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Date:	Mon, 20 Oct 2014 11:26:56 -0400
From:	Rik van Riel <>
To:	Vlastimil Babka <>,
	Andrew Morton <>
	Minchan Kim <>, Mel Gorman <>,
	Joonsoo Kim <>,
	Michal Nazarewicz <>,
	Naoya Horiguchi <>,
	Christoph Lameter <>,
	David Rientjes <>
Subject: Re: [PATCH 4/5] mm, compaction: always update cached scanner positions

On 10/07/2014 11:33 AM, Vlastimil Babka wrote:
> Compaction caches the migration and free scanner positions between compaction
> invocations, so that the whole zone gets eventually scanned and there is no
> bias towards the initial scanner positions at the beginning/end of the zone.
> The cached positions are continuously updated as scanners progress and the
> updating stops as soon as a page is successfully isolated. The reasoning
> behind this is that a pageblock where isolation succeeded is likely to succeed
> again in near future and it should be worth revisiting it.
> However, the downside is that potentially many pages are rescanned without
> successful isolation. At worst, there might be a page where isolation from LRU
> succeeds but migration fails (potentially always). So upon encountering this
> page, cached position would always stop being updated for no good reason.
> It might have been useful to let such page be rescanned with sync compaction
> after async one failed, but this is now handled by caching scanner position
> for async and sync mode separately since commit 35979ef33931 ("mm, compaction:
> add per-zone migration pfn cache for async compaction").
> After this patch, cached positions are updated unconditionally. In
> stress-highalloc benchmark, this has decreased the numbers of scanned pages
> by few percent, without affecting allocation success rates.
> Signed-off-by: Vlastimil Babka <>
> Cc: Minchan Kim <>
> Cc: Mel Gorman <>
> Cc: Joonsoo Kim <>
> Cc: Michal Nazarewicz <>
> Cc: Naoya Horiguchi <>
> Cc: Christoph Lameter <>
> Cc: Rik van Riel <>
> Cc: David Rientjes <>

Acked-by: Rik van Riel <>

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