CAPACITY = 1 << (31 - qShift); |
1164 |
} |
1165 |
|
1166 |
private static long objectFieldOffset(String field, Class<?> klazz) { |
1167 |
try { |
1168 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1169 |
} catch (NoSuchFieldException e) { |
1170 |
// Convert Exception to corresponding Error |
1171 |
NoSuchFieldError error = new NoSuchFieldError(field); |
1172 |
error.initCause(e); |
1173 |
throw error; |
1174 |
} |
1175 |
} |
1176 |
|
1177 |
/** |
1178 |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1179 |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1180 |
* into a jdk. |
1181 |
* |
1182 |
* @return a sun.misc.Unsafe |
1183 |
*/ |
1184 |
private static sun.misc.Unsafe getUnsafe() { |
1185 |
try { |
1186 |
return sun.misc.Unsafe.getUnsafe(); |
1187 |
} catch (SecurityException se) { |
1188 |
try { |
1189 |
return java.security.AccessController.doPrivileged |
1190 |
(new java.security |
1191 |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1192 |
public sun.misc.Unsafe run() throws Exception { |
1193 |
java.lang.reflect.Field f = sun.misc |
1194 |
.Unsafe.class.getDeclaredField("theUnsafe"); |
1195 |
f.setAccessible(true); |
1196 |
return (sun.misc.Unsafe) f.get(null); |
1197 |
}}); |
1198 |
} catch (java.security.PrivilegedActionException e) { |
1199 |
throw new RuntimeException("Could not initialize intrinsics", |
1200 |
e.getCause()); |
1201 |
} |
1202 |
} |
1203 |
} |
1204 |
} |