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

[Steve Langasek]

On Thu, Oct 09, 2014 at 06:15:17PM +0200, Rohan Garg wrote:
> > As I said, the switch to deadline was seen to address existing problems with
> > applications on the unity desktop (when running on an HDD) becoming
> > non-responsive under heavy I/O.  Switching back to cfq is likely to
> > reintroduce this problem.

> Right and this data is fairly out of date right? I mean, is there up to
> date data on whether or not switching to CFQ will definitely cause issues
> in Unity?

It's the most current data we have.  There's no more current data because
users aren't running Unity with cfq on HDDs today.  It's fundamental to this
class of problem that you can't test it without subjecting users to possible
regressions.

I'm not saying that this should block the SRU.  But it *should* inform the
kind of regression testing done as part of the SRU.  In particular, someone
(or multiple someones) will need to test this kubuntu package on a trusty
system with a rotational disk as part of the SRU verification.  I would ask
that the SRU verifiers also test:

 - installing an Ubuntu precise desktop with unity7 (which I believe was the
   last release using cfq by default) and the .0 kernel (3.2.0)
 - using the system for a bit to try to reproduce the original problem -
   application windows becoming grayed out by the WM under normal usage,
   indicating that the app is not responding
 - changing the scheduler to deadline and testing to see if the problem
   persists or resolves itself
 - upgrading to the trusty backport kernel (linux-image-generic-lts-trusty)
 - verifying that the desktop behaves correctly with the default deadline
   scheduler
 - changing the scheduler to cfq and testing whether the original problem
   recurs

> I'd rather work with data which we have right now ( general feedback from
> users suggests that baloo performance is quite bad with deadline and
> improves with the switch to CFQ ) rather than making assumptions based on
> outdated data.

The previous bugs aren't invalidated by being dated, any more than Ubuntu
deltas from Debian should be summarily dropped on merge without examination.

-- 
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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