RFR: 8175375: MemoryPoolMXBean.getCollectionUsage() does not works with -XX:+UseG1GC -XX:+ExplicitGCInvokesConcurrent

[Thomas Schatzl]

Hi Fairoz,

On Wed, 2018-09-05 at 06:55 -0700, Fairoz Matte wrote:
> Hi,
> 
> Please review this change.
> MemoryPoolMXBean.getCollectionUsage() does not works in G1 with "-
> XX:+ExplicitGCInvokesConcurrent", it always returns 0.
> In case of "+ExplicitGCInvokesConcurrent" call to heap usage
> collection doesn't honor full gc.
> With the change of  8175375, it will check for full GC request and
> set the heap usage collection accordingly.
> 
> JBS issue - https://bugs.openjdk.java.net/browse/JDK-8175375 
> Webrev - http://cr.openjdk.java.net/~fmatte/8175375/webrev.00/

  I think this change breaks actual accounting: the G1MonitoringScopes
will immediately be destructed at the end of the if/else blocks, so
they will not be measuring values correctly.

I think you need to set the full_gc parameter of the scope depending on
the gc cause instead.

I am kind of surprised that gccause is GCCause::_java_lang_system_gc
for this young collection, but that may be correct. 

Thanks,
  Thomas