RFR: Borrow from old gen

[Kelvin Nilsen]

On Mon, 7 Feb 2022 21:52:42 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/shenandoahThreadLocalData.hpp line 176:
> 
>> 174:   }
>> 175: 
>> 176:   static void disable_plab_promotions(Thread* thread) {
> 
> This looks wrong - shouldn't it set `_plab_allows_promotion` to false here? It's also not clear to me how this relates to the other changes here. When would we disable plab promotion?

Good catch.  I got careless with copy and paste.  Thanks.

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