linux-kernel - Re: [patch 3/3]cfq-iosched: don't idle if a deep seek queue is slow

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Date:	Tue, 09 Nov 2010 09:36:42 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"czoccolo@...il.com" <czoccolo@...il.com>
Subject: Re: [patch 3/3]cfq-iosched: don't idle if a deep seek queue is slow

On Mon, 2010-11-08 at 23:06 +0800, Jens Axboe wrote:
> On 2010-11-08 15:20, Vivek Goyal wrote:
> > On Mon, Nov 08, 2010 at 10:07:25AM +0800, Shaohua Li wrote:
> >> If a deep seek queue slowly deliver requests but disk is much faster, idle
> >> for the queue just wastes disk throughput. If the queue delevers all requests
> >> before half its slice is used, the patch disable idle for it.
> >> In my test, application delivers 32 requests one time, the disk can accept
> >> 128 requests at maxium and disk is fast. without the patch, the throughput
> >> is just around 30m/s, while with it, the speed is about 80m/s. The disk is
> >> a SSD, but is detected as a rotational disk. I can configure it as SSD, but
> >> I thought the deep seek queue logic should be fixed too, for example,
> >> considering a fast raid.
> >>
> > 
> > Hi Shaohua,
> > 
> > So looks like you are trying to cut down queue idling in the case when
> > device is fast and idling hurts. That's a noble goal, just that detetction
> > of this condition only for deep queues does not seem to cover lots of
> > cases. Manually one can set slice_idle=0 to handle this situation.
> > 
> > What about if you have lots of sequential queues (not deep) and they all
> > will still idle.
> > 
> > Secondly, what if driver is just buffering lots of requests in its device
> > queue and not necessarily device is processing the reuqests faster.
> 
> That is not a valid concern, a driver should never extract more than it
> can process (pretty much) immediately.
> 
> > So I think it is a good idea to cut down on idling if we can find that
> > underlying device is fast and idling on queue might hurt more. But
> > discovering this only using deep queues does not sound very appleaing to
> > me. This is help only a particular workload which is driving deep queues.
> > So if there was a generic mechanism to tackle this, that would be much
> > better.
> 
> Agree, we could use better metrics for this.
Agree we'd better have a better method to measure device speed, but this
seems not easy. Even in a fast device, a request might take long time to
finish when NCQ is enabled. Before we have generic mechanism, we still
need fix some particular cases.

Thanks,
Shaohua

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