[SRU] Set the default IO scheduler to CFQ in Kubuntu Trusty

[Steve Langasek]

Hi Jonathan,

On Wed, Oct 08, 2014 at 03:56:42PM +0100, Jonathan Riddell wrote:
> Me and Rohan would like a second opinion on bug 1378789
>  [SRU] Set the default IO scheduler to CFQ in Kubuntu Trusty
>  https://launchpad.net/bugs/1378789

> The kernel team have changed the scheduler away from upstream Linux
> defaults to deadlock which causes our desktop indexing programme Baloo
> to run very slow and take up lots of resources because it doesn't
> implement ionice suport.  We'd like to update kubuntu-settings with a
> udev rule to change back to CFQ.

> We've changed this in utopic.  There's minimal chance of regressions,
> it's an upstream default recommended by Linux and Baloo.  We're
> getting quite a few reports of people saying their system is sluggish
> and the baloo upstream author is most grumpy.

> Scott would rather wait until after utopic is out where it would get
> more testing, but I doubt there will be much that most users can say
> beyond "it's not horribly slow".

I don't think it's at all appropriate for a desktop environment to install a
udev rule which changes the kernel scheduler.  That's a severe layering
violation, and it means that anyone who installs kubuntu-desktop on an
existing system will significantly change the performance characteristics of
that system.

I also think it's categorically wrong to say that there's minimal chance of
regression.  These schedulers have pretty fundamentally different
characteristics, and where CFQ behaves pathologically for one process (the
indexer), deadline will behave pathologically for others.

I don't think it's at all acceptable to work around a kernel bug in a
kubuntu-settings SRU.  The right fix is to resolve this in the kernel
package instead. (Bug #1310402)  Cc:ing the kernel team.

Also, what exactly do you mean when you say baloo doesn't "implement ionice
support"?  The 'ionice' tool is part of the base system (util-linux).  It
would be a simple matter of packaging to always run baloo under ionice.

-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org
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