linux-kernel - Re: [patch 2/8] mm: vmscan: disregard swappiness shortly before going OOM

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Date:	Thu, 13 Dec 2012 16:29:59 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/8] mm: vmscan: disregard swappiness shortly before
 going OOM

On Thu 13-12-12 10:34:20, Mel Gorman wrote:
> On Wed, Dec 12, 2012 at 04:43:34PM -0500, Johannes Weiner wrote:
> > When a reclaim scanner is doing its final scan before giving up and
> > there is swap space available, pay no attention to swappiness
> > preference anymore.  Just swap.
> > 
> > Note that this change won't make too big of a difference for general
> > reclaim: anonymous pages are already force-scanned when there is only
> > very little file cache left, and there very likely isn't when the
> > reclaimer enters this final cycle.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Ok, I see the motivation for your patch but is the block inside still
> wrong for what you want? After your patch the block looks like this
> 
>                 if (sc->priority || noswap) {
>                         scan >>= sc->priority;
>                         if (!scan && force_scan)
>                                 scan = SWAP_CLUSTER_MAX;
>                         scan = div64_u64(scan * fraction[file], denominator);
>                 }
> 
> if sc->priority == 0 and swappiness==0 then you enter this block but
> fraction[0] for anonymous pages will also be 0 and because of the ordering
> of statements there, scan will be
> 
> scan = scan * 0 / denominator
> 
> so you are still not reclaiming anonymous pages in the swappiness=0
> case. What did I miss?

Yes, now that you have mentioned that I realized that it really doesn't
make any sense. fraction[0] is _always_ 0 for swappiness==0. So we just
made a bigger pressure on file LRUs. So this sounds like a misuse of the
swappiness. This all has been introduced with fe35004f (mm: avoid
swapping out with swappiness==0).

I think that removing swappiness check make sense but I am not sure it
does what the changelog says. It should have said that checking
swappiness doesn't make any sense for small LRUs.
-- 
Michal Hocko
SUSE Labs
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