RFR: 8359348: G1: Improve cpu usage measurements for heap sizing [v8]

[Ivan Walulya]

On Wed, 27 Aug 2025 08:02:27 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

>> Hi,
>> 
>> Please review this patch which takes into account the impact of concurrent GC activity on mutator threads when computing the time spent on GC activity in a time interval. Previously, only the GC pause times were considered, and the overhead imposed by concurrent GC worker threads was not included.
>> 
>> With this change, we now estimate the impact of concurrent GC by dividing `elapsed_gc_cpu_time` by the number of CPUs. This provides an approximation of the additional time attributable to GC activity, assuming a fair CPU resource sharing.  Although this method does not account for contention on other shared resources (such as memory bandwidth or caches), it offers a reasonable estimate for most scenarios.
>> 
>> Testing: Tier 1
>
> Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:
> 
>   cleanup

Thanks for the reviews!

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PR Comment: https://git.openjdk.org/jdk/pull/26351#issuecomment-3227865178