RFR: 8316299: GenShen: Reduce frequency of expedited GC

[Aleksey Shipilev]

On Thu, 14 Sep 2023 15:35:43 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Err, miscomputed. That would be only `1%` of heap size.
>
> Yes.  20% of 5%.  The idea: In the rare scenario that we anticipate that promotion will consume over 20% of the evacuation reserve, we'll expedite the cycle so as to improve the likelihood that GC will replenish young allocation pool before it becomes exhausted.

All right. Feels like it needs to be a tunable, but hardcoded is okay for now.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/325#discussion_r1326146116