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

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Message-ID: <20120426160852.GA12568@alpha.arachsys.com>
Date:	Thu, 26 Apr 2012 17:08:52 +0100
From:	Richard Davies <richard.davies@...stichosts.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Satoru Moriya <satoru.moriya@....com>,
	Jerome Marchand <jmarchan@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"jweiner@...hat.com" <jweiner@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"lwoodman@...hat.com" <lwoodman@...hat.com>,
	"shaohua.li@...el.com" <shaohua.li@...el.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Seiji Aguchi <seiji.aguchi@....com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [RFC][PATCH] avoid swapping out with swappiness==0

KOSAKI Motohiro wrote:
> Christoph Lameter wrote:
> > Richard Davies wrote:
> >
> > > I strongly believe that Linux should have a way to turn off swapping unless
> > > absolutely necessary. This means that users like us can run with swap
> > > present for emergency use, rather than having to disable it because of the
> > > side effects.
> >
> > Agree. And this ooperation mode should be the default behavior given that
> > swapping is a very slow and tedious process these days.
> 
> Even though current patch is not optimal, I don't disagree this opinion. Can
> you please explain your use case? Why don't you use swapoff?

My use case is that I have large (64 or 128GB RAM) qemu-kvm virtualization
hosts, running many (20-50) VMs.

Typically the total memory in use is less than physical memory. In these
cases I would like the virtualization host to run without any swapping. I
have set swappiness==0, but in practise I get big load spikes from swapping.
See http://marc.info/?l=linux-mm&m=133517452117581

I don't want to run swapoff, because sometimes I will need to provision
slightly more VMs than physical memory, and in these cases I would rather
that the system runs with a little swap in use rather than the OOM killer
occurring.

Richard.
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