Will `jcmd PID GC.class_histogram` call a full GC before collecting data?

问题

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

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

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

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

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.

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

回答1:

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

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

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);
}

回答2:

You could try to run jcmd PID GC.run before jcmd PID GC.class_histogram

 jcmd PID help GC.run

 PID:
 GC.run
 Call java.lang.System.gc().

 Impact: Medium: Depends on Java heap size and content.

 Syntax: GC.run

But I'm not sure that there is any guarantee that JVM will actually perform GC upon that request. Javadoc for System.gc() says this:

 When control returns from the method call, the Java Virtual
 Machine has made a best effort to reclaim space from all discarded objects

来源：https://stackoverflow.com/questions/48446767/will-jcmd-pid-gc-class-histogram-call-a-full-gc-before-collecting-data

标签

java

performance

garbage-collection

jvm