RFR: 8271163: G1 uses wrong degree of MT processing since JDK-8270169

[Thomas Schatzl]

On Fri, 23 Jul 2021 10:33:18 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   JDK-8270169 changed the amount of resources supplied by the CM reference processor, assuming that the CM ref processor is only ever used with the marking threads.
> This is unfortunately not true, the CM reference processor is also used by the Remark pause for discovery using the parallel gc worker threads.
> 
> This causes crashes because of the code assuming single threaded operation while it is not.
> 
> The solution proposed here is to just revert JDK-8270169 which introduced  that change. Applies cleanly.
> 
> I also added a comment in the reference processor constructor to why we need to consider both thread counts.
> 
> Thanks,
>   Thomas

Since this is basically a clean backout of JDK-8270169, I'll skip the 24h rule requirement.

Thanks @kimbarrett @albertnetymk for your reviews.

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PR: https://git.openjdk.java.net/jdk/pull/4888