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

Changes requested by wkemper (Committer).

src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.cpp line 93:

> 91:   if (ShenandoahHeap::heap()->mode()->is_generational()) {
> 92:     if (ShenandoahHeap::heap()->is_aging_cycle()) {
> 93:       r->decrement_age();

Why are we decreasing the age here? Putting the region in the collection set will call `make_cset` on the region which sets the age to zero.

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