Hi JC, yes it is good to have for monitoring. However another application we have is that people wanted to trigger actions on an OOM occurrence , like doing some restarts etc .
For these kind of actions they had in our proprietary JVM some kind of queue containing the latest “prominent” errors (OOMs , but also some others). So they periodically checked the queue and in case an OOM is found , they started the desired action . There is (to some extend) a replacement for this in the VM, the XX flags ExitOnOutOfMemoryError and OnOutOfMemoryError=<command> . However, these flags are not very flexible (always exiting is not what all people want), and executing a command forks and execs a shell (see vmError) which is quite some overhead and also a bit problematic in some cases. Best regards, Matthias From: JC Beyler <jcbey...@google.com> Sent: Mittwoch, 19. September 2018 16:49 To: Baesken, Matthias <matthias.baes...@sap.com> Cc: hotspot-dev Source Developers <hotspot-...@openjdk.java.net>; serviceability-dev@openjdk.java.net Subject: Re: OOM counters Hi Matthias, In the case of an OOM, what's the use-case here? Are there cases where you are recuperating often from OOMs that the counters have different values and it's interesting to monitor? Or is it to determine which of the three got incremented by the OOM that was thrown? Finally, when would you call the MemoryMXBean new call: if you are recuperating from OOMs, I could imagine that you might be able to get the counters periodically and monitor this; if you're not, you'd be doing this call during an OOM? Thanks, Jc On Wed, Sep 19, 2018 at 2:13 AM Baesken, Matthias <matthias.baes...@sap.com<mailto:matthias.baes...@sap.com>> wrote: Hello, Currently we have already counters in the Hotspot codebase counting Java heap, Metaspace and class metaspace related OOMs. See declarations: jdk/src/hotspot/share/utilities/exceptions.hpp 107 // Count out of memory errors that are interesting in error diagnosis 108 static volatile int _out_of_memory_error_java_heap_errors; 109 static volatile int _out_of_memory_error_metaspace_errors; 110 static volatile int _out_of_memory_error_class_metaspace_errors; output : jdk/src/hotspot/share/utilities/exceptions.cpp 460void Exceptions::print_exception_counts_on_error(outputStream* st) { 461 print_oom_count(st, "java_heap_errors", _out_of_memory_error_java_heap_errors); 462 print_oom_count(st, "metaspace_errors", _out_of_memory_error_metaspace_errors); 463 print_oom_count(st, "class_metaspace_errors", _out_of_memory_error_class_metaspace_errors); 464 if (_stack_overflow_errors > 0) { 465 st->print_cr("StackOverflowErrors=%d", _stack_overflow_errors); 466 } 467 if (_linkage_errors > 0) { 468 st->print_cr("LinkageErrors=%d", _linkage_errors); 469 } 470} But currently the output is only done from vmError.cpp , in case of a) error reporting b) jcmd vm.info<http://vm.info> (in case exceptions happened, otherwise the section is not printed) (see void VMError::print_vm_info(outputStream* st) { ... } ) It would be interesting for us to get the values of the existing counters via JDK coding. In our proprietary JVM we had a similar mechanism. Is there anything planned for this (or even already something present) ? There exists already a class java/lang/management/MemoryMXBean.java with a management interface for the memory system of the Java virtual machine. It contains some "getter" - methods like public int getObjectPendingFinalizationCount(); public MemoryUsage getHeapMemoryUsage(); Do you think we could add a method (or methods) for accessing the existing OOM counters ( for example, public OomCounters getOutOfMemoryCounters(); ) ? Best regards, Matthias -- Thanks, Jc
