linux-kernel - Re: [PATCH][RF C/T/D] Unmapped page cache control

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Date:	Wed, 17 Mar 2010 19:11:32 +0000
From:	Chris Webb <chris@...chsys.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	Avi Kivity <avi@...hat.com>, balbir@...ux.vnet.ibm.com,
	KVM development list <kvm@...r.kernel.org>,
	Rik van Riel <riel@...riel.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot
 parameter

Vivek Goyal <vgoyal@...hat.com> writes:

> Are you using CFQ in the host? What is the host kernel version? I am not sure
> what is the problem here but you might want to play with IO controller and put
> these guests in individual cgroups and see if you get better throughput even
> with cache=writethrough.

Hi. We're using the deadline IO scheduler on 2.6.32.7. We got better
performance from deadline than from cfq when we last tested, which was
admittedly around the 2.6.30 timescale so is now a rather outdated
measurement.

> If the problem is that if sync writes from different guests get intermixed
> resulting in more seeks, IO controller might help as these writes will now
> go on different group service trees and in CFQ, we try to service requests
> from one service tree at a time for a period before we switch the service
> tree.

Thanks for the suggestion: I'll have a play with this. I currently use
/sys/kernel/uids/N/cpu_share with one UID per guest to divide up the CPU
between guests, but this could just as easily be done with a cgroup per
guest if a side-effect is to provide a hint about IO independence to CFQ.

Best wishes,

Chris.
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