RFR: Borrow from old gen

[Kelvin Nilsen]

On Mon, 7 Feb 2022 22:24:10 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> This commit represents multiple months of incremental performance improvements to allow generational shenandoah to run more efficiently, especially with larger heap sizes and high memory utilization.
>> 
>> Specific improvements are described in individual commit log messages.
>
> src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.hpp line 50:
> 
>> 48:   size_t                _used;
>> 49:   size_t                _region_count;
>> 50:   size_t                _immediate_trash;
> 
> Might consider having collection set itself manage these new members through `add_region` API - rather than having similar looking code in each of the heuristics when the collection set is populated.

This is a good idea.  I'll work on this change.

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PR: https://git.openjdk.java.net/shenandoah/pull/110