What actions are allowed in an JVMTI ResourceExhausted event handler?

[Thomas Stüfe]

Hi Chris,

I do not think so. The MetaspaceSize we see when we fail has the
expected value, it is not downgraded somehow.

Also, would that bug apply in a normal VM? Is UseJVMCICompiler not off
by default?

I think with this bug the chance for this error to happen may increase
but the bug itself (the fact that we post ResourceExhausted from
Compiler Thread) is older and also predates JVMCI. We see this also
with older VMs.

Thanks Thomas
On Wed, Nov 14, 2018 at 6:37 AM Chris Plummer <chris.plummer at oracle.com> wrote:
>
> On 11/13/18 9:32 PM, David Holmes wrote:
> > 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.
> >
> Does the recent fix for https://bugs.openjdk.java.net/browse/JDK-8193126
> address the metaspace failure you are seeing?
>
> Chris
> > 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
> >>
>
>