RFR: 8327097: GenShen: Align PLAB sizes down rather than up

[William Kemper]

On Mon, 26 Feb 2024 21:27:37 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.

test/hotspot/jtreg/ProblemList.txt line 94:

> 92: gc/TestAllocHumongousFragment.java#static 8298781 generic-all
> 93: gc/stress/gclocker/TestExcessGCLockerCollections.java 8229120 generic-all
> 94: gc/shenandoah/TestHumongousThreshold.java 832700 macos-aarch64

Not sure if it matters, but the ticket number looks too short. I also think the os name is `macosx`, based on other disabled tests. If that doesn't work, I think `generic-all` would also be fine temporarily.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/401#discussion_r1507936459