Date: Fri, 20 Jan 2006 09:10:43 +0100
From: Jens Axboe <axboe@suse.de>
To: Nate Diller <nate.diller@gmail.com>
Cc: Andrew Morton <akpm@osdl.org>, cmm@us.ibm.com, seelam@cs.utep.edu,
       linux-kernel@vger.kernel.org, ext2-devel@lists.sourceforge.net
Subject: Re: Fall back io scheduler for 2.6.15?
Message-ID: <20060120081042.GC4213@suse.de>
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On Thu, Jan 19 2006, Nate Diller wrote:
> On 1/16/06, Jens Axboe <axboe@suse.de> wrote:
> > On Fri, Jan 13 2006, Andrew Morton wrote:
> > > Mingming Cao <cmm@us.ibm.com> wrote:
> > > >
> > > > On 2.6.14, the
> > > > fall back io scheduler (if the chosen io scheduler is not found) is set
> > > > to the default io scheduler (anticipatory, in this case), but since
> > > > 2.6.15-rc1, this semanistic is changed to fall back to noop.
> > >
> > > OK.  And I assume that AS wasn't compiled, so that's why it fell back?
> > >
> > > I actually thought that elevator= got removed, now we have
> > > /sys/block/sda/queue/scheduler.  But I guess that's not very useful with
> > > CONFIG_SYSFS=n.
> > >
> > > > Is there any reason to fall back to noop instead of as?  It seems
> > > > anticipatory is much better than noop for ext3 with large sequential
> > > > write tests (i.e, 1G dd test) ...
> > >
> > > I suspect that was an accident.  Jens?
> >
> > It is, it makes more sense to fallback to the default of course.
> 
> Not an accident at all, actually, because the original patch i
> submitted allowed you to select a scheduler as 'default' even if it
> were compiled as a module in kconfig.  Since noop is guaranteed to be
> present in any system, it is the obvious choice if the chosen or
> default scheduler is not loaded.

Yes and that was a bug in that patch. The default scheduler must be
builtin, that's a given. The Kconfig rules should make a default
selection as a module illegal. And they do, they have since been fixed.

> If you change it to fall back to the default, it will oops if the
> default is not available.

It must be.

-- 
Jens Axboe

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