Decommit small free regions when close to OutOfMemory

[cshung]

Sometimes, when we ran out of memory on one heap, it appears that a good portion of the committed bytes are still occupied by the region_free_list of other heaps. As of now, our allocator is incapable of using those.

distribute_free_regions attempts to spread them across all heaps evenly, that was done by counting the number of free regions. Unfortunately, sometimes there are regions that are tiny (with only 1 page committed), and for reason that is not very well understood, sometimes, these tiny regions are clustered in a single heap, leading to a heap running out of free committed memory while the others still have it.

This change decommits those small ones so that this unfortunate scenario is alleviated.

There could be alternative solutions (e.g. stealing from some other heaps during allocation, or maintaining a global region free list that can be shared across heaps). These alternative solutions might give better perf characteristics but they are potentially too complicated for .NET 7 now.

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Issue Details

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Sometimes, when we ran out of memory on one heap, it appears that a good portion of the committed bytes are still occupied by the region_free_list of other heaps. As of now, our allocator is incapable of using those.

This change decommits them so that they could be reused by the desired heap after the compacting GC.

There could be alternative solutions (e.g. stealing from some other heaps during allocation, or maintaining a global region free list that can be shared across heaps). These alternative solutions might give better perf characteristics but they are potentially too complicated for .NET 7 now.

Author:	cshung
Assignees:	cshung
Labels:	area-GC-coreclr
Milestone:	-

[cshung]

We can decommit the other generation regions end as well.

[cshung]

Make the changes to reset last_gc_before_oom REGIONS only