RFR: 8368006: Parallel: Skip full regions in dense prefix during Full GC

Thu Sep 18 18:08:02 UTC 2025

Implement skip-full-region optimization for filler related operations on dense-prefix. I constrained the filler-creation task to a single worker for simpler implementation. During the compaction phase, the most expensive part is almost always the non-dense-prefix part, so letting the rest of workers to start with the expensive tasks seems sensible.

Synthetic bms shows good reduction in full-gc time in debug build; perf-neutral for other bms (specjvm2008,specjbb2015) in release build.

Test: tier1-5

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Commit messages:
 - pgc-skip-full-region

Changes: https://git.openjdk.org/jdk/pull/27369/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368006
  Stats: 91 lines in 1 file changed: 40 ins; 23 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/27369.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27369/head:pull/27369

PR: https://git.openjdk.org/jdk/pull/27369