在收集数据之前，"jcmd PID GC.class_histogram"会调用完整的GC吗? [英] Will `jcmd PID GC.class_histogram` call a full GC before collecting data?

问题描述

运行jcmd PID help GC.heap_dump时，帮助会明确说明将调用完整的GC，除非指定了-all标志:

When I run jcmd PID help GC.heap_dump, the help clearly says that full GC will be called, unless -all flag is specified:

GC.heap_dump ... 影响:高:取决于Java堆的大小和内容.除非指定'-all'选项，否则请求完整的GC. < ...>

GC.heap_dump ... Impact: High: Depends on Java heap size and content. Request a full GC unless the '-all' option is specified. <...>

-all:[可选]检查所有对象，包括不可达的对象(BOOLEAN，false)

-all : [optional] Inspect all objects, including unreachable objects (BOOLEAN, false)

如果我运行jcmd PID help GC.class_histogram，则帮助没有说明强制使用完整的GC，但是"Impact"仍然被认为是高"，并且该选项仍然具有-all标志，其行为完全相同至于GC.heap_dump:

If I run jcmd PID help GC.class_histogram, the help doesn't say anything about forcing a full GC, however "Impact" is still said to be "high", and the option still has an -all flag, which behaves exactly as for GC.heap_dump:

GC.class_histogram ... 影响:高:取决于Java堆的大小和内容.

GC.class_histogram ... Impact: High: Depends on Java heap size and content.

-all:[可选]检查所有对象，包括不可达的对象(BOOLEAN，false)

-all : [optional] Inspect all objects, including unreachable objects (BOOLEAN, false)

我尝试在几个环境中运行此命令，但未调用完整GC.但是，由于它取决于Java堆的大小和内容" ，所以我不确定.

I tried to run this command on couple of environments, and full GC was not called. However, since it "Depends on Java heap size and content" I cannot be sure.

那么jcmd PID GC.class_histogram在某些情况下可以调用完整的GC吗?如果是，那是什么?

So can jcmd PID GC.class_histogram call a full GC in some circumstances? If yes, what are they?

推荐答案

jcmd PID GC.class_histogram默认情况下将导致Full GC.

jcmd PID GC.class_histogram will cause Full GC by default.

如果目标JVM是使用-XX:+PrintGC启动的，您将看到一条日志消息，如

If the target JVM is launched with -XX:+PrintGC, you will see a log message like

// JDK 8:
[Full GC (Heap Inspection Initiated GC)  1397K->331K(126720K), 0.0023298 secs]

// JDK 9:
[15.503s][info   ][gc] GC(0) Pause Full (Heap Inspection Initiated GC) 2M->0M(8M) 8.107ms

但是，使用-all选项时，与GC.heap_dump完全一样，对于GC.class_histogram没有完全GC.在热点来源:

However, with -all option there will be no Full GC for GC.class_histogram, exactly like for GC.heap_dump. Find the proof in HotSpot sources:

void ClassHistogramDCmd::execute(DCmdSource source, TRAPS) {
  VM_GC_HeapInspection heapop(output(),
                              !_all.value() /* request full gc if false */);
  VMThread::execute(&heapop);
}

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