Date: Mon, 2 Feb 2015 16:12:23 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Zhang Yanfei <zhangyanfei.ok@hotmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, Mel Gorman <mgorman@suse.de>,
        David Rientjes <rientjes@google.com>, Rik van Riel <riel@redhat.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/4] mm/compaction: enhance compaction finish condition
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On Sat, Jan 31, 2015 at 11:58:03PM +0800, Zhang Yanfei wrote:
> At 2015/1/30 20:34, Joonsoo Kim wrote:
> > From: Joonsoo <iamjoonsoo.kim@lge.com>
> > 
> > Compaction has anti fragmentation algorithm. It is that freepage
> > should be more than pageblock order to finish the compaction if we don't
> > find any freepage in requested migratetype buddy list. This is for
> > mitigating fragmentation, but, there is a lack of migratetype
> > consideration and it is too excessive compared to page allocator's anti
> > fragmentation algorithm.
> > 
> > Not considering migratetype would cause premature finish of compaction.
> > For example, if allocation request is for unmovable migratetype,
> > freepage with CMA migratetype doesn't help that allocation and
> > compaction should not be stopped. But, current logic regards this
> > situation as compaction is no longer needed, so finish the compaction.
> > 
> > Secondly, condition is too excessive compared to page allocator's logic.
> > We can steal freepage from other migratetype and change pageblock
> > migratetype on more relaxed conditions in page allocator. This is designed
> > to prevent fragmentation and we can use it here. Imposing hard constraint
> > only to the compaction doesn't help much in this case since page allocator
> > would cause fragmentation again.
> 
> Changing both two behaviours in compaction may change the high order allocation
> behaviours in the buddy allocator slowpath, so just as Vlastimil suggested,
> some data from allocator should be necessary and helpful, IMHO.

As Vlastimil said, fragmentation effect should be checked. I will do
it and report the result on next version.

Thanks.

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