RFR: 8315219: G1: Improve allocator pathological case where it keeps doing direct allocations instead of retiring a PLAB

[Ivan Walulya]

Hi all,

Please review this change to improve on how G1 deals with ParallelGCBufferWastePct. Currently, any allocations larger than  ParallelGCBufferWastePct of the current PLAB size are allocated directly without regard for the state of the current PLAB. 

In this change, we directly check whether the current PLAB can be retired i.e. the remaining PLAB space is < ParallelGCBufferWastePct of current PLAB size.

Testing: mach5 Tier 1-6

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

Commit messages:
 - rename
 - initial

Changes: https://git.openjdk.org/jdk/pull/15482/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15482&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315219
  Stats: 21 lines in 2 files changed: 7 ins; 4 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15482.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15482/head:pull/15482

PR: https://git.openjdk.org/jdk/pull/15482