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

[Kelvin Nilsen]

On Fri, 24 Jan 2025 18:30:02 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> 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%.

[netflix-calibrate-better.xlsx](https://github.com/user-attachments/files/18540252/netflix-calibrate-better.xlsx)

The attached spreadsheet provides a summary of performance benefits of this patch.  In the spreadsheet:
  Control represents tip without changes
  Better Penalty represents https://github.com/openjdk/jdk/pull/23305
  Better Progress represents https://github.com/openjdk/jdk/pull/23306 (this PR)
  Better Both represents the combined benefits of Better Penalty and Better Progress.
  
  Compared to Control, "Better Both" results are better across all measures:
  
![image](https://github.com/user-attachments/assets/849291e2-713d-4611-80ad-0c4b43aee2c5)

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PR Comment: https://git.openjdk.org/jdk/pull/23306#issuecomment-2613155202