linux-kernel - Re: [PATCH V4 10/15] mm, compaction: remember position within pageblock in free pages scanner

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Date:	Fri, 25 Jul 2014 13:35:32 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH V4 10/15] mm, compaction: remember position within
 pageblock in free pages scanner

On Wed, Jul 16, 2014 at 03:48:18PM +0200, Vlastimil Babka wrote:
> Unlike the migration scanner, the free scanner remembers the beginning of the
> last scanned pageblock in cc->free_pfn. It might be therefore rescanning pages
> uselessly when called several times during single compaction. This might have
> been useful when pages were returned to the buddy allocator after a failed
> migration, but this is no longer the case.
> 
> This patch changes the meaning of cc->free_pfn so that if it points to a
> middle of a pageblock, that pageblock is scanned only from cc->free_pfn to the
> end. isolate_freepages_block() will record the pfn of the last page it looked
> at, which is then used to update cc->free_pfn.
> 
> In the mmtests stress-highalloc benchmark, this has resulted in lowering the
> ratio between pages scanned by both scanners, from 2.5 free pages per migrate
> page, to 2.25 free pages per migrate page, without affecting success rates.
> 
> With __GFP_NO_KSWAPD allocations, this appears to result in a worse ratio (2.1
> instead of 1.8), but page migration successes increased by 10%, so this could
> mean that more useful work can be done until need_resched() aborts this kind
> of compaction.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Reviewed-by: Zhang Yanfei <zhangyanfei@...fujitsu.com>
> Reviewed-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Acked-by: David Rientjes <rientjes@...gle.com>
> Acked-by: Minchan Kim <minchan@...nel.org>

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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