RFR: Performance improvements for non-generational modes

[William Kemper]

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.

Converted to draft form pending the results of performance testing. I also intend to factor out new heuristic classes for generational mode, and also untangle the allocation paths for generational mode.

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