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

[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%.

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Commit messages:
 - Use freeset to determine goodness of progress
 - Use size-of young generation to assess progress

Changes: https://git.openjdk.org/jdk/pull/23306/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23306&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348595
  Stats: 8 lines in 2 files changed: 4 ins; 0 del; 4 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