工作窃取(work-stealing)算法是指某个线程从其他队列里窃取任务来执行
为什么ForkJoin会存在工作窃取呢?因为我们将任务进行分解成多个子任务的时候。每个子任务的处理时间都不一样。例如分别有子任务A\B。如果子任务A的1ms的时候已经执行,子任务B还在执行。那么如果我们子任务A的线程等待子任务B完毕后在进行汇总,那么子任务A线程就会在浪费执行时间,最终的执行时间就以最耗时的子任务为准。而如果我们的子任务A执行完毕后,处理子任务B的任务,并且执行完毕后将任务归还给子任务B。这样就可以提高执行效率。而这种就是工作窃取。
package com.wf.zhang.juc; import org.junit.Test; import java.time.Duration; import java.time.Instant; import java.util.concurrent.ForkJoinPool; import java.util.concurrent.ForkJoinTask; import java.util.concurrent.RecursiveTask; import java.util.stream.LongStream; public class TestForkJoinPool { public static void main(String[] args) { Instant start = Instant.now(); ForkJoinPool pool = new ForkJoinPool(); ForkJoinTask<Long> task = new ForkJoinSumCalculate(0L, 50000000000L); Long sum = pool.invoke(task); System.out.println(sum); Instant end = Instant.now(); System.out.println("耗费时间为:" + Duration.between(start, end).toMillis());//166-1996-10590 } @Test public void test1(){ Instant start = Instant.now(); long sum = 0L; for (long i = 0L; i <= 50000000000L; i++) { sum += i; } System.out.println(sum); Instant end = Instant.now(); System.out.println("耗费时间为:" + Duration.between(start, end).toMillis());//35-3142-15704 } //java8 新特性 @Test public void test2(){ Instant start = Instant.now(); Long sum = LongStream.rangeClosed(0L, 50000000000L) .parallel() .reduce(0L, Long::sum); System.out.println(sum); Instant end = Instant.now(); System.out.println("耗费时间为:" + Duration.between(start, end).toMillis());//1536-8118 } } class ForkJoinSumCalculate extends RecursiveTask<Long>{ private static final long serialVersionUID = -259195479995561737L; private long start; private long end; private static final long THURSHOLD = 10000L; //临界值 public ForkJoinSumCalculate(long start, long end) { this.start = start; this.end = end; } @Override protected Long compute() { long length = end - start; if(length <= THURSHOLD){ long sum = 0L; for (long i = start; i <= end; i++) { sum += i; } return sum; }else{ long middle = (start + end) / 2; ForkJoinSumCalculate left = new ForkJoinSumCalculate(start, middle); left.fork(); //进行拆分,同时压入线程队列 ForkJoinSumCalculate right = new ForkJoinSumCalculate(middle+1, end); right.fork(); // return left.join() + right.join(); } } }