RFR: 8259647: Add support for JFR event ObjectCountAfterGC to Shenandoah

[earthling-amzn]

On Wed, 3 Feb 2021 20:05:33 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Please review this patch that adds JFR ObjectCountAfterGC event support.
> 
> AFAICT, the event is off by default. If it is enabled, it distorts Shenandoah pause characteristics, since it performs heap walk during final mark pause. 
> 
> When event is disabled:
> `[191.033s][info][gc,stats] Pause Init Mark (G)                 454 us`
> `[191.033s][info][gc,stats] Pause Init Mark (N)                  13 us`
> 
> When event is enabled:
> `[396.631s][info][gc,stats] Pause Final Mark (G)              43199 us`
> `[396.631s][info][gc,stats] Pause Final Mark (N)              42982 us`
> 
> Test:
> - [x] hotspot_gc_shenandoah

That certainly is bad news for pause times. Do you think it'd be feasible to "piggyback" the object count calculation on concurrent marking? Might address https://bugs.openjdk.java.net/browse/JDK-8258431 also.

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PR: https://git.openjdk.java.net/jdk/pull/2386