RFR: 8241678: Remove PerfData sampling via StatSampler [v4]

[Casper Norrbin]

On Thu, 22 May 2025 14:09:37 GMT, Casper Norrbin <cnorrbin at openjdk.org> wrote:

>> Hi everyone,
>> 
>> This change removes the legacy `PerfData` sampling mechanism implemented through the `StatSampler` — an always-on periodic task that runs every 50ms my default. The sampling feature was originally introduced to collect performance counters and timestamps, but has since seen very little use.
>> 
>> For G1/ZGC, the only sampled value is a timestamp (`sun.os.hrt.ticks`). For Serial/Parallel, it also samples some heap space counters, but these are already updated after each GC cycle, making the sampling redundant. With sampling removed, the `PerfDataSamplingInterval` flag becomes obsoleted, as it no longer serves any purpose.
>> 
>> The only thing relying on the sampled timestamps is `jstat`: running `jstat -t` prints an extra column with the time since VM start. To preserve this funcitonality, we can calculate the timestamps as an offset from the already existing `sun.rt.createVmBeginTime` instead.
>
> Casper Norrbin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:
> 
>  - Merge branch 'master' into statsampler-removal
>  - removed last traces of hrt.ticks
>  - Merge branch 'master' into statsampler-removal
>  - feedback fixes
>  - removed the PerfDataSamplingInterval flag
>  - calculate timestamp in jstat instead of sampling
>  - StatSampler + sampling code removed

Thank you everyone for the reviews!

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PR Comment: https://git.openjdk.org/jdk/pull/24872#issuecomment-2916013283