Shenandoah GC概述

文章目录

- 1_介绍
- 2_原理
- - 1.0版本
  - 2.0版本
  - 3_ShenandoahGC的执行流程
  - 4_并发转移阶段 – 并发问题

1_介绍

Shenandoah 是由Red Hat开发的一款低延迟的垃圾收集器，Shenandoah 并发执行大部分 GC 工作，包括并发的整理，堆大小对STW的时间基本没有影响。

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1、下载。Shenandoah只包含在OpenJDK中，默认不包含在内需要单独构建，可以直接下载构建好的。
下载地址：https://builds.shipilev.net/openjdk-jdk-shenandoah/

选择方式如下：
{aarch64, arm32-hflt, mipsel, mips64el, ppc64le, s390x, x86_32, x86_64}：架构，使用arch命令选择对应的的架构。
{server,zero}：虚拟机类型，选择server，包含所有GC的功能。
{release, fastdebug, Slowdebug, optimization}：不同的优化级别，选择release，性能最高。
{gcc*-glibc*, msvc*}：编译器的版本，选择较高的版本性能好一些，如果兼容性有问题（无法启动），选择较低的版本。

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2、配置。将OpenJDK配置到环境变量中，使用java –version进行测试。打印出如下内容代表成功。

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3、添加参数，运行Java程序。

-XX:+UseShenandoahGC 开启Shenandoah GC
-Xlog:gc 打印GC日志

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/** Copyright (c) 2005, 2014, Oracle and/or its affiliates. All rights reserved.* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.** This code is free software; you can redistribute it and/or modify it* under the terms of the GNU General Public License version 2 only, as* published by the Free Software Foundation.  Oracle designates this* particular file as subject to the "Classpath" exception as provided* by Oracle in the LICENSE file that accompanied this code.** This code is distributed in the hope that it will be useful, but WITHOUT* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or* FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License* version 2 for more details (a copy is included in the LICENSE file that* accompanied this code).** You should have received a copy of the GNU General Public License version* 2 along with this work; if not, write to the Free Software Foundation,* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.** Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA* or visit www.oracle.com if you need additional information or have any* questions.*/package org.sample;import com.sun.management.OperatingSystemMXBean;
import org.openjdk.jmh.annotations.*;
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;import java.lang.management.ManagementFactory;
import java.lang.management.MemoryMXBean;
import java.lang.management.MemoryUsage;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;//执行5轮预热，每次持续2秒
@Warmup(iterations = 5, time = 2, timeUnit = TimeUnit.SECONDS)
//输出毫秒单位
@OutputTimeUnit(TimeUnit.MILLISECONDS)
//统计方法执行的平均耗时
@BenchmarkMode(Mode.AverageTime)
//java -jar benchmarks.jar -rf json
@State(Scope.Benchmark)
public class MyBenchmark {//每次测试对象大小 4KB和4MB@Param({"4","4096"})int perSize;private void test(Blackhole blackhole){//每次循环创建堆内存60%对象 JMX获取到Java运行中的实时数据MemoryMXBean memoryMXBean = ManagementFactory.getMemoryMXBean();//获取堆内存大小MemoryUsage heapMemoryUsage = memoryMXBean.getHeapMemoryUsage();//获取到剩余的堆内存大小long heapSize = (long) ((heapMemoryUsage.getMax() - heapMemoryUsage.getUsed()) * 0.6);//计算循环次数long size = heapSize / (1024 * perSize);for (int i = 0; i < 4; i++) {List<byte[]> objects = new ArrayList<>((int)size);for (int j = 0; j < size; j++) {objects.add(new byte[1024 * perSize]);}blackhole.consume(objects);}}@Benchmark@Fork(value = 1,jvmArgsAppend = {"-Xms4g","-Xmx4g","-XX:+UseSerialGC"})public void serialGC(Blackhole blackhole){test(blackhole);}@Benchmark@Fork(value = 1,jvmArgsAppend = {"-Xms4g","-Xmx4g","-XX:+UseParallelGC"})public void parallelGC(Blackhole blackhole){test(blackhole);}@Benchmark@Fork(value = 1,jvmArgsAppend = {"-Xms4g","-Xmx4g"})public void g1(Blackhole blackhole){test(blackhole);}@Benchmark@Fork(value = 1,jvmArgsAppend = {"-Xms4g","-Xmx4g","-XX:+UseShenandoahGC"})public void shenandoahGC(Blackhole blackhole){test(blackhole);}public static void main(String[] args) throws RunnerException {Options opt = new OptionsBuilder().include(MyBenchmark.class.getSimpleName()).forks(1).build();new Runner(opt).run();}
}