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

[Roman Kennke]

On Thu, 4 Feb 2021 18:17:46 GMT, earthling-amzn <github.com+71722661+earthling-amzn 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.

This could certainly be done, in a similar fashion as liveness counting. However, it would have to be done such that it only actually counts objects when JFR is requesting it, and otherwise stays out of the way, because this costs marking performance. Which means doubling the number of mark-loops, and select the correct loop based on whether or not we need object counts.

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