RFR: 8316299: GenShen: Reduce frequency of expedited GC

[William Kemper]

On Thu, 14 Sep 2023 15:21:53 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> We have found that expedited GCs too frequently yield very little benefit because they occur so soon after the previous GC that there has been no accumulation of garbage.
> 
> The primary motivation for expediting GC is to avoid a situation under which there is so much "promotion" work to be performed that the urgent need to reclaim garbage from young-generation is obstructed by this promotion work.
> 
> This PR ends the practice of expediting to support promotion in place.  The effort required to promote a region in place is minimal and unlikely to contend in a major way with young collection efforts.
> 
> This PR also reduces the likelihood that we will expedite for promotion by evacuation, because it requires the amount of accumulated promo potential to exceed a particular threshold.

Changes requested by wkemper (Committer).

src/hotspot/share/gc/shenandoah/heuristics/shenandoahYoungHeuristics.cpp line 149:

> 147:   }
> 148: 
> 149:   size_t promo_in_place_potential = heap->get_promotion_in_place_potential();

Can we also now delete `ShenandoahHeap::get_promotion_in_place_potential` and all the attendant code that maintains the value? This heuristic was the only place reading it.

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PR Review: https://git.openjdk.org/shenandoah/pull/325#pullrequestreview-1627317452
PR Review Comment: https://git.openjdk.org/shenandoah/pull/325#discussion_r1326223128