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

[Ivan Walulya]

> 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

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Changes: https://git.openjdk.org/jdk/pull/26351/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26351&range=02
  Stats: 39 lines in 3 files changed: 37 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26351.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26351/head:pull/26351

PR: https://git.openjdk.org/jdk/pull/26351