Re: [PATCH] mm: consider all swapped back pages in used-once logic

[Johannes Weiner]

Hi Michal,

On Thu, May 17, 2012 at 11:13:53AM +0200, Michal Hocko wrote:
> [64574746 vmscan: detect mapped file pages used only once] made mapped pages
> have another round in inactive list because they might be just short
> lived and so we could consider them again next time. This heuristic
> helps to reduce pressure on the active list with a streaming IO
> worklods.
> This patch fixes a regression introduced by this commit for heavy shmem
> based workloads because unlike Anon pages, which are excluded from this
> heuristic because they are usually long lived, shmem pages are handled
> as a regular page cache.
> This doesn't work quite well, unfortunately, if the workload is mostly
> backed by shmem (in memory database sitting on 80% of memory) with a
> streaming IO in the background (backup - up to 20% of memory). Anon
> inactive list is full of (dirty) shmem pages when watermarks are
> hit. Shmem pages are kept in the inactive list (they are referenced)
> in the first round and it is hard to reclaim anything else so we reach
> lower scanning priorities very quickly which leads to an excessive swap
> out.
> 
> Let's fix this by excluding all swap backed pages (they tend to be long
> lived wrt. the regular page cache anyway) from used-once heuristic and
> rather activate them if they are referenced.

Yes, the algorithm only makes sense for file cache, which is easy to
reclaim.  Thanks for the fix!

> CC: Johannes Weiner <hannes@xxxxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Mel Gorman <mel@xxxxxxxxx>
> CC: Minchan Kim <minchan@xxxxxxxxxx>
> CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> CC: Rik van Riel <riel@xxxxxxxxxx>
> CC: stable [2.6.34+]
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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