RFR: 8258431: Provide a JFR event with live set size estimate [v12]

[Stefan Johansson]

On Tue, 16 Mar 2021 11:06:22 GMT, Jaroslav Bachorik <jbachorik at openjdk.org> wrote:

> > Sure, but for the event to be useful we want the reported value to be as close to the reality as possible. I don't understand why you want the lower bound, can you explain why? I would go for the upper bound, which in that case would be used() at the end of the GC. I know used() is not perfect, but for G1 this is the best "cheap" value we have for liveness at the end of any GC.
> 
> Mostly because `used()` will report all live instances and potential garbage and will make it inconsistent with what the other GCs would report.
> 
The other STW GCs do report the same, right? 

> > So as a middle road I would suggest to update G1CollectedHeap::gc_epilogue(bool full) to include:
> > set_live(used());
> > With this you don't need the changes for the G1FullCollector. The liveness calculated at Remark would be used until the next young collection and I think here is where some improvements could be made. During the mixed phase a better solution would make use of the liveness information we have for the old regions together with what is newly allocated, but this needs further investigation.
> 
> This sounds interesting. Let me try this out.

Glad you like the idea :) I did a quick test locally and it shows the trend ok, even if it is an over estimate of live: 
  live = 1.1 GB
  live = 1.2 GB
  live = 1.5 GB
  live = 1.7 GB
  live = 2.1 GB
  name = "G1Old"
  live = 1.4 GB
  live = 1.6 GB
  live = 1.8 GB
  live = 2.0 GB
  live = 2.3 GB
  live = 2.5 GB
  live = 2.8 GB
  live = 3.1 GB
  live = 3.3 GB
  live = 3.7 GB
  live = 4.0 GB
  live = 4.3 GB
  name = "G1Old"
  live = 1.2 GB
G1Old is from concurrent mark events.

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PR: https://git.openjdk.java.net/jdk/pull/2579