RFR: 8359348: G1: Improve cpu usage measurements for heap sizing

[Albert Mingkun Yang]

On Wed, 16 Jul 2025 13:55:32 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

src/hotspot/share/gc/g1/g1Analytics.cpp line 174:

> 172:   // caches, therefore underestimate the impact of the concurrent GC activity on mutator threads.
> 173:   uint num_cpus = (uint)os::active_processor_count();
> 174:   num_cpus = MIN2(num_cpus, MAX2(ConcGCThreads, ParallelGCThreads));

Why the use of `ParallelGCThreads`? I thought parallel-threads are used only for stw-work but the current context about concurrent-work.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26351#discussion_r2212920774