RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v12]

[Kelvin Nilsen]

On Sat, 23 Mar 2024 03:51:08 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 one additional commit since the last revision:
> 
>   Add detail to log message to help with debugging

Efforts to resolve reviewer comments introduced some regressions.  Converting to draft while I debug these.

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PR Comment: https://git.openjdk.org/shenandoah/pull/401#issuecomment-2016516696