RFR: Balance without cancel

[William Kemper]

On Thu, 28 Apr 2022 04:12:58 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> This commit does load balancing on remembered set scanning that happens during concurrent marking and during concurrent updating of references.  Experiments show that the concurrency of remembered set marking is now much closer to the number of concurrent GC threads (generally within 5%) whereas before this commit, the level of concurrency was often less than 1/4 of the number of concurrent GC threads.
> 
> An additional fix integrated in this commit is to not scan the entirety of humongous object arrays.  We only scan the portions of these arrays that overlay with dirty cards in the remembered set.

Can we break out the changes to the remembered set scan, the refactoring of collection set selection and the scanning of humongous objects into separate, testable changes? I have some concerns about the performance of this change.

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PR: https://git.openjdk.java.net/shenandoah/pull/136