In Java 8, does executors.newWorkStealingPool () also provide a task queue?

Question

Is there a waiting list of tasks used in conjunction with Java 8 Executors.newWorkStealingPool() ?

For example, suppose # the available cores are 2, and Executors.newWorkStealingPool() empty because 2 tasks are already running. Then, what happens if the third task is sent to the performer? Is he queued? And if so, what are the boundaries if they are in the specified queue?

Thanks in advance.

Answer 1

Is there a waiting list of tasks used in conjunction with Java 8 Executors.newWorkStealingPool ()?

Yes, each stream is supported by its own deck. When one thread executes with its tasks, it takes a task from another thread and executes it.

And if so, what are the boundaries if they are in the specified queue?

The maximum size for queues is limited by the number: static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M

When the queue is full, an unchecked exception is thrown: RejectedExecutionException("Queue capacity exceeded")

Answer 2

From grepcode Executors and ForkJoinPool

Executors . newWorkStealingPool returns ForkJoinPool

Performers:

  public static ExecutorService newWorkStealingPool() { return new ForkJoinPool (Runtime.getRuntime().availableProcessors(), ForkJoinPool.defaultForkJoinWorkerThreadFactory, null, true); } 

ForkJoinPool:

 public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory, UncaughtExceptionHandler handler, boolean asyncMode) { this(checkParallelism(parallelism), checkFactory(factory), handler, asyncMode ? FIFO_QUEUE : LIFO_QUEUE, "ForkJoinPool-" + nextPoolId() + "-worker-"); checkPermission(); } 

On execute() :

 public void execute(ForkJoinTask<?> task) { if (task == null) throw new NullPointerException(); externalPush(task); } 

externalPush calls externalSubmit , and you can see the details of WorkQueue in this implementation.

externalSubmit:

// External operations

 /** * Full version of externalPush, handling uncommon cases, as well * as performing secondary initialization upon the first * submission of the first task to the pool. It also detects * first submission by an external thread and creates a new shared * queue if the one at index if empty or contended. * * @param task the task. Caller must ensure non-null. */ 

More information on queue sizes can be found in the WorkQueue class.

  static final class WorkQueue { 

WokrQueue Documentation:

 /** * Queues supporting work-stealing as well as external task * submission. See above for descriptions and algorithms. * Performance on most platforms is very sensitive to placement of * instances of both WorkQueues and their arrays -- we absolutely * do not want multiple WorkQueue instances or multiple queue * arrays sharing cache lines. The @Contended annotation alerts * JVMs to try to keep instances apart. */ @sun.misc.Contended /** * Capacity of work-stealing queue array upon initialization. * Must be a power of two; at least 4, but should be larger to * reduce or eliminate cacheline sharing among queues. * Currently, it is much larger, as a partial workaround for * the fact that JVMs often place arrays in locations that * share GC bookkeeping (especially cardmarks) such that * per-write accesses encounter serious memory contention. */ static final int INITIAL_QUEUE_CAPACITY = 1 << 13; /** * Maximum size for queue arrays. Must be a power of two less * than or equal to 1 << (31 - width of array entry) to ensure * lack of wraparound of index calculations, but defined to a * value a bit less than this to help users trap runaway * programs before saturating systems. */ static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M