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

[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

-------------

Depends on: https://git.openjdk.org/jdk/pull/26323

Commit messages:
 - Merge branch 'NormalizeTiming' into ConcCPUImpact
 - reorder
 - concurrent gc impact

Changes: https://git.openjdk.org/jdk/pull/26351/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26351&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359348
  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