RFR: Improve some defaults and remove unused options for generational mode

Thu Nov 10 18:45:18 UTC 2022

On Thu, 10 Nov 2022 00:47:14 GMT, William Kemper <wkemper at openjdk.org> wrote:

> `ShenandoahPromoteTenuredObjects` and `ShenandoahUsePLABs` are removed.  The default `NewRatio` for generational mode is changed to 1. The default percentage of garbage for regions included in the old generation collection set is lowered to 10.

It shows a substantial reduction in max rss for `lusearch` (which I expect is due to the lowering of the immediate garbage threshold). It also shows reduction in max rss for `tomcat` and `xalan` benchmarks.

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PR: https://git.openjdk.org/shenandoah/pull/169