ParallelGCFailedAllocation 的大“同步"值 [英] Large 'sync' value for ParallelGCFailedAllocation

问题描述

有时我会在我的 gc.log 中看到长时间的停顿

Sometimes I see long pauses in my gc.log

2014-07-18T18:42:26.137+0400: 7846.980: [GC [PSYoungGen: 86267K->10032K(87744K)] 251198K-175254K(272512K), 0.0066190 秒] [时间: 用户=0.08 系统, 实数=0.01 秒]

2014-07-18T18:42:26.137+0400: 7846.980: [GC [PSYoungGen: 86267K->10032K(87744K)] 251198K-175254K(272512K), 0.0066190 secs] [Times: user=0.08 sys=0.00, real=0.01 secs]

应用程序线程停止的总时间:0.6394170秒

Total time for which application threads were stopped: 0.6394170 seconds

应用程序线程停止的总时间:0.0014570 秒

Total time for which application threads were stopped: 0.0014570 seconds

但这些暂停不会影响 GC 时间和 CPU 使用时间.

But these pauses don't affect GC time and CPU usage time.

安全点统计(+XX:PrintSafePointStatistics):

Safepoint statistics (+XX:PrintSafePointStatistics):

vmop [threads: total initial_running wait_to_block] [time: spin block sync cleanup vmop] page_trap_count7846.348:ParallelGCFailedAllocation [370 0 2] [0 0 630 1 7] 0

vmop [threads: total initially_running wait_to_block] [time: spin block sync cleanup vmop] page_trap_count 7846.348: ParallelGCFailedAllocation [ 370 0 2 ] [ 0 0 630 1 7 ] 0

(同步时间 = 630 毫秒)在这种情况下，同步"是什么意思?

(sync time = 630ms) What does 'sync' mean in this context?

推荐答案

sync 这里是到达安全点所花费的总时间.换句话说，就是从安全点请求到所有可运行的 Java 线程最终被阻塞的那一刻之间的延迟.

sync here is the total time spent to reach a safepoint. In other words, it is the latency between the safepoint request and the moment when all runnable Java threads are finally blocked.

由于以下原因之一，安全点同步有时可能需要很长时间:

The safepoint synchronization may sometimes take too long due to one of the following reasons:

• 应用程序没有获得足够的 CPU 时间，因为操作系统正忙于密集的磁盘 I/O，或者当它开始交换时，或者当另一个高优先级进程占用所有 CPU 时间时.
• 一些 Java 线程执行长时间的不可中断操作，例如使用 System.arraycopy()、clone()、ByteBuffer.get() 等复制大量数据.
• MappedByteBuffer I/O.

• The application does not receive enough CPU time because the OS is busy with intensive disk I/O or when it starts swapping or when another high-priority process takes all CPU time.
• Some Java thread performs long non-interruptible operation, e.g. copies large amount of data using System.arraycopy(), clone(), ByteBuffer.get() etc.
• MappedByteBuffer I/O.

要记录在 500 毫秒内无法在安全点上阻塞的线程的名称，请使用

To log the names of the threads that could not get blocked on a safepoint in 500 ms use

-XX:+SafepointTimeout -XX:SafepointTimeoutDelay=500

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