将Java数组中任意范围的元素设置为null的最快方法是什么？

Question

我知道我可以简单地从迭代start到end和清除这些细胞,但我想知道,如果它(可能使用JNI-ED是任何更快的方式可能System.arrayCopy)？

Answer 1

如果我做对了,您需要使数组无效,或者包含对象引用的数组的子范围,以使它们符合GC的条件.而且你有一个常规的Java数组,它可以在堆上存储数据.

回答你的问题,System.arrayCopy是取消数组子范围的最快方法.它比内存更糟糕Arrays.fill,因为你必须分配两倍的内存来保存引用,在最坏的情况下你可以复制一个空数组.虽然如果你需要完全使数组为空,那么更快就是创建一个新的空数组(例如new Object[desiredLength])并用它替换你想要的数组.

Unsafe,DirectByteBuffer,DirectLongBuffer实现不提供一个天真直观的实现任何性能增益(也就是说,如果你只是更换Array用DirectByteBuffer或Unsafe).它们也比批量慢System.arrayCopy.由于这些实现与Java无关Array,因此无论如何它们都超出了您的问题范围.

这是我的JMH基准(通过gist提供的完整基准代码)片段,包括unsafe.setMemory根据@apangin评论的案例; 并包括ByteBuffer.put(long[] src, int srcOffset, int longCount)@ jan-chaefer; 和Arrays.fill@scott-carey 相当的循环来检查是否Arrays.fill可能是JDK 8中的内在函数.

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void arrayFill() {
    Arrays.fill(objectHolderForFill, null);
}

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void arrayFillManualLoop() {
    for (int i = 0, len = objectHolderForFill.length; i < len; i++) {
        objectHolderForLoop[i] = null;
    }
}

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void arrayCopy() {
    System.arraycopy(nullsArray, 0, objectHolderForArrayCopy, 0,
                              objectHolderForArrayCopy.length);
}

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directByteBufferManualLoop() {
    while (referenceHolderByteBuffer.hasRemaining()) {
        referenceHolderByteBuffer.putLong(0);
    }
}

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directByteBufferBatch() {
    referenceHolderByteBuffer.put(nullBytes, 0, nullBytes.length);
}

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directLongBufferManualLoop() {
    while (referenceHolderLongBuffer.hasRemaining()) {
        referenceHolderLongBuffer.put(0L);
    }
}

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directLongBufferBatch() {
    referenceHolderLongBuffer.put(nullLongs, 0, nullLongs.length);
}


@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void unsafeArrayManualLoop() {
    long addr = referenceHolderUnsafe;
    long pos = 0;
    for (int i = 0; i < size; i++) {
        unsafe.putLong(addr + pos, 0L);
        pos += 1 << 3;
    }
}

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void unsafeArraySetMemory() {
    unsafe.setMemory(referenceHolderUnsafe, size*8, (byte) 0);
}

这是我得到的(Java 1.8,JMH 1.13,Core i3-6100U 2.30 GHz,Win10):

100 elements
Benchmark                                       Mode      Cnt   Score   Error    Units
ArrayNullFillBench.arrayCopy                   sample  5234029  39,518 ± 0,991   ns/op
ArrayNullFillBench.directByteBufferBatch       sample  6271334  43,646 ± 1,523   ns/op
ArrayNullFillBench.directLongBufferBatch       sample  4615974  45,252 ± 2,352   ns/op
ArrayNullFillBench.arrayFill                   sample  4745406  76,997 ± 3,547   ns/op
ArrayNullFillBench.arrayFillManualLoop         sample  5549216  78,677 ± 13,013  ns/op
ArrayNullFillBench.unsafeArrayManualLoop       sample  5980381  78,811 ± 2,870   ns/op
ArrayNullFillBench.unsafeArraySetMemory        sample  5985884  85,062 ± 2,096   ns/op
ArrayNullFillBench.directLongBufferManualLoop  sample  4697023  116,242 ±  2,579  ns/op <-- wow
ArrayNullFillBench.directByteBufferManualLoop  sample  7504629  208,440 ± 10,651  ns/op <-- wow

I skipped all** the loop implementations from further tests
** - except arrayFill and arrayFillManualLoop for scale

1000 elements
Benchmark                                 Mode      Cnt    Score   Error    Units
ArrayNullFillBench.arrayCopy              sample  6780681  184,516 ± 14,036  ns/op
ArrayNullFillBench.directLongBufferBatch  sample  4018778  293,325 ± 4,074   ns/op
ArrayNullFillBench.directByteBufferBatch  sample  4063969  313,171 ± 4,861   ns/op
ArrayNullFillBench.arrayFillManualLoop    sample  6270397  543,801 ± 20,325  ns/op
ArrayNullFillBench.arrayFill              sample  6590416  548,250 ± 13,475  ns/op

10000 elements
Benchmark                                 Mode      Cnt     Score   Error    Units
ArrayNullFillBench.arrayCopy              sample  2551851  2024,543 ± 12,533  ns/op
ArrayNullFillBench.directLongBufferBatch  sample  2958517  4469,210 ± 10,376  ns/op
ArrayNullFillBench.directByteBufferBatch  sample  2892258  4526,945 ± 33,443  ns/op
ArrayNullFillBench.arrayFill              sample  2578580  5532,063 ± 20,705  ns/op
ArrayNullFillBench.arrayFillManualLoop    sample  2562569  5550,195 ± 40,666  ns/op

PS说到ByteBuffer和Unsafe- 它们在你的情况下的主要好处是它们可以在堆外存储数据,你可以实现自己的内存释放算法,这样可以比普通的GC更好地解决你的数据结构问题.所以你不需要使它们无效,并且可以随意压缩内存.很可能这些努力不值得,因为现在你可以更容易获得性能较差且更容易出错的代码.