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

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Message-ID: <20110610092922.GE4183@redhat.com>
Date:	Fri, 10 Jun 2011 05:29:22 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
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 Fri, Jun 10, 2011 at 11:20:43AM +0200, Jens Axboe wrote:
> 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.

I was hoping that hardware does some prioritization. Well, in this case
even if hardware maintains FIFO behavior it should be good enough.

But I would not claim anything in this regard as I have never experimented
with it and have no idea that how sync latencies are impacted if we don't
drain the queue before dispathing WRITEs.

I was just wondering that with current generation hardware is it bad
enough that we need to keep this logic around?

Thanks
Vivek
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