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

[Jaroslav Bachorik]

On Thu, 11 Mar 2021 15:42:51 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> I am leaving this as "request changes" for now as the question I had earlier about that after G1 Full gc the value of `_live_estimate` still seems unanswered and there does not seem to be code in this change for this. Is this intentional? (Not even setting the live bytes to `used()` which at that point would be a good estimate)
>> 
>> There is another PR (#2760) that implements something like that although I haven't looked at it in detail.
>> 
>> Otherwise looks okay.
>
> Started reviewing PR #2760, and it implements liveness calculation for G1 full gc. I also suggested [there](https://github.com/openjdk/jdk/pull/2760#discussion_r592449837) to extract this functionality out into an extra CR. Maybe you can work together.

@tschatzl @Hamlin-Li 
Would it be ok to set the live estimate to the `used()` value at the end of `G1FullCollector::phase4_do_compaction()` method to have something suboptimal but working and refine in https://github.com/openjdk/jdk/pull/2760 (or a subsequent ticket/PR once both parts are ready)?

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