RFR: Collect old humongous regions immediately at end of concurrent old mark

[Roman Kennke]

On Fri, 21 May 2021 21:10:13 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Recycle dead humongous objects at the end of concurrent old mark, rather than add them to the cset. This behavior is similar to young gen cset selection. It also increases the amount of free memory in old gen to support evacuations. Shenandoah has a specific code path for recycling humongous objects and (debug builds) will assert when an old region is added to the cset.

Looks good to me, thanks!

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Marked as reviewed by rkennke (Lead).

PR: https://git.openjdk.java.net/shenandoah/pull/44