Subject: Re: [Ext2-devel] Re: Fall back io scheduler for 2.6.15?
From: Mingming Cao <cmm@us.ibm.com>
Reply-To: cmm@us.ibm.com
To: Andrew Morton <akpm@osdl.org>
Cc: seelam@cs.utep.edu, linux-kernel@vger.kernel.org,
       ext2-devel@lists.sourceforge.net, Jens Axboe <axboe@suse.de>
In-Reply-To: <20060113174914.7907bf2c.akpm@osdl.org>
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Content-Type: text/plain
Organization: IBM LTC
Date: Mon, 16 Jan 2006 11:38:18 -0800
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On Fri, 2006-01-13 at 17:49 -0800, 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?
> 

AS was compiled, and AS is set as the default scheduler. But since
2.6.15 doesn't recognize "elevator=as" (we need to say
"elevator=anticipatory"), so in 2.6.15, elevator_setup_default() will
explicitly fall back to "noop" scheduler.

2.6.14 doesn't recognize "elevator=as" either, but it fall back to the
default scheduler instead. Which makes more sense, as Jen pointed out.

> 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?
> 
> 
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