RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v13]

[William Kemper]

On Tue, 26 Mar 2024 15:37:02 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> When adjusting LAB sizes, round down rather than rounding up.  Otherwise, we may violate the ShenandoahHumongousThreshold bound.
>
> Kelvin Nilsen has updated the pull request incrementally with four additional commits since the last revision:
> 
>  - Add a missing include to fix zero build
>  - Add TODO comment for future refactoring of _plab initialization
>  - Bug fixes to address regressions encountered during integration testing
>  - Tighten up assert to assist with debugging

LGTM

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Marked as reviewed by wkemper (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/401#pullrequestreview-1960951683