Question

Java的垃圾收集器（GC）并不总是停止主线程。它可能会阻塞所有线程，或者只是并行运行而不会暂停其他线程。

我可以通过使用来衡量垃圾收集总时间程序结束时garbageCollectorMXBean.getCollectionTime();，或者使用-XX:+PrintGCDetails -XX:+PrintGCTimeStamps -verbose:gc运行VM。

但是，我想测量主线程被阻塞（即暂停）的GC部分，以便我可以测量主线程运行的确切时间。是否可以测量它？

Answer 1

我猜你的问题归结为测量“主线程运行的确切时间”。为了了解这一点，您不需要测量垃圾收集器的阻塞时间。

线程运行的时间与其CPU时间相对应。如果你还想考虑等待I / O的时间，那么这种方法对你不起作用，但是垃圾收集器暂停你的线程的时间应该可以忽略不计。

我为你准备了一个简单的概念证明：

import java.lang.management.ManagementFactory;
import java.lang.management.ThreadMXBean;
import java.util.concurrent.TimeUnit;

public class CpuTime
{
    public static void main(String[] args) {

        // We obtain the thread mx bean and check if it support CPU time measuring.
        final ThreadMXBean threadMxBean = ManagementFactory.getThreadMXBean();
        if (!threadMxBean.isThreadCpuTimeSupported())
            throw new IllegalStateException("CPU time not supported on this platform");

        // This is important because the default value is platform-dependent.
        threadMxBean.setThreadCpuTimeEnabled(true);

        // Now we start the measurement ...
        final long threadId = Thread.currentThread().getId();
        final long wallTimeStart = System.nanoTime();
        final long cpuTimeStart = threadMxBean.getThreadCpuTime(Thread.currentThread().getId());

        // ... and do stupid things to give the garbage collector some work.
        for (int i = 0; i < 1_000_000_000; i++) {
            Long l = new Long(i);
        }

        // Finally we measure the cpu and wall time.
        final long wallTime = System.nanoTime() - wallTimeStart;
        final long cpuTime = threadMxBean.getThreadCpuTime(threadId) - cpuTimeStart;

        System.out.println("CPU time:  " + TimeUnit.NANOSECONDS.toMillis(cpuTime));
        System.out.println("Wall time: " + TimeUnit.NANOSECONDS.toMillis(wallTime));
    }
}

请注意，这种方法需要进行一些调整以获得绝对精确的结果。但它应该让你开始。

Answer 2

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Answer 3

除了GC日志和MBean之外，还有JVM的perf计数器公开相关信息。

jcmd.exe PID PerfCounter.print | grep "sun.rt.safepointTime"命令将显示JVM自启动以来处于Stop-the-World状态的累计经过时间。

Answer 4

您可能需要查看jHiccup，https://www.azul.com/jhiccup/（免费和开源）。这可以衡量应用程序以外的所有内容对应用程序性能的影响。您可以使用此数据来计算由于JVM等效果（如GC）而导致主线程未运行的总时间。但是，这不会因为被其他应用程序线程阻塞而导致主线程暂停。

测量垃圾收集时间的阻塞部分

4 个答案: