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

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Message-ID: <8631DC5930FA9E468F04F3FD3A5D007214AD7315@USINDEM103.corp.hds.com>
Date:	Fri, 14 Dec 2012 19:44:30 +0000
From:	Satoru Moriya <satoru.moriya@....com>
To:	Michal Hocko <mhocko@...e.cz>, Johannes Weiner <hannes@...xchg.org>
CC:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	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:
> On Thu 13-12-12 23:50:30, Johannes Weiner wrote:
>> On Thu, Dec 13, 2012 at 10:25:43PM +0000, Satoru Moriya wrote:
>>>
>>> I introduced swappiness check here with fe35004f because, in some 
>>> cases, we prefer OOM to swap out pages to detect problems as soon as 
>>> possible. Basically, we design the system not to swap out and so if 
>>> it causes swapping, something goes wrong.
>>
>> I might be missing something terribly obvious, but... why do you add 
>> swap space to the system in the first place?  Or in case of cgroups, 
>> why not set the memsw limit equal to the memory limit?
> 
> 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?
> 

Right. One of the reason is what Michal described above and another
reason that I thought is softlimit. softlimit reclaim always works
with priority=0. Therefore, if we set softlimit to one memcg without
swappiness=0, the kernel scans both anonymous and filebacked pages
during soft limit reclaim for the memcg and reclaims them.

Regards,
Satoru
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