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

[Steve Langasek]

On Wed, Oct 08, 2014 at 10:20:23AM -0700, Steve Langasek wrote:
> On Wed, Oct 08, 2014 at 05:42:48PM +0100, Colin Ian King wrote:
> > > 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.

> > Linux supports I/O scheduling priorities since 2.6.13 just with the CFQ
> > io scheduler.

> Sorry, I don't understand.  Do you mean that 'ionice' doesn't help when
> using the deadline scheduler?

Ok, I've caught up on the IRC discussion on this and have a better
understanding now.  To summarize:

 - baloo does use ionice, but ionice has no effect when the deadline
   scheduler is used.  There is also no equivalent to ionice for deadline
   that would let baloo declare that it should be given lower priority.
 - benchmarks for deadline vs. cfq on rotational disks are mixed; and the
   change to use deadline was done in part *because of* applications being
   i/o-starved and unresponsive on the Ubuntu desktop, which was mitigated
   by this switch.
 - due to the lack of per-process userspace controls on the deadline
   scheduler, overriding the kernel scheduler seems to be the only way to
   give a reasonable experience for kubuntu on rotational disks

So while I still don't agree that this is free of risk of regression (e.g.,
a system with both kubuntu and ubuntu desktops installed could see a direct
regression under the ubuntu session as a result of this change), I also
don't see any better way to fix it.  So while I wouldn't be comfortable with
a kubuntu-specific udev rule in an SRU, I also wouldn't try to block it.

It has been pointed out that Ubuntu also has an indexer, zeitgeist, which
apparently doesn't suffer from the same problem.  Perhaps the KDE team would
want to take a look to understand what zeitgeist is doing differently that
makes it compatible with the deadline scheduler.

-- 
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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