RFR: 8348595: GenShen: Fix generational free-memory no-progress check [v2]

[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:

  Respond to reviewer feedback
  
  In testing suggested refinements, I discovered a bug in original
  implementation.  ShenandoahFreeSet::capacity() does not represent the
  size of young generation.  It represents the total size of the young
  regions that had available memory at the time we most recently rebuilt
  the ShenandoahFreeSet.
  
  I am rerunning the performance tests following this suggested change.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23306/files
  - new: https://git.openjdk.org/jdk/pull/23306/files/a850e484..7969515d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23306&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23306&range=00-01

  Stats: 13 lines in 5 files changed: 4 ins; 0 del; 9 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