当我遇到一些令人惊讶的数字时，我正在使用jmh 1.21比较JDK 8和11的性能：
Java version: 1.8.0_192, vendor: Oracle Corporation
Benchmark Mode Cnt Score Error Units
MyBenchmark.emptyMethod avgt 25 0.362 ± 0.001 ns/op
Java version: 9.0.4, vendor: Oracle Corporation
Benchmark Mode Cnt Score Error Units
MyBenchmark.emptyMethod avgt 25 0.362 ± 0.001 ns/op
Java version: 10.0.2, vendor: Oracle Corporation
Benchmark Mode Cnt Score Error Units
MyBenchmark.emptyMethod avgt 25 0.723 ± 0.001 ns/op
Java version: 11.0.1, vendor: Oracle Corporation
Benchmark Mode Cnt Score Error Units
MyBenchmark.emptyMethod avgt 25 0.724 ± 0.002 ns/op
OpenJDK 11和12的性能与OracleJDK 11相似。为简洁起见，我省略了它们的编号。
我知道微基准测试并不表示现实生活中应用程序的性能行为。不过，我很好奇这种差异的来源。 有什么想法吗？
以下是整个基准：
pom.xml ：
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>jmh</groupId>
<artifactId>empty-method</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
<name>JMH benchmark sample: Java</name>
<dependencies>
<dependency>
<groupId>org.openjdk.jmh</groupId>
<artifactId>jmh-core</artifactId>
<version>${jmh.version}</version>
</dependency>
<dependency>
<groupId>org.openjdk.jmh</groupId>
<artifactId>jmh-generator-annprocess</artifactId>
<version>${jmh.version}</version>
<scope>provided</scope>
</dependency>
</dependencies>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<jmh.version>1.21</jmh.version>
<javac.target>1.8</javac.target>
<uberjar.name>benchmarks</uberjar.name>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-enforcer-plugin</artifactId>
<version>1.4.1</version>
<executions>
<execution>
<id>enforce-versions</id>
<goals>
<goal>enforce</goal>
</goals>
<configuration>
<rules>
<requireMavenVersion>
<version>3.0</version>
</requireMavenVersion>
</rules>
</configuration>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.0</version>
<configuration>
<compilerVersion>${javac.target}</compilerVersion>
<source>${javac.target}</source>
<target>${javac.target}</target>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>3.2.1</version>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>shade</goal>
</goals>
<configuration>
<finalName>${uberjar.name}</finalName>
<transformers>
<transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
<mainClass>org.openjdk.jmh.Main</mainClass>
</transformer>
</transformers>
<filters>
<filter>
<!--
Shading signed JARs will fail without this.
http://stackoverflow.com/questions/999489/invalid-signature-file-when-attempting-to-run-a-jar
-->
<artifact>*:*</artifact>
<excludes>
<exclude>META-INF/*.SF</exclude>
<exclude>META-INF/*.DSA</exclude>
<exclude>META-INF/*.RSA</exclude>
</excludes>
</filter>
</filters>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
<pluginManagement>
<plugins>
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<version>2.6.1</version>
</plugin>
<plugin>
<artifactId>maven-deploy-plugin</artifactId>
<version>2.8.2</version>
</plugin>
<plugin>
<artifactId>maven-install-plugin</artifactId>
<version>2.5.2</version>
</plugin>
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>3.1.0</version>
</plugin>
<plugin>
<artifactId>maven-javadoc-plugin</artifactId>
<version>3.0.0</version>
</plugin>
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>3.1.0</version>
</plugin>
<plugin>
<artifactId>maven-site-plugin</artifactId>
<version>3.7.1</version>
</plugin>
<plugin>
<artifactId>maven-source-plugin</artifactId>
<version>3.0.1</version>
</plugin>
<plugin>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.22.0</version>
</plugin>
</plugins>
</pluginManagement>
</build>
</project>
src / main / java / jmh / MyBenchmark.java ：
package jmh;
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 java.util.concurrent.TimeUnit;
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class MyBenchmark
{
@Benchmark
public void emptyMethod()
{
}
}
这是我使用的Windows特定脚本。将其翻译到其他平台应该很简单：
set JAVA_HOME=C:\Program Files\Java\jdk1.8.0_192
call mvn -V -Djavac.target=1.8 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar
set JAVA_HOME=C:\Program Files\Java\jdk-9.0.4
call mvn -V -Djavac.target=9 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar
set JAVA_HOME=C:\Program Files\Java\jdk-10.0.2
call mvn -V -Djavac.target=10 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar
set JAVA_HOME=C:\Program Files\Java\oracle-11.0.1
call mvn -V -Djavac.target=11 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar
我的运行时环境是：
Apache Maven 3.6.0 (97c98ec64a1fdfee7767ce5ffb20918da4f719f3; 2018-10-24T14:41:47-04:00)
Maven home: C:\Program Files\apache-maven-3.6.0\bin\..
Default locale: en_CA, platform encoding: Cp1252
OS name: "windows 10", version: "10.0", arch: "amd64", family: "windows"
更具体地说，我正在运行Microsoft Windows [Version 10.0.17763.195]。
答案 0 :(得分：39)
您正在测量空的基准，而不是空的方法。换句话说，测量处理基准测试本身的最小基础结构代码。这很容易剖析，因为您希望在热路径上仅得到一些说明。 JMH的-prof perfasm或-prof xperfasm会在几秒钟内为您提供最热门的说明。
我认为影响是由于Thread-Local Handshakes (JEP 312)造成的，请参阅：
8u191：0.389±0.029 ns / op [到目前为止很好]
3.60% ↗ ...a2: movzbl 0x94(%r8),%r10d
0.63% │ ...aa: add $0x1,%rbp
32.82% │ ...ae: test %eax,0x1765654c(%rip) ; global safepoint poll
58.14% │ ...b4: test %r10d,%r10d
╰ ...b7: je ...a2
11.0.2：0.585±0.014 ns / op [糟糕，回归]
0.31% ↗ ...70: movzbl 0x94(%r9),%r10d
0.19% │ ...78: mov 0x108(%r15),%r11 ; reading the thread-local poll addr
25.62% │ ...7f: add $0x1,%rbp
35.10% │ ...83: test %eax,(%r11) ; thread-local safepoint poll
34.91% │ ...86: test %r10d,%r10d
╰ ...89: je ...70
11.0.2，-XX：-ThreadLocalHandshakes：0.399±0.048 ns / op [返回8u perf]
5.64% ↗ ...62: movzbl 0x94(%r8),%r10d
0.91% │ ...6a: add $0x1,%rbp
34.36% │ ...6e: test %eax,0x179be88c(%rip) ; global safepoint poll
54.79% │ ...74: test %r10d,%r10d
╰ ...77: je ...62
我认为这种现象在像这样的紧密循环中大都可见。
UPD：希望提供更多详细信息，here。