java为什么要添加系统。出来println能让线程运行得更快吗？

Question

我在尝试各种方法，使大型计算并行运行。除了显式创建和启动Thread实例时，要慢得多，这一点与预期一样有效。我添加了一些println输出，以检查线程是否真的并行运行，但正是这一更改使其更快。速度从“超过一秒”到“几十毫秒”，结果是可重复的

特别奇怪：ArrayList没有慢的问题，println没有明显的区别，但是LinkedList很慢，直到我添加了一些println

我用Java 8在Windows 10上运行这个。我还尝试了Java11和Java12，但速度并没有变慢（所以无意义的输出无法使速度更快）

我的问题不是如何让它更快——我实际上不需要运行这段代码，我可以用另一种方式让它并行（我推荐streams）。我的问题是，如何通过打印一些输出来加快线程的速度，或者如果你更喜欢，那么为什么它一开始速度很慢

我试着改变线程的数量：一个线程就消失了这种奇怪（！）但是有2个或更多线程（我测试了两倍于我机器上的内核数）

我尝试过改变正在处理的列表的大小，但只证明较小的列表可以更快地处理

我尝试改变输出：两个println输出似乎比一个输出更能提高速度，而在线程寿命结束时打印的效果比开始时小

我确保这不是GC效应（巨大的堆，并检查了GC日志）

这是我运行的代码

import java.util.ArrayList;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class ListAddThreads {
    private static final int SIZE    = 20000000;
    private static final int REPEATS = 3;
    private static final int THREADS = Runtime.getRuntime().availableProcessors() / 2;

    private static String log = "none";

    public static void main( String[] args ) {
        if( args.length > 0 ) switch( args[0] ) {
            case "some":
            case "more":
            case "lots":
                log = args[0];
        }

        System.out.println( "Array List:" );
        List<Integer>listA = new ArrayList<>(SIZE);
        for( int i = 0; i < SIZE; i++ )
            listA.add( i );
        for( int i = 0; i < REPEATS; i++ )
            add( listA );

        System.out.println( "\nLinked List:" );
        List<Integer>listL = new LinkedList<>(listA);
        for( int i = 0; i < REPEATS; i++ )
            add( listL );
    }

    private static class AdderThread extends Thread {
        private long          result = 0;
        private List<Integer> list;
        private int           from;
        private int           to;

        AdderThread( List<Integer> list, int from, int to ) {
            this.list = list;
            this.from = from;
            this.to   = to;
        }

        long getResult() {
            return result;
        }

        public void run() {
            switch( log ) {
                case "some":
                    System.err.println( "string literal" );
                    break;
                case "more":
                    System.err.println( Thread.currentThread().getName() + " starting" );
                    break;
                case "lots":
                    System.err.println( "string literal" );
                    System.err.println( Thread.currentThread().getName() + " starting" );
                    break;
            }
            Iterator<Integer> it = list.iterator();
            for( int i = 0; i < to; i++ ) {
                Integer j = it.next();
                if( i >= from )
                    result += j;
            }
        }
    }

    private static void add( List<Integer> list ) {
        long          start  = System.currentTimeMillis();
        AdderThread[] adders = new AdderThread[THREADS];
        int           size   = list.size() / THREADS;

        for( int i = 0; i < THREADS; i++ ) {
            int from = i * size;
            int to   = i == THREADS - 1 ? list.size() : (i + 1) * size;

            adders[i] = new AdderThread( list, from, to );
        }
        for( int i = 0; i < THREADS; i++ ) {
            adders[i].start();
        }
        long result = 0;
        try {
            for( int i = 0; i < THREADS; i++ ) {
                adders[i].join();
                result += adders[i].getResult();
            }
        } catch ( InterruptedException x ) {
            throw new RuntimeException( "Adder thread interrupted.", x );
        }
        System.out.println( "Sum: " + result + ", time: " + (System.currentTimeMillis() - start) );
    }
}

我这样称呼它：

D:\weird> for %G in (none some more lots) do java -Xms4G -Xmx4G -XX:+PrintGCDetails -Xloggc:ListAddThreads_%G_gc.log ListAddThreads %G 2>nul

下面是一些示例输出，显示了LinkedList时间随着输出的增加而加快（重定向到oblivion）：

java -Xms4G -Xmx4G -XX:+PrintGCDetails -Xloggc:ListAddThreads_none_gc.log ListAddThreads none  2>nul
Array List:
Sum: 199999990000000, time: 62
Sum: 199999990000000, time: 157
Sum: 199999990000000, time: 62

Linked List:
Sum: 199999990000000, time: 3343
Sum: 199999990000000, time: 1814
Sum: 199999990000000, time: 1796

java -Xms4G -Xmx4G -XX:+PrintGCDetails -Xloggc:ListAddThreads_some_gc.log ListAddThreads some  2>nul
Array List:
Sum: 199999990000000, time: 50
Sum: 199999990000000, time: 58
Sum: 199999990000000, time: 56

Linked List:
Sum: 199999990000000, time: 2777
Sum: 199999990000000, time: 560
Sum: 199999990000000, time: 527

java -Xms4G -Xmx4G -XX:+PrintGCDetails -Xloggc:ListAddThreads_more_gc.log ListAddThreads more  2>nul
Array List:
Sum: 199999990000000, time: 48
Sum: 199999990000000, time: 56
Sum: 199999990000000, time: 39

Linked List:
Sum: 199999990000000, time: 108
Sum: 199999990000000, time: 93
Sum: 199999990000000, time: 94

java -Xms4G -Xmx4G -XX:+PrintGCDetails -Xloggc:ListAddThreads_lots_gc.log ListAddThreads lots  2>nul
Array List:
Sum: 199999990000000, time: 52
Sum: 199999990000000, time: 68
Sum: 199999990000000, time: 39

Linked List:
Sum: 199999990000000, time: 115
Sum: 199999990000000, time: 89
Sum: 199999990000000, time: 91