OOM counters

[David Holmes]

Hi Matthias,


Those counters, as I recall, were added to the hs_err report purely to 
show if an application/test had been "playing too close to the edge" 
before it crashed. They were never intended as user-inspectable values.

I have no opinion as to whether they should be.

David

On 19/09/2018 7:12 PM, Baesken, Matthias 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  (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
>