长度恒定的 System.arraycopy

2022-09-04 03:49:41

我正在玩JMH(http://openjdk.java.net/projects/code-tools/jmh/),我只是偶然发现了一个奇怪的结果。

我正在对制作数组的浅副本的方法进行基准测试,并且我可以观察到预期的结果(循环遍历数组是一个坏主意,并且性能和 性能之间没有显着差异)。#clone()System#arraycopy()Arrays#copyOf()

除了当数组的长度是硬编码时,它慢了四分之一...等等,什么?这怎么能慢一点呢?System#arraycopy()

有没有人知道可能的原因是什么?

结果(吞吐量):

# JMH 1.11 (released 17 days ago)
# VM version: JDK 1.8.0_05, VM 25.5-b02
# VM invoker: /Library/Java/JavaVirtualMachines/jdk1.8.0_05.jdk/Contents/Home/jre/bin/java
# VM options: -Dfile.encoding=UTF-8 -Duser.country=FR -Duser.language=fr -Duser.variant
# Warmup: 20 iterations, 1 s each
# Measurement: 20 iterations, 1 s each
# Timeout: 10 min per iteration
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Throughput, ops/time

Benchmark                                            Mode  Cnt         Score         Error  Units
ArrayCopyBenchmark.ArraysCopyOf                     thrpt   20  67100500,319 ±  455252,537  ops/s
ArrayCopyBenchmark.ArraysCopyOf_Class               thrpt   20  65246374,290 ±  976481,330  ops/s
ArrayCopyBenchmark.ArraysCopyOf_Class_ConstantSize  thrpt   20  65068143,162 ± 1597390,531  ops/s
ArrayCopyBenchmark.ArraysCopyOf_ConstantSize        thrpt   20  64463603,462 ±  953946,811  ops/s
ArrayCopyBenchmark.Clone                            thrpt   20  64837239,393 ±  834353,404  ops/s
ArrayCopyBenchmark.Loop                             thrpt   20  21070422,097 ±  112595,764  ops/s
ArrayCopyBenchmark.Loop_ConstantSize                thrpt   20  24458867,274 ±  181486,291  ops/s
ArrayCopyBenchmark.SystemArrayCopy                  thrpt   20  66688368,490 ±  582416,954  ops/s
ArrayCopyBenchmark.SystemArrayCopy_ConstantSize     thrpt   20  48992312,357 ±  298807,039  ops/s

基准测试类:

import java.util.Arrays;
import java.util.concurrent.TimeUnit;

import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.OutputTimeUnit;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.Setup;
import org.openjdk.jmh.annotations.State;

@State(Scope.Benchmark)
@BenchmarkMode(Mode.Throughput)
@OutputTimeUnit(TimeUnit.SECONDS)
public class ArrayCopyBenchmark {

    private static final int LENGTH = 32;

    private Object[] array;

    @Setup
    public void before() {
        array = new Object[LENGTH];
        for (int i = 0; i < LENGTH; i++) {
            array[i] = new Object();
        }
    }

    @Benchmark
    public Object[] Clone() {
        Object[] src = this.array;
        return src.clone();
    }

    @Benchmark
    public Object[] ArraysCopyOf() {
        Object[] src = this.array;
        return Arrays.copyOf(src, src.length);
    }

    @Benchmark
    public Object[] ArraysCopyOf_ConstantSize() {
        Object[] src = this.array;
        return Arrays.copyOf(src, LENGTH);
    }

    @Benchmark
    public Object[] ArraysCopyOf_Class() {
        Object[] src = this.array;
        return Arrays.copyOf(src, src.length, Object[].class);
    }

    @Benchmark
    public Object[] ArraysCopyOf_Class_ConstantSize() {
        Object[] src = this.array;
        return Arrays.copyOf(src, LENGTH, Object[].class);
    }

    @Benchmark
    public Object[] SystemArrayCopy() {
        Object[] src = this.array;
        int length = src.length;
        Object[] array = new Object[length];
        System.arraycopy(src, 0, array, 0, length);
        return array;
    }

    @Benchmark
    public Object[] SystemArrayCopy_ConstantSize() {
        Object[] src = this.array;
        Object[] array = new Object[LENGTH];
        System.arraycopy(src, 0, array, 0, LENGTH);
        return array;
    }

    @Benchmark
    public Object[] Loop() {
        Object[] src = this.array;
        int length = src.length;
        Object[] array = new Object[length];
        for (int i = 0; i < length; i++) {
            array[i] = src[i];
        }
        return array;
    }

    @Benchmark
    public Object[] Loop_ConstantSize() {
        Object[] src = this.array;
        Object[] array = new Object[LENGTH];
        for (int i = 0; i < LENGTH; i++) {
            array[i] = src[i];
        }
        return array;
    }
}

答案 1

像往常一样,通过研究生成的代码可以快速回答这些问题。JMH在Linux和Windows上为您提供。如果你在JDK 8u40上运行基准测试,那么你会看到(注意我曾经让分数更容易理解):-prof perfasm-prof xperfasm-bm avgt -tu ns

Benchmark                         Mode  Cnt   Score   Error  Units
ACB.SystemArrayCopy               avgt   25  13.294 ± 0.052  ns/op
ACB.SystemArrayCopy_ConstantSize  avgt   25  16.413 ± 0.080  ns/op

为什么这些基准测试的性能不同?让我们首先进行剖析(我删除了无关紧要的行):-prof perfnorm

Benchmark                                     Mode  Cnt    Score    Error  Units
ACB.SAC                                       avgt   25   13.466 ±  0.070  ns/op
ACB.SAC:·CPI                                  avgt    5    0.602 ±  0.025   #/op
ACB.SAC:·L1-dcache-load-misses                avgt    5    2.346 ±  0.239   #/op
ACB.SAC:·L1-dcache-loads                      avgt    5   24.756 ±  1.438   #/op
ACB.SAC:·L1-dcache-store-misses               avgt    5    2.404 ±  0.129   #/op
ACB.SAC:·L1-dcache-stores                     avgt    5   14.929 ±  0.230   #/op
ACB.SAC:·LLC-loads                            avgt    5    2.151 ±  0.217   #/op
ACB.SAC:·branches                             avgt    5   17.795 ±  1.003   #/op
ACB.SAC:·cycles                               avgt    5   56.677 ±  3.187   #/op
ACB.SAC:·instructions                         avgt    5   94.145 ±  6.442   #/op

ACB.SAC_ConstantSize                          avgt   25   16.447 ±  0.084  ns/op
ACB.SAC_ConstantSize:·CPI                     avgt    5    0.637 ±  0.016   #/op
ACB.SAC_ConstantSize:·L1-dcache-load-misses   avgt    5    2.357 ±  0.206   #/op
ACB.SAC_ConstantSize:·L1-dcache-loads         avgt    5   25.611 ±  1.482   #/op
ACB.SAC_ConstantSize:·L1-dcache-store-misses  avgt    5    2.368 ±  0.123   #/op
ACB.SAC_ConstantSize:·L1-dcache-stores        avgt    5   25.593 ±  1.610   #/op
ACB.SAC_ConstantSize:·LLC-loads               avgt    5    1.050 ±  0.038   #/op
ACB.SAC_ConstantSize:·branches                avgt    5   17.853 ±  0.697   #/op
ACB.SAC_ConstantSize:·cycles                  avgt    5   66.680 ±  2.049   #/op
ACB.SAC_ConstantSize:·instructions            avgt    5  104.759 ±  4.831   #/op

因此,以某种方式执行更多的L1-dcache存储,但LLC负载少了一个。嗯,这就是我们正在寻找的,在不断的情况下有更多的商店。 方便地突出显示装配中的热零件:ConstantSize-prof perfasm

default:

  4.32%    6.36%   0x00007f7714bda2dc: movq   $0x1,(%rax)            ; alloc
  0.09%    0.04%   0x00007f7714bda2e3: prefetchnta 0x100(%r9)
  2.95%    1.48%   0x00007f7714bda2eb: movl   $0xf80022a9,0x8(%rax)
  0.38%    0.18%   0x00007f7714bda2f2: mov    %r11d,0xc(%rax)
  1.56%    3.02%   0x00007f7714bda2f6: prefetchnta 0x140(%r9)
  4.73%    2.71%   0x00007f7714bda2fe: prefetchnta 0x180(%r9)

ConstantSize:

  0.58%    1.22%   0x00007facf921132b: movq   $0x1,(%r14)            ; alloc
  0.84%    0.72%   0x00007facf9211332: prefetchnta 0xc0(%r10)
  0.11%    0.13%   0x00007facf921133a: movl   $0xf80022a9,0x8(%r14)
  0.21%    0.68%   0x00007facf9211342: prefetchnta 0x100(%r10)
  0.50%    0.87%   0x00007facf921134a: movl   $0x20,0xc(%r14)
  0.53%    0.82%   0x00007facf9211352: mov    $0x10,%ecx
  0.04%    0.14%   0x00007facf9211357: xor    %rax,%rax
  0.34%    0.76%   0x00007facf921135a: shl    $0x3,%rcx
  0.50%    1.17%   0x00007facf921135e: rex.W rep stos %al,%es:(%rdi) ; zeroing
 29.49%   52.09%   0x00007facf9211361: prefetchnta 0x140(%r10)
  1.03%    0.53%   0x00007facf9211369: prefetchnta 0x180(%r10)  

所以有讨厌消耗了大量时间。这会将新分配的数组清零。在测试中,JVM 无法关联您正在覆盖整个目标数组,因此在深入研究实际的数组副本之前,它必须将其置零。rex.W rep stos %al,%es:(%rdi)ConstantSize

如果您查看 JDK 9b82(最新可用)上生成的代码,那么您将看到它将两种模式折叠在非零副本中,如 您所见,并且还可以通过以下方式确认:-prof perfasm-prof perfnorm

Benchmark                                     Mode  Cnt    Score    Error  Units
ACB.SAC                                       avgt   50   14.156 ±  0.492  ns/op
ACB.SAC:·CPI                                  avgt    5    0.612 ±  0.144   #/op
ACB.SAC:·L1-dcache-load-misses                avgt    5    2.363 ±  0.341   #/op
ACB.SAC:·L1-dcache-loads                      avgt    5   28.350 ±  2.181   #/op
ACB.SAC:·L1-dcache-store-misses               avgt    5    2.287 ±  0.607   #/op
ACB.SAC:·L1-dcache-stores                     avgt    5   16.922 ±  3.402   #/op
ACB.SAC:·branches                             avgt    5   21.242 ±  5.914   #/op
ACB.SAC:·cycles                               avgt    5   67.168 ± 20.950   #/op
ACB.SAC:·instructions                         avgt    5  109.931 ± 35.905   #/op

ACB.SAC_ConstantSize                          avgt   50   13.763 ±  0.067  ns/op
ACB.SAC_ConstantSize:·CPI                     avgt    5    0.625 ±  0.024   #/op
ACB.SAC_ConstantSize:·L1-dcache-load-misses   avgt    5    2.376 ±  0.214   #/op
ACB.SAC_ConstantSize:·L1-dcache-loads         avgt    5   28.285 ±  2.127   #/op
ACB.SAC_ConstantSize:·L1-dcache-store-misses  avgt    5    2.335 ±  0.223   #/op
ACB.SAC_ConstantSize:·L1-dcache-stores        avgt    5   16.926 ±  1.467   #/op
ACB.SAC_ConstantSize:·branches                avgt    5   19.469 ±  0.869   #/op
ACB.SAC_ConstantSize:·cycles                  avgt    5   62.395 ±  3.898   #/op
ACB.SAC_ConstantSize:·instructions            avgt    5   99.891 ±  5.435   #/op

当然,所有这些用于数组复制的纳米基准都容易受到矢量化复制存根中奇怪的对齐引起的性能差异的影响,但这是另一个(恐怖)故事,我没有勇气去讲述。


答案 2