无法了解Java线程所消耗的时间

发布于02月15日

我创建了1000个线程，每个线程大约需要10秒才能完成.在这10秒内，线程不会Hibernate ，而是执行简单的数学运算. 然而，在我的PC(英特尔i7 6核)上，该程序仍然在20秒内完成.我不明白是怎么回事.我预计这至少需要大约1000/6=167秒.知道这是怎么回事吗？

public class test
{
   public static void main(String args[]) throws Exception
   {
       long startTime = System.currentTimeMillis();
       Thread[] threads = new Thread[1000];
       for(int i = 0; i<1000; i++){
           threads[i] = new Thread(()->doSomething(10));
           threads[i].start();
       }
       for(Thread t : threads){ 
           t.join(); 
       }
       long endTime = System.currentTimeMillis();
       System.out.println("Time taken "+(endTime - startTime)/1000);
   }
   
   public static int doSomething(int seconds){
    long st = System.currentTimeMillis();
    long usageTimeInMillis = seconds*1000L;
    long startTime = System.currentTimeMillis();
    int i = 0;
    while ((System.currentTimeMillis() - startTime) < usageTimeInMillis) { i++; } 
    long lt = System.currentTimeMillis();
    System.out.println("Done "+Thread.currentThread().getId()+" in "+(lt-st)/1000+" seconds ");
    return i;
   }
}

以下是部分输出:

Done 48 in 10 seconds 
Done 36 in 10 seconds 
Done 597 in 10 seconds 
...
Done 206 in 10 seconds 
Done 217 in 10 seconds 
....
Done 462 in 10 seconds 
Time taken 17

输出显示每个线程确实运行了大约10秒.那为什么1000个线程在17秒内就完成了？

package work.basil.example.threading; import java.time.*; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.stream.IntStream; public class BusyBusy { public static void main ( String args[] ) throws Exception { System.out.println ( "Demo start at " + Instant.now ( ) ); final long demoStartNanos = System.nanoTime ( ); final int countTasks = 1_000; try ( ExecutorService executorService = Executors.newFixedThreadPool ( countTasks ) ; // We want one task per thread. So one thousand tasks means one thousand threads. ) { IntStream .rangeClosed ( 1 , countTasks ) .mapToObj ( TenSecondTask :: new ) .forEach ( executorService :: submit ); } Duration demoDuration = Duration.ofNanos ( System.nanoTime ( ) - demoStartNanos ); System.out.println ( "Demo end at " + Instant.now ( ) + ". demoDuration = " + demoDuration ); } } class TenSecondTask implements Runnable { private final int sequenceNumber; public TenSecondTask ( final int sequenceNumber ) { this.sequenceNumber = sequenceNumber; } @Override public void run ( ) { final long tenSecondsInNanos = Duration.ofSeconds ( 10 ).toNanos ( ); final long taskStartNanos = System.nanoTime ( ); long countLoops = 0; while ( ( System.nanoTime ( ) - taskStartNanos ) < tenSecondsInNanos ) { countLoops++; } System.out.println ( "Task # " + this.sequenceNumber + " in thread ID " + Thread.currentThread ( ).threadId ( ) + " done at " + Instant.now ( ) + ". Loops: " + String.format ( "%,d" , countLoops ) ); } }

Demo start at 2024-02-15T09:24:13.773544Z Task # 6 in thread ID 26 done at 2024-02-15T09:24:23.779903Z. Loops: 328,186 Task # 11 in thread ID 31 done at 2024-02-15T09:24:23.812401Z. Loops: 349,456 Task # 13 in thread ID 33 done at 2024-02-15T09:24:23.839549Z. Loops: 356,229 Task # 20 in thread ID 40 done at 2024-02-15T09:24:23.883941Z. Loops: 379,757 … Task # 991 in thread ID 1019 done at 2024-02-15T09:24:33.561808Z. Loops: 981,788 Task # 992 in thread ID 1020 done at 2024-02-15T09:24:33.566229Z. Loops: 1,034,678 Task # 993 in thread ID 1021 done at 2024-02-15T09:24:33.572045Z. Loops: 1,172,157 Task # 994 in thread ID 1022 done at 2024-02-15T09:24:33.576395Z. Loops: 1,067,262 Task # 995 in thread ID 1023 done at 2024-02-15T09:24:33.582312Z. Loops: 1,034,725 Task # 996 in thread ID 1024 done at 2024-02-15T09:24:33.586528Z. Loops: 1,134,316 Task # 997 in thread ID 1025 done at 2024-02-15T09:24:33.592593Z. Loops: 1,112,738 Task # 998 in thread ID 1026 done at 2024-02-15T09:24:34.158249Z. Loops: 6,455,185 Task # 999 in thread ID 1027 done at 2024-02-15T09:24:34.158381Z. Loops: 6,484,593 Task # 1000 in thread ID 1028 done at 2024-02-15T09:24:34.168723Z. Loops: 7,411,728 Demo end at 2024-02-15T09:24:34.168958Z. demoDuration = PT20.392301916S

无法了解Java线程所消耗的时间

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