RFR: 8354517: Parallel: JDK-8339668 causes up to 3.7x slowdown in openjdk.bench.vm.gc.systemgc

Thu May 22 06:39:26 UTC 2025

Before JDK-8339668, full-gc marking array-chunk size uses `ObjArrayMarkingStride`. This patch restores the old behavior (performance).

The fix is extract out the chunk-size from `PartialArraySplitter` and use the either `ParGCArrayScanChunk` or `ObjArrayMarkingStride`, depending on the context -- the former is used during young-gc while the latter full-gc.

Test: tier1-3; checked perf regression is gone

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Commit messages:
 - pgc-split-array

Changes: https://git.openjdk.org/jdk/pull/25382/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25382&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354517
  Stats: 9 lines in 5 files changed: 3 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/25382.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25382/head:pull/25382

PR: https://git.openjdk.org/jdk/pull/25382