在多线程中使用LongAdder时得到了错误的值

Question

我正在做这个练习：

生成1000个线程，每个线程递增一个计数器100,000次。比较使用AtomicLong和LongAdder的性能。

以下是我的执行情况：

import java.io.*;
import java.util.*;
import java.nio.file.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;

public class AtomicLongVsLongAddr {

    // 9. Generate 1,000 threads, each of which increments a counter
    // 100,000 times. Compare the performance of using AtomicLong
    // versus LongAdder.

    AtomicLong al = new AtomicLong(0);
    LongAdder la = new LongAdder();

    public class AtomicLongThread extends Thread {
        @Override
        public void run() {
            for (int i = 0; i < 100000; i ++) {
                al.incrementAndGet();
            }
        }
    }

    public class LongAdderThread extends Thread {
        @Override
        public void run() {
            for (int i = 0; i < 100000; i ++) {
                la.increment();
            }
        }
    }

    public static void main(String[] args) {
        AtomicLongVsLongAddr vs = new AtomicLongVsLongAddr();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 1000; i ++) {
            (vs.new AtomicLongThread()).start();
        }
        long endTime = System.currentTimeMillis();
        System.out.printf("AtomicLong--Number: %s, Time: %d\n", vs.al, endTime - startTime);
        startTime = System.currentTimeMillis();
        for (int i = 0; i < 1000; i ++) {
            (vs.new LongAdderThread()).start();
        }
        long res = vs.la.sum();
        endTime = System.currentTimeMillis();
        System.out.printf("LongAdder--Number: %s, Time: %d\n", res, endTime - startTime);

    }

}

每次运行这个程序时，我都会得到类似于标准输出的如下内容：

AtomicLong--Number: 100000000, Time: 2330
LongAdder--Number: 99882179, Time: 469

显然，我在LongAdder中得到了一个错误的值，但是我不知道我在哪里做错了。

你能帮帮我吗?

已更新

在在座各位和@Ravindra Ranwala的帮助下，我更新了上述练习的答案：

import java.io.*;
import java.util.*;
import java.nio.file.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;

public class AtomicLongVsLongAddr {

    // 9. Generate 1,000 threads, each of which increments a counter
    // 100,000 times. Compare the performance of using AtomicLong
    // versus LongAdder.

    AtomicLong al = new AtomicLong(0);
    LongAdder la = new LongAdder();

    public class AtomicLongThread extends Thread {
        @Override
        public void run() {
            for (int i = 0; i < 100000; i ++) {
                al.incrementAndGet();
            }
        }
    }

    public class LongAdderThread extends Thread {
        @Override
        public void run() {
            for (int i = 0; i < 100000; i ++) {
                la.increment();
            }
        }
    }

    public static void main(String[] args) {
        try{
            long startTime;
            long endTime;
            AtomicLongVsLongAddr vs = new AtomicLongVsLongAddr();
            Thread[] t = new Thread[1000];
            for (int i = 0; i < 1000; i ++) {
                t[i] = vs.new AtomicLongThread();
            }
            startTime = System.currentTimeMillis();
            for (int i = 0; i < 1000; i ++) {
                t[i].start();
                t[i].join();
            }
            endTime = System.currentTimeMillis();
            System.out.printf("AtomicLong--Number: %s, Time: %d\n", vs.al, endTime - startTime);
            for (int i = 0; i < 1000; i ++) {
                t[i] = vs.new LongAdderThread();
            }
            startTime = System.currentTimeMillis();
            for (int i = 0; i < 1000; i ++) {
                t[i].start();
                t[i].join();
            }
            long res = vs.la.sum();
            endTime = System.currentTimeMillis();
            System.out.printf("LongAdder--Number: %s, Time: %d\n", res, endTime - startTime);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

如果还有什么不对的地方，请帮我指出。谢谢大家。

Answer 1

调用所有线程上的Thread.join并等待所有线程完成。您的主线程似乎在其他增加两个变量完成的线程之前退出。你在这里得到的是一些中间的结果。

这是密码，

public static void main(String[] args) throws InterruptedException {
    final List<Thread> adderThreads = new ArrayList<>();
    final List<Thread> atomicThreads = new ArrayList<>();
    AtomicLongVsLongAddr vs = new AtomicLongVsLongAddr();
    long startTime = System.currentTimeMillis();
    for (int i = 0; i < 1000; i++) {
        final AtomicLongThread atomicThread = vs.new AtomicLongThread();
        atomicThreads.add(atomicThread);
        atomicThread.start();
    }

    for (Thread thread : atomicThreads) {
        thread.join();
    }
    long endTime = System.currentTimeMillis();
    System.out.printf("AtomicLong--Number: %s, Time: %d\n", vs.al, endTime - startTime);
    startTime = System.currentTimeMillis();
    for (int i = 0; i < 1000; i++) {
        final LongAdderThread adderThread = vs.new LongAdderThread();
        adderThreads.add(adderThread);
        adderThread.start();
    }
    for (Thread thread : adderThreads) {
        thread.join();
    }
    long res = vs.la.sum();
    endTime = System.currentTimeMillis();
    System.out.printf("LongAdder--Number: %s, Time: %d\n", res, endTime - startTime);

}

Answer 2

您的代码不是同步的--主线程在完成计数器线程之前将退出/继续，从而造成差异。