RFR: 8324649: Shenandoah: refactor implementation of free set [v14]

[Aleksey Shipilev]

On Fri, 26 Jan 2024 18:36:40 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> We want to put these Collector regions into the service of the Mutator threads as quickly as possible in order to avoid Mutator allocation failures.  We also want to complete update-refs as quickly as possible, because that will put a lot more memory into the hands of the Mutators.
>> 
>> The implementation of move_collector_sets_to_mutator() takes the heap lock when it reassigns a region's purpose from Collector to Mutator.
>
> Okay.

Cobbling together update-refs and freeset management adds to already high complexity. We need to do this as separate task somewhere in `op_updaterefs()`. Also, what does this move for _non-concurrent_ update refs, like degen GC?

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PR Review Comment: https://git.openjdk.org/jdk/pull/17561#discussion_r1509365091