What actions are allowed in an JVMTI ResourceExhausted event handler?

[David Holmes]

Hi Thomas,

On 14/11/2018 6:50 am, Thomas Stüfe wrote:
> Hi all,
> 
> We have a client using CloudFoundry and its "jvmkill" agent. That is a
> tiny JVMTI agent (see https://github.com/cloudfoundry/jvmkill) which
> subscribes to the JVMTI ResourceExhausted Event. In the handler it
> then does call JVMTI FollowReferences() to produce a heap histogram.
> 
> The thing is, at our client we seem to run out of Metaspace in a
> compiler thread. That thread normally would swallow the Metaspace OOM
> and just bailout from the compilation. But as part of the metaspace
> OOME handling the ResourceExhausted event gets posted, the handler
> then uses JVMTI FollowReferences() and attempts to print out the heap
> histogram, then runs into a guarantee since the compiler thread cannot
> call java methods.
> 
> My question is: are there any limitations about what one can do inside
> a ResourceExhausted event handler?

Not specified no. But the reality of JVM TI is that you can't anticipate 
every execution context and there are times when there are implicit 
constraints imposed by the implementation.

In this case I think we have a mismatch between the fact we post the 
event from the compiler thread, but that a compiler thread is not a true 
"Java thread" and so can not execute arbitrary JNI or JVM TI code, or in 
particular can not lead to executing Java code. I think we should not be 
posting the event from the compiler thread in this case.

Cheers,
David

> I checked the https://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html
> documentation, but I cannot find any mentioning of limitations in that
> case.
> 
> Thanks and Best Regards, Thomas
>