Re: [RFC PATCH 5/6] mm, compaction: stop when number of free pages goes below watermark

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Jul 27, 2017 at 06:07:00PM +0200, Vlastimil Babka wrote:
> When isolating free pages as miration targets in __isolate_free_page(),

s/miration/migration/

> compaction respects the min watermark. Although it checks that there's enough
> free pages above the watermark in __compaction_suitable() before starting to
> compact, parallel allocation may result in their depletion. Compaction will
> detect this only after needlessly scanning many pages for migration,
> potentially wasting CPU time.
> 
> After this patch, we check if we are still above the watermark in
> __compact_finished(). For kcompactd, we check the low watermark instead of min
> watermark, because that's the point when kswapd is woken up and it's better to
> let kswapd finish freeing memory before doing kcompactd work.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Otherwise I cannot see a problem. Some compaction opportunities might be
"missed" but they're ones that potentially cause increased direct
reclaim or kswapd reclaim activity.

-- 
Mel Gorman
SUSE Labs

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