RFR: 8349766: GenShen: Bad progress after degen does not always need full gc

[Kelvin Nilsen]

On Tue, 11 Feb 2025 03:31:51 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> In generational mode, only upgrade to full GC from degenerated GC if we've done two degenerated cycles in a row and both indicated bad progress.  Otherwise, start another concurrent GC, which will most likely degenerate also.  But this degenerated cycle will reclaim floating garbage within the young generation much more quickly than a full GC would have done.

Some detailed results running the workload mentioned in JBS ticket on tip:

![Screenshot 2025-02-10 at 7 10 18 PM](https://github.com/user-attachments/assets/c06606a6-ec21-4e40-b117-915ddfc0d1f6)

These are results running the same workload with the changes of this PR:

![Screenshot 2025-02-10 at 7 35 47 PM](https://github.com/user-attachments/assets/432c227e-9bf4-4f21-8099-1b39b5af364a)

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PR Comment: https://git.openjdk.org/jdk/pull/23552#issuecomment-2649732684
PR Comment: https://git.openjdk.org/jdk/pull/23552#issuecomment-2649733471