linux-kernel - Re: Over-eager swapping

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Message-ID: <20100819151351.GA23611@balbir.in.ibm.com>
Date:	Thu, 19 Aug 2010 20:43:51 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Chris Webb <chris@...chsys.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Wu Fengguang <fengguang.wu@...el.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: Over-eager swapping

* Chris Webb <chris@...chsys.com> [2010-08-19 10:25:36]:

> Balbir Singh <balbir@...ux.vnet.ibm.com> writes:
> 
> > Can you give an idea of what the meminfo inside the guest looks like.
> 
> Sorry for the slow reply here. Unfortunately not, as these guests are run on
> behalf of customers. They install them with operating systems of their
> choice, and run them on our service.
>

Thanks for clarifying.
 
> > Have you looked at
> > http://kerneltrap.org/mailarchive/linux-kernel/2010/6/8/4580772
> 
> Yes, I've been watching this discussions with interest. Our application is
> one where we have little to no control over what goes on inside the guests,
> but these sorts of things definitely make sense where the two are under the
> same administrative control.
>

Not necessarily, in some cases you can use a guest that uses lesser
page cache, but that might not matter in your case at the moment.
 
> > Do we have reason to believe the problem can be solved entirely in the
> > host?
> 
> It's not clear to me why this should be difficult, given that the total size
> of vm allocated to guests (and system processes) is always strictly less
> than the total amount of RAM available in the host. I do understand that it
> won't allow for as impressive overcommit (except by ksm) or be as efficient,
> because file-backed guest pages won't get evicted by pressure in the host as
> they are indistinguishable from anonymous pages.
>
> After all, a solution that isn't ideal, but does work, is to turn off swap
> completely! This is what we've been doing to date. The only problem with
> this is that we can't dip into swap in an emergency if there's no swap there
> at all.

If you are not overcommitting it should work, in my experiments I've
seen a lot of memory used by the host as page cache on behalf of the
guest. I've done my experiments using cgroups to identify accurate
usage.

-- 
	Three Cheers,
	Balbir
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