linux-kernel - Re: [PATCH v2 4/4] mm, compaction: direct freepage allocation for async direct compaction

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Date:	Mon, 4 Apr 2016 10:31:59 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v2 4/4] mm, compaction: direct freepage allocation for
 async direct compaction

On Thu, Mar 31, 2016 at 10:50:36AM +0200, Vlastimil Babka wrote:
> The goal of direct compaction is to quickly make a high-order page available
> for the pending allocation. The free page scanner can add significant latency
> when searching for migration targets, although to succeed the compaction, the
> only important limit on the target free pages is that they must not come from
> the same order-aligned block as the migrated pages.
> 

What prevents the free pages being allocated from behind the migration
scanner? Having compaction abort when the scanners meet misses
compaction opportunities but it avoids the problem of Compactor A using
pageblock X as a migration target and Compactor B using pageblock X as a
migration source.

-- 
Mel Gorman
SUSE Labs