jcmd GC.run to invoke GC fails if -XX:+DisableExplicitGC is set.

[Kevin Walls]

Hi,

jcmd GC.run to invoke GC fails if -XX:+DisableExplicitGC is set: this 
seems like a mistake?

This behaviour is obstructive for a live app that _needs_ a GC, and was 
started with -XX:+DisableExplicitGC.

DisableExplicitGC to protect from Java code calling System.gc frequently 
makes sense, but if I can attach and run a dcmd, I should have 
permission to inspect and maintain the JVM, including invoking a GC.  
(This is as the user who owns the process and can kill it off.)

The behaviour (checking DisableExplicitGC in SystemGCDCmd::execute) 
comes in with:

https://bugs.openjdk.java.net/browse/JDK-8004095
8004095: Add support for JMX interface to Diagnostic Framework and Commands

The JMX relation I suppose suggests we didn't want JMX to override 
DisableExplicitGC by way of using a jcmd/DCmd.

But also, we now have:
8072913: [REDO] GCCause should distinguish jcmd GC.run from System.gc()
Summary: GCCause which is caused by GC.run diagnostic command should be 
different from System.gc() .

..at least the causes are distinct.

I don't think we document this clearly.  Our comment in globals.hpp is 
""Ignore calls to System.gc()".  I don't think we say anywhere that jcmd 
is subject to being disabled by the flag.

Interested to hear any reason in favour of the current behaviour!  If 
there's nothing, I'll log a bug and ask for review of the change to 
remove it...

Thanks
Kevin