RFR: Enforce max regions [v3]

[William Kemper]

On Wed, 7 Dec 2022 16:14:18 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> This commit enforces upper bounds on the number of ShenandoahHeapRegions affiliated with each generation.  Prior to this change, enforcement of generation sizes was by usage alone.  This allowed situations in which so many sparsely populated regions were affiliated with old-gen that there were insufficient FREE regions available to satisfy legitimate young-gen allocation requests.  This was resulting in excessive TLAB allocation failures and degenerated collections.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix white space and add an assertion

Looks good - thank you!

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Marked as reviewed by wkemper (Committer).

PR: https://git.openjdk.org/shenandoah/pull/179