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

[Zhengyu Gu]

On Thu, 4 Feb 2021 18:44:24 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> 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.

It dose not just count number of objects, but number of objects by type,  much more than liveness counting. Just add a branch in hot marking loop, I can foresee negative impact on performance.

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