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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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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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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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* records (in field stolen) the most recent task it stole |
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* from some other worker. Plus, it records (in field joining) the |
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* 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 maintain per-task bookkeeping. This |
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* requires a linear scan of workers array to locate stealers, |
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* which isolates cost to when it is needed, rather than adding to |
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* per-task overhead. (2) It is "shallow", ignoring nesting and |
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* potentially cyclic mutual steals. (3) It is intentionally |
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* racy: field joining is updated only while actively joining, |
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* which means that we could miss links in the chain during |
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* long-lived tasks, GC stalls etc. (4) We fall back to |
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* suspending the worker and if necessary replacing it with a |
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* spare (see ForkJoinPool.tryAwaitJoin). |
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* |
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* Efficient implementation of these algorithms currently relies on |
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dl |
1.1 |
* an uncomfortable amount of "Unsafe" mechanics. To maintain |
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* correct orderings, reads and writes of variable base require |
114 |
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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* scanWhileJoining.) Because they are protected by volatile base |
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* reads, reads of the queue array and its slots by other threads |
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* do not need volatile load semantics, but writes (in push) |
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* require store order and CASes (in pop and deq) require |
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* (volatile) CAS semantics. (Michael, Saraswat, and Vechev's |
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* algorithm has similar properties, but without support for |
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* nulling slots.) Since these combinations aren't supported |
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* using ordinary volatiles, the only way to accomplish these |
126 |
dl |
1.28 |
* efficiently is to use direct Unsafe calls. (Using external |
127 |
dl |
1.1 |
* AtomicIntegers and AtomicReferenceArrays for the indices and |
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* array is significantly slower because of memory locality and |
129 |
dl |
1.28 |
* indirection effects.) |
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jsr166 |
1.29 |
* |
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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 |
135 |
dl |
1.1 |
* contention effects across multiple queues (especially in the |
136 |
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* presence of GC cardmarking). Also, to improve thread-locality, |
137 |
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. |
142 |
dl |
1.1 |
*/ |
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/** |
145 |
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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/** |
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* The timeout value for suspending spares. Spare workers that |
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* remain unsignalled for more than this time may be trimmed |
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* (killed and removed from pool). Since our goal is to avoid |
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* long-term thread buildup, the exact value of timeout does not |
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* matter too much so long as it avoids most false-alarm timeouts |
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* under GC stalls or momentarily high system load. |
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*/ |
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private static final long SPARE_KEEPALIVE_NANOS = |
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5L * 1000L * 1000L * 1000L; // 5 secs |
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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). |
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dl |
1.1 |
*/ |
177 |
dl |
1.5 |
private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 28; |
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dl |
1.1 |
|
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/** |
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jsr166 |
1.16 |
* The pool this thread works in. Accessed directly by ForkJoinTask. |
181 |
dl |
1.1 |
*/ |
182 |
dl |
1.5 |
final ForkJoinPool pool; |
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dl |
1.1 |
|
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/** |
185 |
dl |
1.35 |
* The task most recently stolen from another worker |
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*/ |
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private volatile ForkJoinTask<?> stolen; |
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/** |
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* The task currently being joined, set only when actively |
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* trying to helpStealer. |
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*/ |
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private volatile ForkJoinTask<?> joining; |
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/** |
196 |
dl |
1.1 |
* The work-stealing queue array. Size must be a power of two. |
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dl |
1.31 |
* Initialized in onStart, to improve memory locality. |
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dl |
1.1 |
*/ |
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private ForkJoinTask<?>[] queue; |
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/** |
202 |
dl |
1.31 |
* Index (mod queue.length) of least valid queue slot, which is |
203 |
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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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/** |
208 |
dl |
1.1 |
* Index (mod queue.length) of next queue slot to push to or pop |
209 |
dl |
1.31 |
* 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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dl |
1.1 |
|
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/** |
217 |
dl |
1.31 |
* 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. |
221 |
dl |
1.1 |
*/ |
222 |
dl |
1.31 |
private volatile int runState; |
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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 |
228 |
dl |
1.1 |
|
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/** |
230 |
dl |
1.31 |
* Number of LockSupport.park calls to block this thread for |
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* suspension or event waits. Used for internal instrumention; |
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* currently not exported but included because volatile write upon |
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* park also provides a workaround for a JVM bug. |
234 |
dl |
1.1 |
*/ |
235 |
dl |
1.35 |
volatile int parkCount; |
236 |
dl |
1.1 |
|
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/** |
238 |
dl |
1.31 |
* Number of steals, transferred and reset in pool callbacks pool |
239 |
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* when idle Accessed directly by pool. |
240 |
dl |
1.1 |
*/ |
241 |
dl |
1.31 |
int stealCount; |
242 |
dl |
1.1 |
|
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/** |
244 |
dl |
1.5 |
* Seed for random number generator for choosing steal victims. |
245 |
dl |
1.31 |
* Uses Marsaglia xorshift. Must be initialized as nonzero. |
246 |
dl |
1.1 |
*/ |
247 |
dl |
1.5 |
private int seed; |
248 |
dl |
1.1 |
|
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/** |
250 |
dl |
1.31 |
* 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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255 |
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/** |
256 |
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* True if use local fifo, not default lifo, for local polling. |
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* Shadows value from ForkJoinPool, which resets it if changed |
258 |
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* pool-wide. |
259 |
dl |
1.1 |
*/ |
260 |
dl |
1.35 |
private final boolean locallyFifo; |
261 |
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262 |
dl |
1.1 |
/** |
263 |
dl |
1.5 |
* Index of this worker in pool array. Set once by pool before |
264 |
dl |
1.31 |
* running, and accessed directly by pool to locate this worker in |
265 |
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* its workers array. |
266 |
dl |
1.1 |
*/ |
267 |
dl |
1.5 |
int poolIndex; |
268 |
dl |
1.1 |
|
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/** |
270 |
dl |
1.31 |
* The last pool event waited for. Accessed only by pool in |
271 |
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* callback methods invoked within this thread. |
272 |
dl |
1.1 |
*/ |
273 |
dl |
1.31 |
int lastEventCount; |
274 |
dl |
1.1 |
|
275 |
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/** |
276 |
dl |
1.31 |
* Encoded index and event count of next event waiter. Used only |
277 |
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* by ForkJoinPool for managing event waiters. |
278 |
dl |
1.7 |
*/ |
279 |
dl |
1.31 |
volatile long nextWaiter; |
280 |
dl |
1.7 |
|
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/** |
282 |
dl |
1.1 |
* Creates a ForkJoinWorkerThread operating in the given pool. |
283 |
jsr166 |
1.11 |
* |
284 |
dl |
1.1 |
* @param pool the pool this thread works in |
285 |
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* @throws NullPointerException if pool is null |
286 |
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*/ |
287 |
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protected ForkJoinWorkerThread(ForkJoinPool pool) { |
288 |
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this.pool = pool; |
289 |
dl |
1.35 |
this.locallyFifo = pool.locallyFifo; |
290 |
dl |
1.31 |
// To avoid exposing construction details to subclasses, |
291 |
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// remaining initialization is in start() and onStart() |
292 |
dl |
1.1 |
} |
293 |
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|
294 |
dl |
1.31 |
/** |
295 |
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* Performs additional initialization and starts this thread |
296 |
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*/ |
297 |
dl |
1.35 |
final void start(int poolIndex, UncaughtExceptionHandler ueh) { |
298 |
dl |
1.31 |
this.poolIndex = poolIndex; |
299 |
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if (ueh != null) |
300 |
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setUncaughtExceptionHandler(ueh); |
301 |
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setDaemon(true); |
302 |
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start(); |
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} |
304 |
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|
305 |
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// Public/protected methods |
306 |
dl |
1.2 |
|
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/** |
308 |
jsr166 |
1.11 |
* Returns the pool hosting this thread. |
309 |
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* |
310 |
dl |
1.2 |
* @return the pool |
311 |
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*/ |
312 |
dl |
1.4 |
public ForkJoinPool getPool() { |
313 |
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return pool; |
314 |
dl |
1.2 |
} |
315 |
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|
316 |
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/** |
317 |
dl |
1.4 |
* Returns the index number of this thread in its pool. The |
318 |
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* returned value ranges from zero to the maximum number of |
319 |
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* threads (minus one) that have ever been created in the pool. |
320 |
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* This method may be useful for applications that track status or |
321 |
dl |
1.5 |
* collect results per-worker rather than per-task. |
322 |
jsr166 |
1.11 |
* |
323 |
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* @return the index number |
324 |
dl |
1.2 |
*/ |
325 |
dl |
1.4 |
public int getPoolIndex() { |
326 |
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return poolIndex; |
327 |
dl |
1.2 |
} |
328 |
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|
329 |
dl |
1.7 |
/** |
330 |
dl |
1.31 |
* Initializes internal state after construction but before |
331 |
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* processing any tasks. If you override this method, you must |
332 |
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* invoke super.onStart() at the beginning of the method. |
333 |
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* Initialization requires care: Most fields must have legal |
334 |
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* default values, to ensure that attempted accesses from other |
335 |
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* threads work correctly even before this thread starts |
336 |
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* processing tasks. |
337 |
dl |
1.7 |
*/ |
338 |
dl |
1.31 |
protected void onStart() { |
339 |
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int rs = seedGenerator.nextInt(); |
340 |
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seed = rs == 0? 1 : rs; // seed must be nonzero |
341 |
dl |
1.5 |
|
342 |
dl |
1.35 |
// Allocate name string and arrays in this thread |
343 |
dl |
1.31 |
String pid = Integer.toString(pool.getPoolNumber()); |
344 |
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String wid = Integer.toString(poolIndex); |
345 |
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setName("ForkJoinPool-" + pid + "-worker-" + wid); |
346 |
dl |
1.5 |
|
347 |
dl |
1.31 |
queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY]; |
348 |
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} |
349 |
dl |
1.5 |
|
350 |
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/** |
351 |
dl |
1.31 |
* Performs cleanup associated with termination of this worker |
352 |
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* thread. If you override this method, you must invoke |
353 |
|
|
* {@code super.onTermination} at the end of the overridden method. |
354 |
jsr166 |
1.21 |
* |
355 |
dl |
1.31 |
* @param exception the exception causing this thread to abort due |
356 |
|
|
* to an unrecoverable error, or {@code null} if completed normally |
357 |
dl |
1.5 |
*/ |
358 |
dl |
1.31 |
protected void onTermination(Throwable exception) { |
359 |
|
|
try { |
360 |
dl |
1.35 |
stolen = null; |
361 |
|
|
joining = null; |
362 |
dl |
1.31 |
cancelTasks(); |
363 |
|
|
setTerminated(); |
364 |
|
|
pool.workerTerminated(this); |
365 |
|
|
} catch (Throwable ex) { // Shouldn't ever happen |
366 |
|
|
if (exception == null) // but if so, at least rethrown |
367 |
|
|
exception = ex; |
368 |
|
|
} finally { |
369 |
|
|
if (exception != null) |
370 |
|
|
UNSAFE.throwException(exception); |
371 |
dl |
1.5 |
} |
372 |
|
|
} |
373 |
|
|
|
374 |
|
|
/** |
375 |
|
|
* This method is required to be public, but should never be |
376 |
|
|
* called explicitly. It performs the main run loop to execute |
377 |
|
|
* ForkJoinTasks. |
378 |
dl |
1.1 |
*/ |
379 |
dl |
1.5 |
public void run() { |
380 |
|
|
Throwable exception = null; |
381 |
|
|
try { |
382 |
|
|
onStart(); |
383 |
|
|
mainLoop(); |
384 |
|
|
} catch (Throwable ex) { |
385 |
|
|
exception = ex; |
386 |
|
|
} finally { |
387 |
|
|
onTermination(exception); |
388 |
|
|
} |
389 |
dl |
1.1 |
} |
390 |
|
|
|
391 |
dl |
1.31 |
// helpers for run() |
392 |
|
|
|
393 |
dl |
1.1 |
/** |
394 |
dl |
1.31 |
* Find and execute tasks and check status while running |
395 |
dl |
1.1 |
*/ |
396 |
dl |
1.5 |
private void mainLoop() { |
397 |
dl |
1.34 |
boolean ran = false; // true if ran task in last loop iter |
398 |
|
|
boolean prevRan = false; // true if ran on last or previous step |
399 |
dl |
1.31 |
ForkJoinPool p = pool; |
400 |
|
|
for (;;) { |
401 |
dl |
1.34 |
p.preStep(this, prevRan); |
402 |
dl |
1.31 |
if (runState != 0) |
403 |
|
|
return; |
404 |
|
|
ForkJoinTask<?> t; // try to get and run stolen or submitted task |
405 |
dl |
1.34 |
if ((t = scan()) != null || (t = pollSubmission()) != null) { |
406 |
dl |
1.31 |
t.tryExec(); |
407 |
|
|
if (base != sp) |
408 |
|
|
runLocalTasks(); |
409 |
dl |
1.35 |
stolen = null; |
410 |
dl |
1.34 |
prevRan = ran = true; |
411 |
|
|
} |
412 |
|
|
else { |
413 |
|
|
prevRan = ran; |
414 |
|
|
ran = false; |
415 |
dl |
1.31 |
} |
416 |
dl |
1.5 |
} |
417 |
dl |
1.1 |
} |
418 |
|
|
|
419 |
dl |
1.5 |
/** |
420 |
dl |
1.31 |
* Runs local tasks until queue is empty or shut down. Call only |
421 |
|
|
* while active. |
422 |
dl |
1.5 |
*/ |
423 |
dl |
1.31 |
private void runLocalTasks() { |
424 |
|
|
while (runState == 0) { |
425 |
|
|
ForkJoinTask<?> t = locallyFifo? locallyDeqTask() : popTask(); |
426 |
|
|
if (t != null) |
427 |
|
|
t.tryExec(); |
428 |
|
|
else if (base == sp) |
429 |
|
|
break; |
430 |
|
|
} |
431 |
dl |
1.5 |
} |
432 |
dl |
1.1 |
|
433 |
|
|
/** |
434 |
dl |
1.31 |
* If a submission exists, try to activate and take it |
435 |
dl |
1.5 |
* |
436 |
dl |
1.31 |
* @return a task, if available |
437 |
dl |
1.1 |
*/ |
438 |
dl |
1.31 |
private ForkJoinTask<?> pollSubmission() { |
439 |
|
|
ForkJoinPool p = pool; |
440 |
|
|
while (p.hasQueuedSubmissions()) { |
441 |
|
|
if (active || (active = p.tryIncrementActiveCount())) { |
442 |
|
|
ForkJoinTask<?> t = p.pollSubmission(); |
443 |
|
|
return t != null ? t : scan(); // if missed, rescan |
444 |
dl |
1.5 |
} |
445 |
|
|
} |
446 |
dl |
1.31 |
return null; |
447 |
dl |
1.1 |
} |
448 |
|
|
|
449 |
dl |
1.31 |
/* |
450 |
|
|
* Intrinsics-based atomic writes for queue slots. These are |
451 |
|
|
* basically the same as methods in AtomicObjectArray, but |
452 |
|
|
* specialized for (1) ForkJoinTask elements (2) requirement that |
453 |
|
|
* nullness and bounds checks have already been performed by |
454 |
|
|
* callers and (3) effective offsets are known not to overflow |
455 |
|
|
* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't |
456 |
|
|
* need corresponding version for reads: plain array reads are OK |
457 |
|
|
* because they protected by other volatile reads and are |
458 |
|
|
* confirmed by CASes. |
459 |
|
|
* |
460 |
|
|
* Most uses don't actually call these methods, but instead contain |
461 |
|
|
* inlined forms that enable more predictable optimization. We |
462 |
|
|
* don't define the version of write used in pushTask at all, but |
463 |
|
|
* instead inline there a store-fenced array slot write. |
464 |
dl |
1.1 |
*/ |
465 |
|
|
|
466 |
|
|
/** |
467 |
dl |
1.31 |
* CASes slot i of array q from t to null. Caller must ensure q is |
468 |
|
|
* non-null and index is in range. |
469 |
dl |
1.1 |
*/ |
470 |
dl |
1.31 |
private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i, |
471 |
|
|
ForkJoinTask<?> t) { |
472 |
|
|
return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null); |
473 |
dl |
1.1 |
} |
474 |
|
|
|
475 |
dl |
1.5 |
/** |
476 |
dl |
1.31 |
* Performs a volatile write of the given task at given slot of |
477 |
|
|
* array q. Caller must ensure q is non-null and index is in |
478 |
|
|
* range. This method is used only during resets and backouts. |
479 |
dl |
1.5 |
*/ |
480 |
dl |
1.31 |
private static final void writeSlot(ForkJoinTask<?>[] q, int i, |
481 |
|
|
ForkJoinTask<?> t) { |
482 |
|
|
UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t); |
483 |
dl |
1.5 |
} |
484 |
|
|
|
485 |
dl |
1.31 |
// queue methods |
486 |
dl |
1.1 |
|
487 |
|
|
/** |
488 |
dl |
1.31 |
* Pushes a task. Call only from this thread. |
489 |
jsr166 |
1.11 |
* |
490 |
jsr166 |
1.10 |
* @param t the task. Caller must ensure non-null. |
491 |
dl |
1.1 |
*/ |
492 |
|
|
final void pushTask(ForkJoinTask<?> t) { |
493 |
|
|
ForkJoinTask<?>[] q = queue; |
494 |
dl |
1.31 |
int mask = q.length - 1; // implicit assert q != null |
495 |
dl |
1.34 |
int s = sp++; // ok to increment sp before slot write |
496 |
|
|
UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t); |
497 |
|
|
if ((s -= base) == 0) |
498 |
|
|
pool.signalWork(); // was empty |
499 |
|
|
else if (s == mask) |
500 |
|
|
growQueue(); // is full |
501 |
dl |
1.1 |
} |
502 |
|
|
|
503 |
|
|
/** |
504 |
|
|
* Tries to take a task from the base of the queue, failing if |
505 |
dl |
1.31 |
* empty or contended. Note: Specializations of this code appear |
506 |
dl |
1.35 |
* in locallyDeqTask and elsewhere. |
507 |
jsr166 |
1.11 |
* |
508 |
|
|
* @return a task, or null if none or contended |
509 |
dl |
1.1 |
*/ |
510 |
dl |
1.7 |
final ForkJoinTask<?> deqTask() { |
511 |
dl |
1.5 |
ForkJoinTask<?> t; |
512 |
dl |
1.1 |
ForkJoinTask<?>[] q; |
513 |
dl |
1.31 |
int b, i; |
514 |
|
|
if ((b = base) != sp && |
515 |
dl |
1.1 |
(q = queue) != null && // must read q after b |
516 |
dl |
1.35 |
(t = q[i = (q.length - 1) & b]) != null && base == b && |
517 |
dl |
1.31 |
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) { |
518 |
dl |
1.1 |
base = b + 1; |
519 |
|
|
return t; |
520 |
|
|
} |
521 |
|
|
return null; |
522 |
|
|
} |
523 |
|
|
|
524 |
|
|
/** |
525 |
dl |
1.31 |
* Tries to take a task from the base of own queue. Assumes active |
526 |
|
|
* status. Called only by current thread. |
527 |
dl |
1.23 |
* |
528 |
|
|
* @return a task, or null if none |
529 |
|
|
*/ |
530 |
|
|
final ForkJoinTask<?> locallyDeqTask() { |
531 |
dl |
1.31 |
ForkJoinTask<?>[] q = queue; |
532 |
|
|
if (q != null) { |
533 |
|
|
ForkJoinTask<?> t; |
534 |
|
|
int b, i; |
535 |
|
|
while (sp != (b = base)) { |
536 |
dl |
1.35 |
if ((t = q[i = (q.length - 1) & b]) != null && base == b && |
537 |
dl |
1.31 |
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, |
538 |
|
|
t, null)) { |
539 |
dl |
1.23 |
base = b + 1; |
540 |
|
|
return t; |
541 |
|
|
} |
542 |
|
|
} |
543 |
|
|
} |
544 |
|
|
return null; |
545 |
|
|
} |
546 |
|
|
|
547 |
|
|
/** |
548 |
dl |
1.31 |
* Returns a popped task, or null if empty. Assumes active status. |
549 |
|
|
* Called only by current thread. (Note: a specialization of this |
550 |
dl |
1.33 |
* code appears in popWhileJoining.) |
551 |
dl |
1.1 |
*/ |
552 |
|
|
final ForkJoinTask<?> popTask() { |
553 |
dl |
1.31 |
int s; |
554 |
dl |
1.33 |
ForkJoinTask<?>[] q; |
555 |
|
|
if (base != (s = sp) && (q = queue) != null) { |
556 |
dl |
1.31 |
int i = (q.length - 1) & --s; |
557 |
|
|
ForkJoinTask<?> t = q[i]; |
558 |
|
|
if (t != null && UNSAFE.compareAndSwapObject |
559 |
|
|
(q, (i << qShift) + qBase, t, null)) { |
560 |
|
|
sp = s; |
561 |
dl |
1.5 |
return t; |
562 |
|
|
} |
563 |
dl |
1.1 |
} |
564 |
|
|
return null; |
565 |
|
|
} |
566 |
|
|
|
567 |
|
|
/** |
568 |
dl |
1.33 |
* Specialized version of popTask to pop only if topmost element |
569 |
|
|
* is the given task. Called only by current thread while |
570 |
|
|
* active. |
571 |
jsr166 |
1.11 |
* |
572 |
|
|
* @param t the task. Caller must ensure non-null. |
573 |
dl |
1.1 |
*/ |
574 |
|
|
final boolean unpushTask(ForkJoinTask<?> t) { |
575 |
dl |
1.31 |
int s; |
576 |
dl |
1.33 |
ForkJoinTask<?>[] q; |
577 |
|
|
if (base != (s = sp) && (q = queue) != null && |
578 |
|
|
UNSAFE.compareAndSwapObject |
579 |
|
|
(q, (((q.length - 1) & --s) << qShift) + qBase, t, null)) { |
580 |
dl |
1.31 |
sp = s; |
581 |
dl |
1.1 |
return true; |
582 |
|
|
} |
583 |
|
|
return false; |
584 |
|
|
} |
585 |
|
|
|
586 |
|
|
/** |
587 |
dl |
1.23 |
* Returns next task or null if empty or contended |
588 |
dl |
1.1 |
*/ |
589 |
dl |
1.2 |
final ForkJoinTask<?> peekTask() { |
590 |
dl |
1.1 |
ForkJoinTask<?>[] q = queue; |
591 |
dl |
1.7 |
if (q == null) |
592 |
|
|
return null; |
593 |
|
|
int mask = q.length - 1; |
594 |
jsr166 |
1.15 |
int i = locallyFifo ? base : (sp - 1); |
595 |
dl |
1.7 |
return q[i & mask]; |
596 |
dl |
1.1 |
} |
597 |
|
|
|
598 |
|
|
/** |
599 |
|
|
* Doubles queue array size. Transfers elements by emulating |
600 |
|
|
* steals (deqs) from old array and placing, oldest first, into |
601 |
|
|
* new array. |
602 |
|
|
*/ |
603 |
|
|
private void growQueue() { |
604 |
|
|
ForkJoinTask<?>[] oldQ = queue; |
605 |
|
|
int oldSize = oldQ.length; |
606 |
|
|
int newSize = oldSize << 1; |
607 |
|
|
if (newSize > MAXIMUM_QUEUE_CAPACITY) |
608 |
|
|
throw new RejectedExecutionException("Queue capacity exceeded"); |
609 |
|
|
ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize]; |
610 |
|
|
|
611 |
|
|
int b = base; |
612 |
|
|
int bf = b + oldSize; |
613 |
|
|
int oldMask = oldSize - 1; |
614 |
|
|
int newMask = newSize - 1; |
615 |
|
|
do { |
616 |
|
|
int oldIndex = b & oldMask; |
617 |
|
|
ForkJoinTask<?> t = oldQ[oldIndex]; |
618 |
|
|
if (t != null && !casSlotNull(oldQ, oldIndex, t)) |
619 |
|
|
t = null; |
620 |
dl |
1.31 |
writeSlot(newQ, b & newMask, t); |
621 |
dl |
1.1 |
} while (++b != bf); |
622 |
dl |
1.5 |
pool.signalWork(); |
623 |
dl |
1.1 |
} |
624 |
|
|
|
625 |
|
|
/** |
626 |
dl |
1.31 |
* Computes next value for random victim probe in scan(). Scans |
627 |
|
|
* don't require a very high quality generator, but also not a |
628 |
|
|
* crummy one. Marsaglia xor-shift is cheap and works well enough. |
629 |
|
|
* Note: This is manually inlined in scan() |
630 |
|
|
*/ |
631 |
|
|
private static final int xorShift(int r) { |
632 |
|
|
r ^= r << 13; |
633 |
|
|
r ^= r >>> 17; |
634 |
|
|
return r ^ (r << 5); |
635 |
|
|
} |
636 |
|
|
|
637 |
|
|
/** |
638 |
dl |
1.5 |
* Tries to steal a task from another worker. Starts at a random |
639 |
|
|
* index of workers array, and probes workers until finding one |
640 |
|
|
* with non-empty queue or finding that all are empty. It |
641 |
|
|
* randomly selects the first n probes. If these are empty, it |
642 |
dl |
1.31 |
* resorts to a circular sweep, which is necessary to accurately |
643 |
|
|
* set active status. (The circular sweep uses steps of |
644 |
|
|
* approximately half the array size plus 1, to avoid bias |
645 |
|
|
* stemming from leftmost packing of the array in ForkJoinPool.) |
646 |
dl |
1.1 |
* |
647 |
|
|
* This method must be both fast and quiet -- usually avoiding |
648 |
|
|
* memory accesses that could disrupt cache sharing etc other than |
649 |
dl |
1.31 |
* those needed to check for and take tasks (or to activate if not |
650 |
|
|
* already active). This accounts for, among other things, |
651 |
|
|
* updating random seed in place without storing it until exit. |
652 |
dl |
1.1 |
* |
653 |
|
|
* @return a task, or null if none found |
654 |
|
|
*/ |
655 |
dl |
1.5 |
private ForkJoinTask<?> scan() { |
656 |
dl |
1.31 |
ForkJoinPool p = pool; |
657 |
dl |
1.33 |
ForkJoinWorkerThread[] ws; // worker array |
658 |
|
|
int n; // upper bound of #workers |
659 |
|
|
if ((ws = p.workers) != null && (n = ws.length) > 1) { |
660 |
|
|
boolean canSteal = active; // shadow active status |
661 |
|
|
int r = seed; // extract seed once |
662 |
|
|
int mask = n - 1; |
663 |
|
|
int j = -n; // loop counter |
664 |
|
|
int k = r; // worker index, random if j < 0 |
665 |
|
|
for (;;) { |
666 |
|
|
ForkJoinWorkerThread v = ws[k & mask]; |
667 |
|
|
r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift |
668 |
|
|
if (v != null && v.base != v.sp) { |
669 |
dl |
1.35 |
if (canSteal || // ensure active status |
670 |
|
|
(canSteal = active = p.tryIncrementActiveCount())) { |
671 |
|
|
int b = v.base; // inline specialized deqTask |
672 |
|
|
ForkJoinTask<?>[] q; |
673 |
|
|
if (b != v.sp && (q = v.queue) != null) { |
674 |
|
|
ForkJoinTask<?> t; |
675 |
|
|
int i = (q.length - 1) & b; |
676 |
|
|
long u = (i << qShift) + qBase; // raw offset |
677 |
|
|
if ((t = q[i]) != null && v.base == b && |
678 |
|
|
UNSAFE.compareAndSwapObject(q, u, t, null)) { |
679 |
|
|
stolen = t; |
680 |
|
|
v.base = b + 1; |
681 |
|
|
seed = r; |
682 |
|
|
++stealCount; |
683 |
|
|
return t; |
684 |
|
|
} |
685 |
|
|
} |
686 |
dl |
1.1 |
} |
687 |
dl |
1.33 |
j = -n; |
688 |
|
|
k = r; // restart on contention |
689 |
dl |
1.1 |
} |
690 |
dl |
1.33 |
else if (++j <= 0) |
691 |
|
|
k = r; |
692 |
|
|
else if (j <= n) |
693 |
|
|
k += (n >>> 1) | 1; |
694 |
|
|
else |
695 |
|
|
break; |
696 |
dl |
1.1 |
} |
697 |
dl |
1.31 |
} |
698 |
|
|
return null; |
699 |
dl |
1.1 |
} |
700 |
|
|
|
701 |
dl |
1.31 |
// Run State management |
702 |
|
|
|
703 |
|
|
// status check methods used mainly by ForkJoinPool |
704 |
|
|
final boolean isTerminating() { return (runState & TERMINATING) != 0; } |
705 |
|
|
final boolean isTerminated() { return (runState & TERMINATED) != 0; } |
706 |
|
|
final boolean isSuspended() { return (runState & SUSPENDED) != 0; } |
707 |
|
|
final boolean isTrimmed() { return (runState & TRIMMED) != 0; } |
708 |
|
|
|
709 |
dl |
1.1 |
/** |
710 |
dl |
1.31 |
* Sets state to TERMINATING, also resuming if suspended. |
711 |
|
|
*/ |
712 |
|
|
final void shutdown() { |
713 |
|
|
for (;;) { |
714 |
|
|
int s = runState; |
715 |
|
|
if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended |
716 |
|
|
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
717 |
|
|
(s & ~SUSPENDED) | |
718 |
|
|
(TRIMMED|TERMINATING))) { |
719 |
|
|
LockSupport.unpark(this); |
720 |
|
|
break; |
721 |
|
|
} |
722 |
|
|
} |
723 |
|
|
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
724 |
|
|
s | TERMINATING)) |
725 |
|
|
break; |
726 |
|
|
} |
727 |
|
|
} |
728 |
|
|
|
729 |
|
|
/** |
730 |
|
|
* Sets state to TERMINATED. Called only by this thread. |
731 |
|
|
*/ |
732 |
|
|
private void setTerminated() { |
733 |
|
|
int s; |
734 |
|
|
do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset, |
735 |
|
|
s = runState, |
736 |
|
|
s | (TERMINATING|TERMINATED))); |
737 |
|
|
} |
738 |
|
|
|
739 |
|
|
/** |
740 |
dl |
1.35 |
* Instrumented version of park used by ForkJoinPool.awaitEvent |
741 |
dl |
1.5 |
*/ |
742 |
dl |
1.31 |
final void doPark() { |
743 |
|
|
++parkCount; |
744 |
|
|
LockSupport.park(this); |
745 |
dl |
1.1 |
} |
746 |
|
|
|
747 |
|
|
/** |
748 |
dl |
1.31 |
* If suspended, tries to set status to unsuspended. |
749 |
|
|
* Caller must unpark to actually resume |
750 |
jsr166 |
1.11 |
* |
751 |
dl |
1.31 |
* @return true if successful |
752 |
dl |
1.7 |
*/ |
753 |
dl |
1.31 |
final boolean tryUnsuspend() { |
754 |
dl |
1.35 |
int s = runState; |
755 |
|
|
if ((s & SUSPENDED) != 0) |
756 |
|
|
return UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
757 |
|
|
s & ~SUSPENDED); |
758 |
|
|
return false; |
759 |
dl |
1.7 |
} |
760 |
|
|
|
761 |
|
|
/** |
762 |
dl |
1.31 |
* Sets suspended status and blocks as spare until resumed, |
763 |
|
|
* shutdown, or timed out. |
764 |
jsr166 |
1.11 |
* |
765 |
dl |
1.31 |
* @return false if trimmed |
766 |
dl |
1.1 |
*/ |
767 |
dl |
1.31 |
final boolean suspendAsSpare() { |
768 |
|
|
for (;;) { // set suspended unless terminating |
769 |
|
|
int s = runState; |
770 |
|
|
if ((s & TERMINATING) != 0) { // must kill |
771 |
|
|
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
772 |
|
|
s | (TRIMMED | TERMINATING))) |
773 |
|
|
return false; |
774 |
|
|
} |
775 |
|
|
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
776 |
|
|
s | SUSPENDED)) |
777 |
|
|
break; |
778 |
|
|
} |
779 |
dl |
1.35 |
boolean timed; |
780 |
|
|
long nanos; |
781 |
|
|
long startTime; |
782 |
|
|
if (poolIndex < pool.parallelism) { |
783 |
|
|
timed = false; |
784 |
|
|
nanos = 0L; |
785 |
|
|
startTime = 0L; |
786 |
|
|
} |
787 |
|
|
else { |
788 |
|
|
timed = true; |
789 |
|
|
nanos = SPARE_KEEPALIVE_NANOS; |
790 |
|
|
startTime = System.nanoTime(); |
791 |
|
|
} |
792 |
|
|
pool.accumulateStealCount(this); |
793 |
dl |
1.31 |
lastEventCount = 0; // reset upon resume |
794 |
|
|
interrupted(); // clear/ignore interrupts |
795 |
|
|
while ((runState & SUSPENDED) != 0) { |
796 |
|
|
++parkCount; |
797 |
dl |
1.35 |
if (!timed) |
798 |
|
|
LockSupport.park(this); |
799 |
|
|
else if ((nanos -= (System.nanoTime() - startTime)) > 0) |
800 |
dl |
1.31 |
LockSupport.parkNanos(this, nanos); |
801 |
|
|
else { // try to trim on timeout |
802 |
|
|
int s = runState; |
803 |
|
|
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, |
804 |
|
|
(s & ~SUSPENDED) | |
805 |
|
|
(TRIMMED|TERMINATING))) |
806 |
|
|
return false; |
807 |
|
|
} |
808 |
dl |
1.1 |
} |
809 |
dl |
1.31 |
return true; |
810 |
|
|
} |
811 |
|
|
|
812 |
|
|
// Misc support methods for ForkJoinPool |
813 |
|
|
|
814 |
|
|
/** |
815 |
|
|
* Returns an estimate of the number of tasks in the queue. Also |
816 |
|
|
* used by ForkJoinTask. |
817 |
|
|
*/ |
818 |
|
|
final int getQueueSize() { |
819 |
|
|
return -base + sp; |
820 |
dl |
1.1 |
} |
821 |
dl |
1.5 |
|
822 |
dl |
1.31 |
/** |
823 |
dl |
1.5 |
* Removes and cancels all tasks in queue. Can be called from any |
824 |
|
|
* thread. |
825 |
dl |
1.1 |
*/ |
826 |
dl |
1.5 |
final void cancelTasks() { |
827 |
dl |
1.31 |
while (base != sp) { |
828 |
|
|
ForkJoinTask<?> t = deqTask(); |
829 |
|
|
if (t != null) |
830 |
|
|
t.cancelIgnoringExceptions(); |
831 |
|
|
} |
832 |
dl |
1.1 |
} |
833 |
|
|
|
834 |
|
|
/** |
835 |
jsr166 |
1.11 |
* Drains tasks to given collection c. |
836 |
|
|
* |
837 |
dl |
1.7 |
* @return the number of tasks drained |
838 |
|
|
*/ |
839 |
dl |
1.22 |
final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) { |
840 |
dl |
1.7 |
int n = 0; |
841 |
dl |
1.31 |
while (base != sp) { |
842 |
|
|
ForkJoinTask<?> t = deqTask(); |
843 |
|
|
if (t != null) { |
844 |
|
|
c.add(t); |
845 |
|
|
++n; |
846 |
|
|
} |
847 |
dl |
1.7 |
} |
848 |
|
|
return n; |
849 |
|
|
} |
850 |
|
|
|
851 |
dl |
1.31 |
// Support methods for ForkJoinTask |
852 |
|
|
|
853 |
dl |
1.7 |
/** |
854 |
dl |
1.35 |
* Possibly runs some tasks and/or blocks, until task is done. |
855 |
|
|
* |
856 |
|
|
* @param joinMe the task to join |
857 |
|
|
*/ |
858 |
|
|
final void joinTask(ForkJoinTask<?> joinMe) { |
859 |
|
|
ForkJoinTask<?> prevJoining = joining; |
860 |
|
|
joining = joinMe; |
861 |
|
|
while (joinMe.status >= 0) { |
862 |
|
|
int s = sp; |
863 |
|
|
if (s == base) { |
864 |
|
|
nonlocalJoinTask(joinMe); |
865 |
|
|
break; |
866 |
|
|
} |
867 |
|
|
// process local task |
868 |
|
|
ForkJoinTask<?> t; |
869 |
|
|
ForkJoinTask<?>[] q = queue; |
870 |
|
|
int i = (q.length - 1) & --s; |
871 |
|
|
long u = (i << qShift) + qBase; // raw offset |
872 |
|
|
if ((t = q[i]) != null && |
873 |
|
|
UNSAFE.compareAndSwapObject(q, u, t, null)) { |
874 |
|
|
/* |
875 |
|
|
* This recheck (and similarly in nonlocalJoinTask) |
876 |
|
|
* handles cases where joinMe is independently |
877 |
|
|
* cancelled or forced even though there is other work |
878 |
|
|
* available. Back out of the pop by putting t back |
879 |
|
|
* into slot before we commit by setting sp. |
880 |
|
|
*/ |
881 |
|
|
if (joinMe.status < 0) { |
882 |
|
|
UNSAFE.putObjectVolatile(q, u, t); |
883 |
|
|
break; |
884 |
|
|
} |
885 |
|
|
sp = s; |
886 |
|
|
t.tryExec(); |
887 |
|
|
} |
888 |
|
|
} |
889 |
|
|
joining = prevJoining; |
890 |
|
|
} |
891 |
|
|
|
892 |
|
|
/** |
893 |
|
|
* Tries to locate and help perform tasks for a stealer of the |
894 |
|
|
* given task (or in turn one of its stealers), blocking (via |
895 |
|
|
* pool.tryAwaitJoin) upon failure to find work. Traces |
896 |
|
|
* stolen->joining links looking for a thread working on |
897 |
|
|
* a descendant of the given task and with a non-empty queue to |
898 |
|
|
* steal back and execute tasks from. Inhibits mutual steal chains |
899 |
|
|
* and scans on outer joins upon nesting to avoid unbounded |
900 |
|
|
* growth. Restarts search upon encountering inconsistencies. |
901 |
|
|
* Tries to block if two passes agree that there are no remaining |
902 |
|
|
* targets. |
903 |
|
|
* |
904 |
|
|
* @param joinMe the task to join |
905 |
|
|
*/ |
906 |
|
|
private void nonlocalJoinTask(ForkJoinTask<?> joinMe) { |
907 |
|
|
ForkJoinPool p = pool; |
908 |
|
|
int scans = p.parallelism; // give up if too many retries |
909 |
|
|
ForkJoinTask<?> bottom = null; // target seen when can't descend |
910 |
|
|
restart: while (joinMe.status >= 0) { |
911 |
|
|
ForkJoinTask<?> target = null; |
912 |
|
|
ForkJoinTask<?> next = joinMe; |
913 |
|
|
while (scans >= 0 && next != null) { |
914 |
|
|
--scans; |
915 |
|
|
target = next; |
916 |
|
|
next = null; |
917 |
|
|
ForkJoinWorkerThread v = null; |
918 |
|
|
ForkJoinWorkerThread[] ws = p.workers; |
919 |
|
|
int n = ws.length; |
920 |
|
|
for (int j = 0; j < n; ++j) { |
921 |
|
|
ForkJoinWorkerThread w = ws[j]; |
922 |
|
|
if (w != null && w.stolen == target) { |
923 |
|
|
v = w; |
924 |
|
|
break; |
925 |
|
|
} |
926 |
|
|
} |
927 |
|
|
if (v != null && v != this) { |
928 |
|
|
ForkJoinTask<?> prevStolen = stolen; |
929 |
|
|
int b; |
930 |
|
|
ForkJoinTask<?>[] q; |
931 |
|
|
while ((b = v.base) != v.sp && (q = v.queue) != null) { |
932 |
|
|
int i = (q.length - 1) & b; |
933 |
|
|
long u = (i << qShift) + qBase; |
934 |
|
|
ForkJoinTask<?> t = q[i]; |
935 |
|
|
if (target.status < 0) |
936 |
|
|
continue restart; |
937 |
|
|
if (t != null && v.base == b && |
938 |
|
|
UNSAFE.compareAndSwapObject(q, u, t, null)) { |
939 |
|
|
if (joinMe.status < 0) { |
940 |
|
|
UNSAFE.putObjectVolatile(q, u, t); |
941 |
|
|
return; // back out |
942 |
|
|
} |
943 |
|
|
stolen = t; |
944 |
|
|
v.base = b + 1; |
945 |
|
|
t.tryExec(); |
946 |
|
|
stolen = prevStolen; |
947 |
|
|
} |
948 |
|
|
if (joinMe.status < 0) |
949 |
|
|
return; |
950 |
|
|
} |
951 |
|
|
next = v.joining; |
952 |
|
|
} |
953 |
|
|
if (target.status < 0) |
954 |
|
|
continue restart; // inconsistent |
955 |
|
|
if (joinMe.status < 0) |
956 |
|
|
return; |
957 |
|
|
} |
958 |
|
|
|
959 |
|
|
if (bottom != target) |
960 |
|
|
bottom = target; // recheck landing spot |
961 |
|
|
else if (p.tryAwaitJoin(joinMe) < 0) |
962 |
|
|
return; // successfully blocked |
963 |
|
|
Thread.yield(); // tame spin in case too many active |
964 |
|
|
} |
965 |
|
|
} |
966 |
|
|
|
967 |
|
|
/** |
968 |
dl |
1.31 |
* Returns an estimate of the number of tasks, offset by a |
969 |
|
|
* function of number of idle workers. |
970 |
|
|
* |
971 |
|
|
* This method provides a cheap heuristic guide for task |
972 |
|
|
* partitioning when programmers, frameworks, tools, or languages |
973 |
|
|
* have little or no idea about task granularity. In essence by |
974 |
|
|
* offering this method, we ask users only about tradeoffs in |
975 |
|
|
* overhead vs expected throughput and its variance, rather than |
976 |
|
|
* how finely to partition tasks. |
977 |
|
|
* |
978 |
|
|
* In a steady state strict (tree-structured) computation, each |
979 |
|
|
* thread makes available for stealing enough tasks for other |
980 |
|
|
* threads to remain active. Inductively, if all threads play by |
981 |
|
|
* the same rules, each thread should make available only a |
982 |
|
|
* constant number of tasks. |
983 |
|
|
* |
984 |
|
|
* The minimum useful constant is just 1. But using a value of 1 |
985 |
|
|
* would require immediate replenishment upon each steal to |
986 |
|
|
* maintain enough tasks, which is infeasible. Further, |
987 |
|
|
* partitionings/granularities of offered tasks should minimize |
988 |
|
|
* steal rates, which in general means that threads nearer the top |
989 |
|
|
* of computation tree should generate more than those nearer the |
990 |
|
|
* bottom. In perfect steady state, each thread is at |
991 |
|
|
* approximately the same level of computation tree. However, |
992 |
|
|
* producing extra tasks amortizes the uncertainty of progress and |
993 |
|
|
* diffusion assumptions. |
994 |
|
|
* |
995 |
|
|
* So, users will want to use values larger, but not much larger |
996 |
|
|
* than 1 to both smooth over transient shortages and hedge |
997 |
|
|
* against uneven progress; as traded off against the cost of |
998 |
|
|
* extra task overhead. We leave the user to pick a threshold |
999 |
|
|
* value to compare with the results of this call to guide |
1000 |
|
|
* decisions, but recommend values such as 3. |
1001 |
|
|
* |
1002 |
|
|
* When all threads are active, it is on average OK to estimate |
1003 |
|
|
* surplus strictly locally. In steady-state, if one thread is |
1004 |
|
|
* maintaining say 2 surplus tasks, then so are others. So we can |
1005 |
|
|
* just use estimated queue length (although note that (sp - base) |
1006 |
|
|
* can be an overestimate because of stealers lagging increments |
1007 |
|
|
* of base). However, this strategy alone leads to serious |
1008 |
|
|
* mis-estimates in some non-steady-state conditions (ramp-up, |
1009 |
|
|
* ramp-down, other stalls). We can detect many of these by |
1010 |
|
|
* further considering the number of "idle" threads, that are |
1011 |
|
|
* known to have zero queued tasks, so compensate by a factor of |
1012 |
|
|
* (#idle/#active) threads. |
1013 |
dl |
1.1 |
*/ |
1014 |
dl |
1.31 |
final int getEstimatedSurplusTaskCount() { |
1015 |
|
|
return sp - base - pool.idlePerActive(); |
1016 |
dl |
1.5 |
} |
1017 |
|
|
|
1018 |
|
|
/** |
1019 |
dl |
1.31 |
* Gets and removes a local task. |
1020 |
jsr166 |
1.16 |
* |
1021 |
dl |
1.31 |
* @return a task, if available |
1022 |
dl |
1.5 |
*/ |
1023 |
dl |
1.31 |
final ForkJoinTask<?> pollLocalTask() { |
1024 |
dl |
1.35 |
while (sp != base) { |
1025 |
dl |
1.31 |
if (active || (active = pool.tryIncrementActiveCount())) |
1026 |
|
|
return locallyFifo? locallyDeqTask() : popTask(); |
1027 |
dl |
1.1 |
} |
1028 |
dl |
1.31 |
return null; |
1029 |
dl |
1.1 |
} |
1030 |
|
|
|
1031 |
|
|
/** |
1032 |
dl |
1.31 |
* Gets and removes a local or stolen task. |
1033 |
|
|
* |
1034 |
|
|
* @return a task, if available |
1035 |
dl |
1.2 |
*/ |
1036 |
dl |
1.31 |
final ForkJoinTask<?> pollTask() { |
1037 |
|
|
ForkJoinTask<?> t; |
1038 |
|
|
return (t = pollLocalTask()) != null ? t : scan(); |
1039 |
dl |
1.2 |
} |
1040 |
|
|
|
1041 |
|
|
/** |
1042 |
jsr166 |
1.16 |
* Runs tasks until {@code pool.isQuiescent()}. |
1043 |
dl |
1.1 |
*/ |
1044 |
dl |
1.5 |
final void helpQuiescePool() { |
1045 |
|
|
for (;;) { |
1046 |
dl |
1.31 |
ForkJoinTask<?> t = pollLocalTask(); |
1047 |
dl |
1.35 |
if (t != null || (t = scan()) != null) { |
1048 |
dl |
1.31 |
t.tryExec(); |
1049 |
dl |
1.35 |
stolen = null; |
1050 |
|
|
} |
1051 |
dl |
1.31 |
else { |
1052 |
|
|
ForkJoinPool p = pool; |
1053 |
|
|
if (active) { |
1054 |
|
|
active = false; // inactivate |
1055 |
|
|
do {} while (!p.tryDecrementActiveCount()); |
1056 |
|
|
} |
1057 |
|
|
if (p.isQuiescent()) { |
1058 |
|
|
active = true; // re-activate |
1059 |
|
|
do {} while (!p.tryIncrementActiveCount()); |
1060 |
|
|
return; |
1061 |
|
|
} |
1062 |
|
|
} |
1063 |
dl |
1.5 |
} |
1064 |
dl |
1.1 |
} |
1065 |
|
|
|
1066 |
jsr166 |
1.20 |
// Unsafe mechanics |
1067 |
|
|
|
1068 |
|
|
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1069 |
|
|
private static final long runStateOffset = |
1070 |
|
|
objectFieldOffset("runState", ForkJoinWorkerThread.class); |
1071 |
dl |
1.31 |
private static final long qBase = |
1072 |
|
|
UNSAFE.arrayBaseOffset(ForkJoinTask[].class); |
1073 |
jsr166 |
1.20 |
private static final int qShift; |
1074 |
|
|
|
1075 |
|
|
static { |
1076 |
|
|
int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class); |
1077 |
|
|
if ((s & (s-1)) != 0) |
1078 |
|
|
throw new Error("data type scale not a power of two"); |
1079 |
|
|
qShift = 31 - Integer.numberOfLeadingZeros(s); |
1080 |
|
|
} |
1081 |
|
|
|
1082 |
|
|
private static long objectFieldOffset(String field, Class<?> klazz) { |
1083 |
|
|
try { |
1084 |
|
|
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1085 |
|
|
} catch (NoSuchFieldException e) { |
1086 |
|
|
// Convert Exception to corresponding Error |
1087 |
|
|
NoSuchFieldError error = new NoSuchFieldError(field); |
1088 |
|
|
error.initCause(e); |
1089 |
|
|
throw error; |
1090 |
|
|
} |
1091 |
|
|
} |
1092 |
|
|
|
1093 |
|
|
/** |
1094 |
|
|
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1095 |
|
|
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1096 |
|
|
* into a jdk. |
1097 |
|
|
* |
1098 |
|
|
* @return a sun.misc.Unsafe |
1099 |
|
|
*/ |
1100 |
jsr166 |
1.17 |
private static sun.misc.Unsafe getUnsafe() { |
1101 |
jsr166 |
1.6 |
try { |
1102 |
jsr166 |
1.17 |
return sun.misc.Unsafe.getUnsafe(); |
1103 |
jsr166 |
1.6 |
} catch (SecurityException se) { |
1104 |
|
|
try { |
1105 |
|
|
return java.security.AccessController.doPrivileged |
1106 |
jsr166 |
1.20 |
(new java.security |
1107 |
|
|
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1108 |
jsr166 |
1.17 |
public sun.misc.Unsafe run() throws Exception { |
1109 |
jsr166 |
1.20 |
java.lang.reflect.Field f = sun.misc |
1110 |
|
|
.Unsafe.class.getDeclaredField("theUnsafe"); |
1111 |
|
|
f.setAccessible(true); |
1112 |
|
|
return (sun.misc.Unsafe) f.get(null); |
1113 |
jsr166 |
1.6 |
}}); |
1114 |
|
|
} catch (java.security.PrivilegedActionException e) { |
1115 |
jsr166 |
1.17 |
throw new RuntimeException("Could not initialize intrinsics", |
1116 |
|
|
e.getCause()); |
1117 |
jsr166 |
1.6 |
} |
1118 |
|
|
} |
1119 |
|
|
} |
1120 |
dl |
1.1 |
} |