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

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Date:	Fri, 14 Dec 2012 17:13:45 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Rik van Riel <riel@...hat.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Satoru Moriya <satoru.moriya@....com>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch 2/8] mm: vmscan: disregard swappiness shortly before
 going OOM

On Fri 14-12-12 10:43:55, Rik van Riel wrote:
> On 12/14/2012 03:37 AM, Michal Hocko wrote:
> 
> >I can answer the later. Because memsw comes with its price and
> >swappiness is much cheaper. On the other hand it makes sense that
> >swappiness==0 doesn't swap at all. Or do you think we should get back to
> >_almost_ doesn't swap at all?
> 
> swappiness==0 will swap in emergencies, specifically when we have
> almost no page cache left, we will still swap things out:
> 
>         if (global_reclaim(sc)) {
>                 free  = zone_page_state(zone, NR_FREE_PAGES);
>                 if (unlikely(file + free <= high_wmark_pages(zone))) {
>                         /*
>                          * If we have very few page cache pages, force-scan
>                          * anon pages.
>                          */
>                         fraction[0] = 1;
>                         fraction[1] = 0;
>                         denominator = 1;
>                         goto out;
> 
> This makes sense, because people who set swappiness==0 but
> do have swap space available would probably prefer some
> emergency swapping over an OOM kill.

Yes, but this is the global reclaim path. I was arguing about
swappiness==0 & memcg. As this patch doesn't make a big difference for
the global case (as both the changelog and you mentioned) then we should
focus on whether this is desirable change for the memcg path. I think it
makes sense to keep "no swapping at all for memcg semantic" as we have
it currently.
-- 
Michal Hocko
SUSE Labs
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