Question

我已经设置了一个JMH基准测试来测量从空数组中使用null，Arrays.fill的更快System.arraycopy，将DirectByteBuffer归零或将unsafe内存块归零试图回答这个question 让我们暂且不谈直接分配的内存是一种罕见的情况，并讨论我的基准测试结果。

这里是JMH基准代码段（full code available via a gist），包括@apangin在原始帖子unsafe.setMemory和byteBuffer.put(byte[], offset, length)中建议的longBuffer.put(long[], offset, length)案例@扬谢弗：

@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void arrayFill() {
    Arrays.fill(objectHolderForFill, 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.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 (except arrayFill for scale) from further tests

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.arrayFill              sample  6862928  518,886 ± 6,372   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  5689507  5028,592 ± 9,074   ns/op

请您澄清以下问题：

1. Why `unsafeArraySetMemory` is a bit but slower than `unsafeArrayManualLoop`?
2. Why directByteBuffer is 2.5X-5X slower than others?

Answer 1

为什么unsafeArraySetMemory有点但比unsafeArrayManualLoop慢？

我的猜测是它没有针对精确设置多个长片进行优化。它必须检查你是否有东西，而不是8的倍数。

为什么directByteBuffer比其他人快一个数量级？

一个数量级大约是10倍，大约慢2.5倍。它必须检查每次访问并更新字段而不是局部变量。

注意：我发现JVM并不总是使用Unsafe循环展开代码。您可以尝试自己动手，看看它是否有帮助。

注意：本机代码可以使用XMM 128位指令，并且越来越多地使用它，这就是复制可能如此之快的原因。可以在Java 10中访问XMM指令。

Answer 2

比较有点不公平。在使用Array.fill和System.arraycopy时，您正在使用单个操作，但您在putLong案例中使用了DirectByteBuffer的循环和多次调用。如果你看一下putLong的实现，你会发现有很多事情要做，例如检查可访问性。您应该尝试使用像put(long[] src, int srcOffset, int longCount)这样的批处理操作，看看会发生什么。

为什么直接记忆＆＃39;阵列＆＃39;比通常的Java数组更清晰？

2 个答案: