RFR: Performance improvements for non-generational modes

[Kelvin Nilsen]

On Sat, 18 Feb 2023 01:56:10 GMT, William Kemper <wkemper at openjdk.org> wrote:

> There are a basket of changes here:
> * Make heuristics a bit less aggressive:
>    * Revert threshold triggers to use `available` rather than adjusted head room.
>    * Remove code that adjusted average gc time, it was causing wild fluctuations and making heuristic behave erratically
> * Skip over more generational mode constraints on the allocation path
> * Disable `ShenadoahEvacLABRatio` by default as it causes unused bytes in survivors regions, which in turn leads to these regions to being included in the mutator view of free memory. This contributed to higher fragmentation rates.
> * More detailed debugging messages under the `gc+free` tags.

I'll need to be careful not to overwrite these improvements with my next merge.  Some of the same code is touched.

Thanks.

src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp line 314:

> 312:           range(1.0,100.0)                                                  \
> 313:                                                                             \
> 314:   product(uintx, ShenandoahMaxEvacLABRatio, 0, EXPERIMENTAL,                \

Surprised this would have an effect on single-generation performance.   Have we measured the costs/benefits of setting this default differently?  If 0 works best, maybe we should remove this entirely?

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Marked as reviewed by kdnilsen (Committer).

PR: https://git.openjdk.org/shenandoah/pull/220