伪填充问题 - zzh

发生场景

多核 cpu 下多个线程之间改写连续不同资源。

发生机制

因为 cpu 往往使用高速缓冲来加快对变量的读取，而 mesi 协议会导致不同 cpu 上的线程修改对应的高速缓存时，会使其它 cpu 上的相应的高速缓存行失效（如果存在），那么另一个 cpu 修改该高速缓冲行中的变量时（即使不是同一个变量）就会从内存中进行读取，这样又会导致原来 cpu 的对应缓冲行失效，如此反复，导致 cpu 对于变量的修改读取都需要经过内存而无法有效利用高速缓冲，从而减慢速度。

代码实现

发生伪填充

public final class FalseSharing
        implements Runnable
{
    public final static int NUM_THREADS = 2; // change
    public final static long ITERATIONS = 500 * 1000 * 1000;
    private final int arrayIndex;

    private static VolatileLong[] longs = new VolatileLong[NUM_THREADS];
    static
    {
        for (int i = 0; i < longs.length; i++)
        {
            longs[i] = new VolatileLong();
        }
    }

    public FalseSharing(final int arrayIndex)
    {
        this.arrayIndex = arrayIndex;
    }

    public static void main(final String[] args) throws Exception
    {
        final long start = System.nanoTime();
        runTest();
        System.out.println("duration = " + (System.nanoTime() - start));
    }

    private static void runTest() throws InterruptedException
    {
        Thread[] threads = new Thread[NUM_THREADS];

        for (int i = 0; i < threads.length; i++)
        {
            threads[i] = new Thread(new FalseSharing(i));
        }

        for (Thread t : threads)
        {
            t.start();
        }

        for (Thread t : threads)
        {
            t.join();
        }
    }

    public void run()
    {
        long i = ITERATIONS + 1;
        while (0 != --i)
        {
            longs[arrayIndex].value = i;
        }
    }

    public final static class VolatileLong
    {
        public volatile long value = 0;
    }
}

未发生伪填充

public final class FalseSharing
        implements Runnable
{
    public final static int NUM_THREADS = 2; // change
    public final static long ITERATIONS = 500 * 1000 * 1000;
    private final int arrayIndex;

    private static VolatileLong[] longs = new VolatileLong[NUM_THREADS];
    static
    {
        for (int i = 0; i < longs.length; i++)
        {
            longs[i] = new VolatileLong();
        }
    }

    public FalseSharing(final int arrayIndex)
    {
        this.arrayIndex = arrayIndex;
    }

    public static void main(final String[] args) throws Exception
    {
        final long start = System.nanoTime();
        runTest();
        System.out.println("duration = " + (System.nanoTime() - start));
    }

    private static void runTest() throws InterruptedException
    {
        Thread[] threads = new Thread[NUM_THREADS];

        for (int i = 0; i < threads.length; i++)
        {
            threads[i] = new Thread(new FalseSharing(i));
        }

        for (Thread t : threads)
        {
            t.start();
        }

        for (Thread t : threads)
        {
            t.join();
        }
    }

    public void run()
    {
        long i = ITERATIONS + 1;
        while (0 != --i)
        {
            longs[arrayIndex].value = i;
        }
    }

    @sun.misc.Contended
    public final static class VolatileLong
    {
        public volatile long value = 0;
    }
}

其中需要加入 jvm 参数 - XX:-RestrictContended 。

实验结果

发生伪填充

未发生伪填充