Re: [PATCH 4/5] mm, compaction: always update cached scanner positions

[Rik van Riel <riel@redhat.com>]

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 <vbabka@suse.cz>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Michal Nazarewicz <mina86@mina86.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: David Rientjes <rientjes@google.com>

Acked-by: Rik van Riel <riel@redhat.com>

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