Re: [PATCH 09/14] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction

From: Mel Gorman
Date: Tue Dec 18 2018 - 08:52:08 EST

On Tue, Dec 18, 2018 at 01:36:42PM +0100, Vlastimil Babka wrote:
> On 12/15/18 12:03 AM, Mel Gorman wrote:
> > When pageblocks get fragmented, watermarks are artifically boosted to pages
> > are reclaimed to avoid further fragmentation events. However, compaction
> > is often either fragmentation-neutral or moving movable pages away from
> > unmovable/reclaimable pages. As the actual watermarks are preserved,
> > allow compaction to ignore the boost factor.
> Right, I should have realized that when reviewing the boost patch. I
> think it would be useful to do the same change in
> __compaction_suitable() as well. Compaction has its own "gap".

That gap is somewhat static though so I'm a bit more wary of it. However,
the check in __isolate_free_page looks too agressive. We isolate in
units of COMPACT_CLUSTER_MAX yet the watermark check there is based on
the allocation request. That means for THP that we check if 512 pages
can be allocated when only somewhere between 1 and 32 is needed for that
compaction cycle to complete. Adjusting that might be more appropriate?

Mel Gorman