我正在尝试理解为什么使用Blackhole.consumeCPU()
是明智的?
我在Google上找到了一些关于Blackhole.consumeCPU()
的信息
有时候,当我们在多个线程上运行基准测试时,我们还想烧掉一些CPU周期来模拟运行我们的代码时的CPU业务。这不能是Thread.sleep,因为我们真正想要烧掉CPU。Blackhole.consumeCPU(long)使我们能够做到这一点。
我的示例代码:
import java.util.concurrent.TimeUnit;
import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Level;
import org.openjdk.jmh.annotations.Measurement;
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;
import org.openjdk.jmh.annotations.Warmup;
import org.openjdk.jmh.infra.Blackhole;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder;
@State(Scope.Thread)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class StringConcatAvgBenchmark {
StringBuilder stringBuilder1;
StringBuilder stringBuilder2;
StringBuffer stringBuffer1;
StringBuffer stringBuffer2;
String string1;
String string2;
/*
* re-initializing the value after every iteration
*/
@Setup(Level.Iteration)
public void init() {
stringBuilder1 = new StringBuilder("foo");
stringBuilder2 = new StringBuilder("bar");
stringBuffer1 = new StringBuffer("foo");
stringBuffer2 = new StringBuffer("bar");
string1 = new String("foo");
string2 = new String("bar");
}
@Benchmark
@Warmup(iterations = 10)
@Measurement(iterations = 100)
@BenchmarkMode(Mode.AverageTime)
public StringBuilder stringBuilder() {
// operation is very thin and so consuming some CPU
Blackhole.consumeCPU(100);
return stringBuilder1.append(stringBuilder2);
// to avoid dead code optimization returning the value
}
@Benchmark
@Warmup(iterations = 10)
@Measurement(iterations = 100)
@BenchmarkMode(Mode.AverageTime)
public StringBuffer stringBuffer() {
Blackhole.consumeCPU(100);
// to avoid dead code optimization returning the value
return stringBuffer1.append(stringBuffer2);
}
@Benchmark
@Warmup(iterations = 10)
@Measurement(iterations = 100)
@BenchmarkMode(Mode.AverageTime)
public String stringPlus() {
Blackhole.consumeCPU(100);
return string1 + string2;
}
@Benchmark
@Warmup(iterations = 10)
@Measurement(iterations = 100)
@BenchmarkMode(Mode.AverageTime)
public String stringConcat() {
Blackhole.consumeCPU(100);
// to avoid dead code optimization returning the value
return string1.concat(string2);
}
public static void main(String[] args) throws RunnerException {
Options options = new OptionsBuilder()
.include(StringConcatAvgBenchmark.class.getSimpleName())
.threads(1).forks(1).shouldFailOnError(true).shouldDoGC(true)
.jvmArgs("-server").build();
new Runner(options).run();
}
}
为什么使用
blackhole.consumeCPU(100)
后,这个基准测试的结果会更好?
更新:
使用blackhole.consumeCPU(100)
的输出结果:
Benchmark Mode Cnt Score Error Units
StringBenchmark.stringBuffer avgt 10 398,843 ± 38,666 ns/op
StringBenchmark.stringBuilder avgt 10 387,543 ± 40,087 ns/op
StringBenchmark.stringConcat avgt 10 410,256 ± 33,194 ns/op
StringBenchmark.stringPlus avgt 10 386,472 ± 21,704 ns/op
没有 blackhole.consumeCPU(100)
的输出:
Benchmark Mode Cnt Score Error Units
StringBenchmark.stringBuffer avgt 10 51,225 ± 19,254 ns/op
StringBenchmark.stringBuilder avgt 10 49,548 ± 4,126 ns/op
StringBenchmark.stringConcat avgt 10 50,373 ± 1,408 ns/op
StringBenchmark.stringPlus avgt 10 87,942 ± 1,701 ns/op
我现在想我知道为什么他们使用这个了,因为没有一些延迟的话基准测试会太快了。
通过 blackhole.consumeCPU(100)
你可以更好地测量每个基准测试并获得更显著的结果。
是这样吗?