http://mechanical-sympathy.blogspot.co.uk/2011/07/false-sharing.html

public final class FalseSharing
    implements Runnable
    {
        public final static int NUM_THREADS = 4; // change
        public final static long ITERATIONS = 500L * 1000L * 1000L;
        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 = 0L;
            public long p1,p2,p3,p4,p5,p6; // comment out
        }
    }

该示例演示了多个线程使彼此的高速缓存行无效所经历的减速,即使每个线程仅仅更新一个变量.

BlockqFigure 1. above illustrates the issue of false sharing. A thread running on core 1 wants to update variable X while a thread on core 2 wants to update variable Y. Unfortunately these two hot variables reside in the same cache line. Each thread will race for ownership of the cache line so they can update it. If core 1 gets ownership then the cache sub-system will need to invalidate the corresponding cache line for core 2. When Core 2 gets ownership and performs its update,then core 1 will be told to invalidate its copy of the cache line. This will ping pong back and forth via the L3 cache greatly impacting performance. The issue would be further exacerbated if competing cores are on different sockets and additionally have to cross the socket interconnect.

我的问题如下.如果要更新的所有变量都是易失性的,为什么这个填充会导致性能提升？我的理解是,volatile变量总是写入并读取到主存储器.因此,我假设在此示例中对任何变量的每次写入和读取都将导致当前核心高速缓存行的刷新.

所以根据我的理解.如果线程1使线程二的高速缓存行无效,那么在它从其自己的高速缓存行读取值之前,这将不会成为线程2的替代.它读取的值是一个易失性值,因此这有效地使缓存变脏,从而导致从主存储器读取.

我的理解在哪里出错了？

谢谢