JFR Event for Full GCs

[Thomas Schatzl]

Hi,

On Wed, 2018-12-05 at 12:23 -0500, Andrew Azores wrote:
> Hmm. It's along the right lines (and I had overlooked it before) but
> I don't think this event meets the requirement. I modified my JMC
> testing rule to look for those events while running my reproducer
> applet with -XX:+UseG1GC -XX:+PrintGCDetails. My run saw one
> OldGarbageCollection event emitted but the GC log output does not
> show any Full GC events. I also tried reading through the JFR and GC
> sources again to trace the execution path where this event is
> emitted, and although I wasn't 100% confident, it didn't look to me
> like this event is 1:1 with Full collections - perhaps a Full
> collection implies this event emission but not vice-versa? From my
> limited understanding of the GC phases and generations this would
> seem to make sense.

  iirc a completed marking cycle also counts as "full" collection in
some cases as it completely considers the old generation too.
PrintGCDetails should show them. 

(Young collections may actually also reclaim space in old gen but does
not send this event - so actually this OldGarbageCollection event may
actually be considered as some kind of backwards compatibility hack for
G1 in any case)

Thanks,
  Thomas