java - 在 Java 8 中，Executors.newWorkStealingPool() 是否也提供了任务队列？

Question

是否有与 Java 8 结合使用的待处理任务队列Executors.newWorkStealingPool()？

例如，假设 #available cores 是 2，并且Executors.newWorkStealingPool()是空的，因为 2 个任务已经在运行。那么如果将第三个任务提交给工作窃取执行者会发生什么？是排队吗？如果是，那么所述队列上的界限是什么？

提前致谢。

Answer 1

是否有与 Java 8 的 Executors.newWorkStealingPool() 一起使用的待处理任务队列？

是的，每个线程都有自己的双端队列支持。当一个线程完成它的任务时，它会从其他线程的双端队列中获取任务并执行它。

如果是，那么所述队列上的界限是什么？

队列的最大大小受数量限制：static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M

当队列已满时，会抛出未经检查的异常：RejectedExecutionException("Queue capacity exceeded")

Answer 2

来自Executors和ForkJoinPool的 grepcode

Executors. newWorkStealingPool返回ForkJoinPool

执行者：

 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();
    }

开execute()：

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

externalPush调用externalSubmit，您可以WorkQueue在该实现中查看详细信息。

外部提交：

// 外部操作

/**
 * 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.

 */

WorkQueue您可以在课堂上找到有关队列大小的更多详细信息

 static final class WorkQueue {

文档WokrQueue：

/**
     * 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