linux-kernel - Re: [RFC][PATCH] avoid swapping out with swappiness==0

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Message-ID: <20120305215602.GA1693@redhat.com>
Date:	Mon, 5 Mar 2012 22:56:02 +0100
From:	Johannes Weiner <jweiner@...hat.com>
To:	Satoru Moriya <satoru.moriya@....com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	"lwoodman@...hat.com" <lwoodman@...hat.com>,
	"shaohua.li@...el.com" <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: Re: [RFC][PATCH] avoid swapping out with swappiness==0

On Fri, Mar 02, 2012 at 12:36:40PM -0500, Satoru Moriya wrote:
> Sometimes we'd like to avoid swapping out anonymous memory
> in particular, avoid swapping out pages of important process or
> process groups while there is a reasonable amount of pagecache
> on RAM so that we can satisfy our customers' requirements.
> 
> OTOH, we can control how aggressive the kernel will swap memory pages
> with /proc/sys/vm/swappiness for global and
> /sys/fs/cgroup/memory/memory.swappiness for each memcg.
> 
> But with current reclaim implementation, the kernel may swap out
> even if we set swappiness==0 and there is pagecache on RAM.
> 
> This patch changes the behavior with swappiness==0. If we set
> swappiness==0, the kernel does not swap out completely
> (for global reclaim until the amount of free pages and filebacked
> pages in a zone has been reduced to something very very small
> (nr_free + nr_filebacked < high watermark)).
> 
> Any comments are welcome.

Last time I tried that (getting rid of sc->may_swap, using
!swappiness), it was rejected it as there were users who relied on
swapping very slowly with this setting.

KOSAKI-san, do I remember correctly?  Do you still think it's an
issue?

Personally, I still think it's illogical that !swappiness allows
swapping and would love to see this patch go in.
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