linux-kernel - Re: CFQ: async queue blocks the whole system

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Message-ID: <4DF1E1EB.8010808@kernel.dk>
Date:	Fri, 10 Jun 2011 11:20:43 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Shaohua Li <shaohua.li@...el.com>, Tao Ma <tm@....ma>,
	linux-kernel@...r.kernel.org
Subject: Re: CFQ: async queue blocks the whole system

On 2011-06-10 11:17, Vivek Goyal wrote:
> On Fri, Jun 10, 2011 at 09:19:12AM +0800, Shaohua Li wrote:
> 
> [..]
>>> If there is no major advantage of draining sync requests before async
>>> is dispatched, I think this should be an easy fix.
>> I thought this is to avoid sync latency if we switch from an async
>> queue to sync queue later.
> 
> Is it about the sync request latency which has already been dispatched? I 
> really wish that driver and disk should do some prioritazation for reads
> here and CFQ does not have to jump through hoops like drain sync requests
> before async requests are dispatched.

That would never work. Are you suggesting putting that logic in all
drivers? Or relying on hardware to get the fairness right? Not going to
happen.

-- 
Jens Axboe

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