JAVA - 多线程Mergesort的排序速度

Question

我用JAVA实现了一个多线程的MergeSort，用不同的线程数测试了算法的运行时间。我在双核处理器上运行代码，算法在 4 或 8 个线程下运行最快。这对我来说没有意义——我有两个核心。这是我的源代码。

public static void parallelMergeSort(int[] a, int NUM_THREADS)
{
    if(NUM_THREADS <= 1)
    {
        mergeSort(a);
        return;
    }

    int mid = a.length / 2;

    int[] left = Arrays.copyOfRange(a, 0, mid);
    int[] right = Arrays.copyOfRange(a, mid, a.length);

    Thread leftSorter = mergeSortThread(left, NUM_THREADS);
    Thread rightSorter = mergeSortThread(right, NUM_THREADS);

    leftSorter.start();
    rightSorter.start();

    try {
        leftSorter.join();
        rightSorter.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    merge(left, right, a);
}

private static Thread mergeSortThread(int[] a, int NUM_THREADS)
{
    return new Thread()
    {
        @Override
        public void run()
        {
            parallelMergeSort(a, NUM_THREADS / 2);
        }
    };
}

public static void mergeSort(int[] a)
{
    if(a.length <= 1) return;

    int mid = a.length / 2;

    int[] left = Arrays.copyOfRange(a, 0, mid);
    int[] right = Arrays.copyOfRange(a, mid, a.length);

    mergeSort(left);
    mergeSort(right);

    merge(left, right, a);
}


private static void merge(int[] a, int[] b, int[] r)
{
    int i = 0, j = 0, k = 0;
    while(i < a.length && j < b.length)
    {
        if(a[i] < b[j])
            r[k++] = a[i++];
        else
            r[k++] = b[j++];
    }

    while(i < a.length)
        r[k++] = a[i++];

    while(j < b.length)
        r[k++] = b[j++];
}

我测试了使用不同数量的线程运行它并在几毫秒内得到了以下结果:

Serial Sort Run Time: 5368.
Parallel Sort Run Time with 2 Threads: 3202.
Parallel Sort Run Time with 4 Threads: 2408.
Parallel Sort Run Time with 8 Threads: 2544.
Parallel Sort Run Time with 16 Threads: 2738.
Parallel Sort Run Time with 32 Threads: 2909.
Parallel Sort Run Time with 64 Threads: 3078.
Parallel Sort Run Time with 128 Threads: 3777.

为什么这个算法在双核 cpu 上用 4 个线程运行得最快？

Answer 1

Why would this algorithm run fastest with 4 threads on a dual core cpu?

这可能只是基准测试不佳造成的。例如，如果您的基准测试没有适当考虑 JVM 预热效果，则结果将不可靠。

---

除此之外，“超线程”的解释是合理的:

Core i3, i5 and i7

First introduced in 2008, the Core i3, i5 and i7 models constitute Intel's current line of desktop PC processors. They cover a wide range of clock speeds, from 1.2 GHz for the i3 Mobile to 3.6 GHz in the fastest i7 processors. All processors in the series are 64-bit designs and have a minimum of two cores each; other than the quad-core i5 models, they all benefit from Hyper-Threading technology.

来源:"What Intel Processors Have Hyper Threading ？

但是，HT 在您的处理器上(显然)可用的事实并不意味着它实际上已启用。这将取决于 BIOS 设置等。

• https://unix.stackexchange.com/questions/33450/checking-if-hyperthreading-is-enabled-or-not