RFR: 8349906: G1: Improve initial survivor rate for newly used young regions

[Thomas Schatzl]

Hi all,

  please review this change that tries to improve the survivor rate initial values for newly expanded regions.

Currently G1 uses `InitialSurvivorRate` as survivor rate for such regions, but it is typically a pretty bad choice because

* it's rather conservative, estimating that 40% of region contents will survive
* such a conservative value is kind of bad particularly in cases for regions that are expanded late in the mutator phase because they are not frequently updated (and with our running weights changes get propagated over a very long time), i.e. this 40% sticks for a long time
* it is a random value, i.e. not particularly specific to the application.

The suggestion is to use the survivor rate for the last region we know the survivor rate already.

Testing: gha, tier1-7 (with other changes)

Hth,
  Thomas

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Commit messages:
 - * remove whitespace
 - 8349906

Changes: https://git.openjdk.org/jdk/pull/23584/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23584&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349906
  Stats: 12 lines in 1 file changed: 8 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23584.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23584/head:pull/23584

PR: https://git.openjdk.org/jdk/pull/23584