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

[Mel Gorman <mgorman@suse.de>]

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@suse.cz>
> Reviewed-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
> Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Acked-by: David Rientjes <rientjes@google.com>
> Acked-by: Minchan Kim <minchan@kernel.org>

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

-- 
Mel Gorman
SUSE Labs

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