RFR: 8316297: GenShen: Degenerated GCs fail to make progress

[William Kemper]

On Thu, 14 Sep 2023 14:32:42 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> A recent change to the implementation of ShenandoahYoungGeneration::available() resulted in a budgeting shortfall during degenerated GC.  The problem is that we degenerate after mutator allocations fail, at which time young available memory is in very short supply.  The change to available() caused us to only see the mutator budget, which has been depleted.  This resulted in a very high probability that degenerated GC would fail to make progress, forcing degenerated GC to upgrade to full GC.
> 
> This PR makes the Collector reserve memory visible to the function that computes evacuation budgets, thereby increasing the likelihood of a successful degenerated cycle and obviating the need for a Full GC.

Marked as reviewed by wkemper (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/324#pullrequestreview-1627320170