linux-kernel - Re: [PATCH] cfq-iosched: quantum check tweak --resend

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Date:	Mon, 1 Mar 2010 16:22:50 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"czoccolo@...il.com" <czoccolo@...il.com>,
	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	"guijianfeng@...fujitsu.com" <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH] cfq-iosched: quantum check tweak --resend

On Mon, Mar 01, 2010 at 04:19:20PM +0800, Jens Axboe wrote:
> On Mon, Mar 01 2010, Shaohua Li wrote:
> > On Mon, Mar 01, 2010 at 04:02:34PM +0800, Jens Axboe wrote:
> > > On Mon, Mar 01 2010, Shaohua Li wrote:
> > > > Currently a queue can only dispatch up to 4 requests if there are other queues.
> > > > This isn't optimal, device can handle more requests, for example, AHCI can
> > > > handle 31 requests. I can understand the limit is for fairness, but we could
> > > > do a tweak: if the queue still has a lot of slice left, sounds we could
> > > > ignore the limit. Test shows this boost my workload (two thread randread of
> > > > a SSD) from 78m/s to 100m/s.
> > > > Thanks for suggestions from Corrado and Vivek for the patch.
> > > 
> > > As mentioned before, I think we definitely want to ensure that we drive
> > > the full queue depth whenever possible. I think your patch is a bit
> > > dangerous, though. The problematic workload here is a buffered write,
> > > interleaved with the occasional sync reader. If the sync reader has to
> > > endure 32 requests every time, latency rises dramatically for him.
> > the patch still matains a hardlimit for dispatched request. For a async,
> > the limit is cfq_slice_async/cfq_slice_idle = 5. For sync, the limit is 8.
> > And we only pipe out such number of requests at the begining of a slice.
> > For the workload you mentioned here, we only dispatch 1 extra request.
> 
> OK, that sound appropriate. Final question - why change the quantum and
> use quantum/2?
This is suggested by Vivek. In this way quantum is still the hard limit and
doesn't surprise users. we do throttling at 1/2 quantum (softlimit) and
then stop at quantum (hard limit)

Thanks,
Shaohua
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