RFR: Borrow from old gen [v2]

[William Kemper]

On Tue, 8 Feb 2022 16:22:22 GMT, Kelvin Nilsen <kdnilsen 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.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Respond to reviewer feedback

src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.inline.hpp line 64:

> 62: }
> 63: 
> 64: #ifdef KELVIN_VERBOSE

Some instrumentation sneaked in here.

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