如何在 JVM 中强制/重现 FullGC?

问题描述

有没有办法在 JVM 中强制/重现 FullGC x 秒?基本上我需要这个来验证某些基于心跳的应用程序(zookeeper 的客户端)中问题的根本原因

Is there a way to force/reproduce FullGC in JVM for x seconds ? Basically I needed this to verify root cause of an issue in certain heart beat based app ( a client of zookeeper)

EDIT:是否 unix 命令 kill -STOP 和 kill -CONT 模拟 FullGC(停止世界行为)?

EDIT: Does unix command kill -STOP <pid> and kill -CONT <pid> simulate FullGC ( stop the world behaviour) ?

推荐答案

您可以在 HotSpot JVM 上模拟一个很长的 stop-the-world 事件，从用户的角度来看类似于 FullGC.

You can simulate a very long stop-the-world event on HotSpot JVMs which is similar to FullGC from user's point of view.

HotSpot 不会将 safepoints 计算在内int 循环，因为它假设它们将足够快"终止(在这种情况下，服务器编译器将生成更优化的循环代码).即使是停止世界也必须等到这个循环结束.在下面的例子中，我们有一个非常紧凑的循环，它在没有安全点轮询的情况下进行小而昂贵的计算:

HotSpot doesn't put safepoints into counted int loops, because it assumes that they will terminate just "fast enough"(In this case server compiler will generate more optimal loop code). Even a stop-the-world will have to wait until this loop will finish. In the following example we have very tight loop which do small but expensive computations without safepoint polling:

public static double slowpoke(int iterations) {
    double d = 0;
    for (int j = 1; j < iterations; j++) {
        d += Math.log(Math.E * j);
    }
    return d;
}

为了像暂停一样重现 FullGC，你可以使用这样的方法:

In order to reproduce FullGC like pause you can use something like this:

public class SafepointTest {

    public static double slowpoke(int iterations) {
        double d = 0;
        for (int j = 1; j < iterations; j++) {
            d += Math.log(Math.E * j);
        }
        return d;
    }

    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread() {
            @Override
            public void run() {
                double sideEffect = 0;
                for (int i = 0; i < 10000; i++) {
                    sideEffect = slowpoke(999999999);
                }
                System.out.println("result = " + sideEffect);
            }
        };
        thread.start();

        new Thread(){
            @Override
            public void run() {
                long timestamp = System.currentTimeMillis();
                while (true){
                    System.out.println("Delay " + (System.currentTimeMillis() - timestamp));
                    timestamp = System.currentTimeMillis();
                    //trigger stop-the-world
                    System.gc();
                }
            }
        }.start();
        thread.join();
    }
}

结果:

Delay 5
Delay 4
Delay 30782
Delay 21819
Delay 21966
Delay 22812
Delay 22264
Delay 21988

为了增加延迟，只需更改slowpoke(int iterations)函数的参数值.

In order to increase delay just change argument value for slowpoke(int iterations) function.

以下是有用的诊断命令:

Here is useful diagnostic commands:

• -XX:+PrintGCApplicationStoppedTime 这实际上会将所有安全点的暂停时间报告到 GC 日志中.不幸的是，此选项的输出缺少时间戳.
• -XX:+PrintSafepointStatistics –XX:PrintSafepointStatisticsCount=1 这两个选项将强制 JVM 在每个安全点之后报告原因和时间.

• -XX:+PrintGCApplicationStoppedTime this will actually report pause time for all safepoints into GC log. Unfortunately output from this option lacks timestamps.
• -XX:+PrintSafepointStatistics –XX:PrintSafepointStatisticsCount=1 this two options will force JVM to report reason and timings after each safepoint.

关于从用户的角度来看，kill -STOP 和 kill -CONT 与 STW 具有相同的语义，即应用程序不响应任何请求.但是，这需要访问命令行并且不消耗资源(CPU、内存).

Regarding to from user's point of view kill -STOP and kill -CONT have the same semantics as STW i.e. application doesn't respond on any request. However, this requires access to command line and doesn't consume resources(CPU, memory).

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