RFR: 8241678: Remove PerfData sampling via StatSampler [v2]

[Albert Mingkun Yang]

On Tue, 29 Apr 2025 09:48:05 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 incrementally with one additional commit since the last revision:
> 
>   feedback fixes

Reviewed only `hotspot` changes. Not familiar with other parts.

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Marked as reviewed by ayang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24872#pullrequestreview-2802821239