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

[Thomas Schatzl]

On Thu, 11 Mar 2021 14:50:10 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Jaroslav Bachorik has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Remove unused field
>
> 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.

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