RFR: 8348595: GenShen: Fix generational free-memory no-progress check [v3]

[Kelvin Nilsen]

> At the end of a degenerated GC, we check whether sufficient progress has been made in replenishing the memory available to the mutator. The test for good progress is implemented as a ratio of free memory against the total heap size.
> 
> For generational Shenandoah, the ratio should be computed against the size of the young generation. Note that the size of the generational collection set is based on young generation size rather than total heap size.
> 
> This issue first identified in GenShen GC logs, where a large number of degenerated cycles were upgrading to full GC because the free-set progress was short of desired by 10-25%.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Add comments suggested by reviewers

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23306/files
  - new: https://git.openjdk.org/jdk/pull/23306/files/7969515d..8f644cdb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23306&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23306&range=01-02

  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23306.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23306/head:pull/23306

PR: https://git.openjdk.org/jdk/pull/23306