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

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Message-ID: <4BA09C9D.1030608@redhat.com>
Date:	Wed, 17 Mar 2010 11:10:53 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Christoph Hellwig <hch@....de>
CC:	Chris Webb <chris@...chsys.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>,
	Kevin Wolf <kwolf@...hat.com>
Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

On 03/17/2010 10:49 AM, Christoph Hellwig wrote:
> On Tue, Mar 16, 2010 at 01:08:28PM +0200, Avi Kivity wrote:
>    
>> If the batch size is larger than the virtio queue size, or if there are
>> no flushes at all, then yes the huge write cache gives more opportunity
>> for reordering.  But we're already talking hundreds of requests here.
>>      
> Yes.  And rememember those don't have to come from the same host.  Also
> remember that we rather limit execssive reodering of O_DIRECT requests
> in the I/O scheduler because they are "synchronous" type I/O while
> we don't do that for pagecache writeback.
>    

Maybe we should relax that for kvm.  Perhaps some of the problem comes 
from the fact that we call io_submit() once per request.

> And we don't have unlimited virtio queue size, in fact it's quite
> limited.
>    

That can be extended easily if it fixes the problem.

-- 
error compiling committee.c: too many arguments to function

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