RFR: Improve humongous remset scan [v2]

[William Kemper]

On Wed, 15 Jun 2022 00:28:12 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> These changes allow humongous objects residing within the remembered set to be scanned by muitiple concurrent GC worker threads, allowing more efficient parallelization of the remembered set scanning effort.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improvements requested by reviewer

Marked as reviewed by wkemper (Committer).

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PR: https://git.openjdk.org/shenandoah/pull/144