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

[Mikael Gerdin]

Hi Kevin,

On 2017-08-22 16:38, Kevin Walls wrote:
> 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...

There were some discussions earlier around this area and I came up with 
the idea of having a "force" option to the GC.run command to override 
DisableExplicitGC.
The comments in globals.hpp are a notoriously bad spec for the flags 
since they are only ever present in debug builds of the JVM.

Thanks
/Mikael

> 
> Thanks
> Kevin
> 
> 
>