RFR: 8352414: JFR: JavaMonitorDeflateEvent crashes when deflated monitor object is dead [v2]

[Aleksey Shipilev]

On Mon, 24 Mar 2025 11:51:14 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> How do you match up the timestamps? Because without the oop, the connection between the two events is lost.

True, but even the timestamp+event is a useful bit of info. If there are X monitors recorded by stats event at 13:13, then 100+ inflations happened at 13:14, and then 50+ deflations happened at 13:15, then I can plausibly guess the momentary monitor population is X+50, even if deflation events gives me no precise mapping for dead objects was deflated.

> Are we measuring how long time it takes to deflate a monitor? How useful is that information?

It is useful to know _when_ deflations happened, as this shows if deflater thread is actually performing well. We have seen "memory leaks" due to deflation policy bugs when monitor deflater was essentially stuck / outpaced by inflations. Pretty sure there are still lingering issues when monitor population spikes in a very short time frame, so it is useful to know inflations/deflations at individual events scale.

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PR Comment: https://git.openjdk.org/jdk/pull/24121#issuecomment-2747947463