RFR: Loan from old should align on region size

[William Kemper]

On Wed, 1 Feb 2023 02:57:07 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> This fixes an error detected by a recently added assertion on a different branch.  When loaning memory from old-gen to young-gen, it is important the total amount of the loan be an integral number of ShenandoahHeapRegions.  The total loan represents a sum of memory loaned to support young evacuations and memory loaned to support mutator allocations made while GC is active.  The sum of these two values is a multiple of region size.  The previous version of the code accidentally loaned only the memory dedicated to supporting mutator allocations, which is not necessarily a multiple of the region size.

I wonder if the missing `old_bytes_loaned_for_young_evac` explains the observation that the loans were often too conservative, resulting in evacuation failures on some workloads. Is the `old_bytes_loaned_for_young_evac` always less than the size of a region? or could it cover multiple regions?

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Marked as reviewed by wkemper (Committer).

PR: https://git.openjdk.org/shenandoah/pull/213