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

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Date:	Tue, 21 Sep 2010 07:55:07 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"jaxboe@...ionio.com" <jaxboe@...ionio.com>,
	"czoccolo@...il.com" <czoccolo@...il.com>
Subject: Re: [patch]cfq-iosched: don't idle if a deep seek queue is slow

On Mon, 2010-09-20 at 22:15 +0800, Vivek Goyal wrote:
> On Mon, Sep 20, 2010 at 04:53:34PM +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 you seem to be addressing the issue of storage being fast enough and
> a single queue not being able to keep the storage busy.
> 
> But we have the same issue for non-deep queues for a fast storage. This
> patch will not solve that.
right.

> I think CFQ idling in general is a problem on faster storage. For SSDs we
> can statically detect non-rotational media and disable idling. For faster
> RAIDs we need to find an intellligent way of detection and disable idling.
> 
> One of the suggestions at this year's LSF was to keep idling on only for
> SATA disks and for any SCSI disks we can think of disabling idling by
> default. May be with the help of udev rule.
That makes sense, but shouldn't kernel has some reasonable setting?
completely depending on userspace isn't reliable.

Thanks,
Shaohua

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