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

[Kevin Walls]

Hi Mikael, thanks yes, it could be a separate cmd GC.runForce... However 
I was thinking if you can get as far as having your jcmd executed, you 
really _do_ want to run that collection.  Whatever behaviour you were 
protecting against when you chose the command-line arguments, you would 
only ever want to override if you run the jcmd to invoke GC... 8-)

I'll convert this to a review request for removing that check, will post 
that shortly.  This would be changing the behaviour, but I don't think 
it contradicts anything we document, and we seem to have added the check 
without documenting it.

Thanks
Kevin


On 28/08/2017 16:01, Mikael Gerdin wrote:
> 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
>>
>>
>>