我在玩JMH(http://openjdk.java.net/projects/code-tools/jmh/),却偶然发现了一个奇怪的结果。
我正在对制作数组的浅拷贝进行基准测试,并且可以观察到预期的结果(遍历数组是一个坏主意,在性能方面,#clone()
,System#arraycopy()
和Arrays#copyOf()
之间没有显着差异)。
当数组的长度被硬编码时,除了System#arraycopy()
慢了四分之一之外,等等。怎么会变慢呢?
有谁知道可能是什么原因?
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
和基准类:
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;
}
}
最佳答案
像往常一样,通过研究生成的代码可以快速回答此类问题。 JMH在Linux上为您提供-prof perfasm
,在Windows上为您提供-prof xperfasm
。如果您在JDK 8u40上运行基准测试,那么您将看到(请注意,我使用-bm avgt -tu ns
来使得分更容易理解):
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表现不同?首先让我们做
-prof perfnorm
进行剖析(我删除了没关系的行):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,
ConstantSize
以某种方式可以执行更多的L1-dcache存储,但是可以减少一个LLC负载。嗯,这就是我们想要的,在恒定的情况下,更多的商店。 -prof perfasm
方便地突出显示装配中的热零件: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)
因此,讨厌的
rex.W rep stos %al,%es:(%rdi)
会花费大量时间。这会将新分配的数组清零。在ConstantSize
测试中,JVM无法关联您正在覆盖整个目标阵列,因此必须先将其预调零,然后再进入实际的阵列副本。如果您查看JDK 9b82(最新可用的)上生成的代码,那么您会看到它将两种模式折叠成非零副本,如
-prof perfasm
所示,也可以用-prof perfnorm
确认: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,所有这些用于阵列复制的纳米基准都容易受到矢量化复制存根中比对引起的奇怪性能差异的影响,但这是另一个(恐怖的)故事,我没有勇气讲。
关于java - 恒定长度的System.arraycopy,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/32834869/