RFR: 8359348: G1: Improve cpu usage measurements for heap sizing [v3]

[Thomas Schatzl]

On Thu, 17 Jul 2025 13:16:29 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 with a new target base due to a merge or a rebase. The pull request now contains nine commits:
> 
>  - cleanup after merge
>  - Merge remote-tracking branch 'upstream/master' into ConcCPUImpact
>  - Merge branch 'NormalizeTiming' into ConcCPUImpact
>  - Thomas suggestion
>  - Thomas Review
>  - reorder
>  - concurrent gc impact
>  - clean out
>  - init

src/hotspot/share/gc/g1/g1Analytics.hpp line 137:

> 135:   void set_concurrent_gc_cpu_time_ms(double ms) {
> 136:     _concurrent_gc_cpu_time_ms = ms;
> 137:   }

This method is unused.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26351#discussion_r2235527507