Is Java's System.arraycopy() efficient for small arrays?

Question

Is Java\'s System.arraycopy() efficient for small arrays, or does the fact that it\'s a native method make it likely to be substantially less efficient than a s

Answer 1

Here is my benchmark code:

public void test(int copySize, int copyCount, int testRep) {
    System.out.println("Copy size = " + copySize);
    System.out.println("Copy count = " + copyCount);
    System.out.println();
    for (int i = testRep; i > 0; --i) {
        copy(copySize, copyCount);
        loop(copySize, copyCount);
    }
    System.out.println();
}

public void copy(int copySize, int copyCount) {
    int[] src = newSrc(copySize + 1);
    int[] dst = new int[copySize + 1];
    long begin = System.nanoTime();
    for (int count = copyCount; count > 0; --count) {
        System.arraycopy(src, 1, dst, 0, copySize);
        dst[copySize] = src[copySize] + 1;
        System.arraycopy(dst, 0, src, 0, copySize);
        src[copySize] = dst[copySize];
    }
    long end = System.nanoTime();
    System.out.println("Arraycopy: " + (end - begin) / 1e9 + " s");
}

public void loop(int copySize, int copyCount) {
    int[] src = newSrc(copySize + 1);
    int[] dst = new int[copySize + 1];
    long begin = System.nanoTime();
    for (int count = copyCount; count > 0; --count) {
        for (int i = copySize - 1; i >= 0; --i) {
            dst[i] = src[i + 1];
        }
        dst[copySize] = src[copySize] + 1;
        for (int i = copySize - 1; i >= 0; --i) {
            src[i] = dst[i];
        }
        src[copySize] = dst[copySize];
    }
    long end = System.nanoTime();
    System.out.println("Man. loop: " + (end - begin) / 1e9 + " s");
}

public int[] newSrc(int arraySize) {
    int[] src = new int[arraySize];
    for (int i = arraySize - 1; i >= 0; --i) {
        src[i] = i;
    }
    return src;
}

From my tests, calling test() with copyCount = 10000000 (1e7) or greater allows the warm-up to be achieved during the first copy/loop call, so using testRep = 5 is enough; With copyCount = 1000000 (1e6) the warm-up need at least 2 or 3 iterations so testRep shall be increased in order to obtain usable results.

With my configuration (CPU Intel Core 2 Duo E8500 @ 3.16GHz, Java SE 1.6.0_35-b10 and Eclipse 3.7.2) it appears from the benchmark that:

• When copySize = 24, System.arraycopy() and the manual loop take almost the same time (sometimes one is very slightly faster than the other, other times it’s the contrary),
• When copySize < 24, the manual loop is faster than System.arraycopy() (slightly faster with copySize = 23, really faster with copySize < 5),
• When copySize > 24, System.arraycopy() is faster than the manual loop (slightly faster with copySize = 25, the ratio loop-time/arraycopy-time increasing as copySize increases).

Note: I’m not English native speaker, please excuse all my grammar/vocabulary errors.