On Wed, Nov 14, 2018 at 6:58 AM David Holmes <david.hol...@oracle.com> wrote: > > On 14/11/2018 3:37 pm, Thomas Stüfe wrote: > > On Wed, Nov 14, 2018, 06:32 David Holmes <david.hol...@oracle.com > > <mailto:david.hol...@oracle.com> 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. > > > > Cheers, > > David > > > > > > Hi David, > > > > Yes I thought so too. I'll prepare a fix. > > My thought on the fix is that we need to check if > Thread::current()->can_call_java(). And that should probably be inside > the JvmtiExport::should_post_xxx implementation. >
I wonder whether that may be too harsh. JVMTI agents may not necessarily call into java as reaction to ResourceExhausted. I would have limited this to !CompilerThread, and only in Metaspace. Also, looking at CompilerThread::can_call_java(), I see that we return true for jvmci compilers. Still do we want to post it there? But I am not sure. What do you think? > Cheers, > David > > > Thanks, Thomas > > > > > > > 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 > > > > >
