RFR: Improve mixed collection logging

[Kelvin Nilsen]

On Thu, 18 May 2023 23:16:55 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Not including any old regions in a mixed collection should be rare. It could indicate a bug. This change will log the relevant state when this happens.

There are some other changes to this code in expand-old-on-demand branch as I had observed some sub-optimal behavior during test and debug.  If you're chasing an issue here, you might take a peak at that.

src/hotspot/share/gc/shenandoah/heuristics/shenandoahOldHeuristics.cpp line 130:

> 128:       _old_generation->transition_to(ShenandoahOldGeneration::WAITING_FOR_FILL);
> 129:     } else {
> 130:       log_info(gc)("No regions selected for mixed collection. "

When this fails, it may be useful to understand a bit more about why.  For example: heap->get_old_evac_reserve() is the evacuation budget.  Knowing how much is live (and how much garbage and "lost evacuation capacity" is in the rejected candidates (or in the first of the rejected candidates) might also be informative.

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/281#pullrequestreview-1434497817
PR Review Comment: https://git.openjdk.org/shenandoah/pull/281#discussion_r1199005139