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

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Date:	Fri, 14 Dec 2012 10:43:55 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>
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 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.

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