RFR: 8351142: Add JFR monitor deflation and statistics events [v3]

[Erik Gahlin]

On Thu, 6 Mar 2025 12:24:10 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> We already have JFR JavaMonitorInflate event, which tells when the monitor is inflated. We are missing JavaMonitorDeflate event, which would tell us when the monitor is deflated. This makes it hard to see the monitor lifecycle, and/or estimate the population of currently inflated monitors. I believe we should add JavaMonitorDeflate event. It would also be useful to have the statistics for the number of currently used/deflating monitors. Deflation event alone would require post-processing to investigate this, so it would be good to have the statistics event as well.
>> 
>> This would also replace two of the RT counters that are going away in [JDK-8348829](https://bugs.openjdk.org/browse/JDK-8348829).
>> 
>> Monitor deflation is done asynchronously in `MonitorDeflationThread`, so the additional overhead of recording the deflation events would likely be performance neutral. We still only enable the statistics event by default to be on a safer side.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `jdk_jfr`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:
> 
>  - Touch up descriptions
>  - Fix test in release builds
>  - Merge branch 'master' into JDK-8351142-jfr-deflate-event
>  - Merge branch 'master' into JDK-8351142-jfr-deflate-event
>  - Test updates
>  - Rework statistics event to be actually statistics
>  - Filter JFR HiddenWait consistently
>  - Event metadata touchups
>  - Separate statistics event as well
>  - Fix

Looks good overall, but I'm not sure we should add maxCount.

I'm hesitant because the peak value can easily be calculated, which we already do for other events (CPULoad, NetworkUtilization, NativeMemoryUsage etc) in "jfr view".

-------------

PR Comment: https://git.openjdk.org/jdk/pull/23900#issuecomment-2706457566