RFR: Improve humongous remset scan

[Kelvin Nilsen]

On Tue, 14 Jun 2022 20:09:14 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.

Before this change, a single thread would scan the entire humongous object, even if the object spans multiple regions.  With this change, different worker threads will be able to scan different parts of the humongous object.  The other improvement here is that when a thread is scanning a humongous object, it only scans the portions of the humongous object that are spanned by DIRTY remembered set cards.  Previously, the entire humongous object was scanned by a single thread.

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