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

[Steve Langasek]

On Thu, Oct 16, 2014 at 04:23:36PM +0200, Rohan Garg wrote:
> > A consistent kernel and performance expectations across Ubuntu is also
> > worthwhile. I don't want someone screw up database/VM/etc. workloads
> > benchmarks simply
> > because they happen to have kubuntu-desktop installed or left around

> Are we honestly optimizing for benchmarks now? I'd rather optimize for
> a good user experience, which in the case of Kubuntu requires the use
> of CFQ. I couldn't care less about
> random-news-site-running-random-benchmarks.

I don't think Dimitri actually meant "benchmarks" here.  I think "workload
performance" is the real question.

There seems to be a grave misunderstanding that the scheduler change was
working around a bug in Unity (cf. 
https://blogs.kde.org/2014/10/15/ubuntus-linux-scheduler-or-why-baloo-might-be-slowing-your-system-1404). 
This was *never* about a bug in Unity; Unity simply has a feature that
provides visual indication to the user when an application is unresponsive,
and with CFQ on HDD, many users found their apps were unresponsive quite a
lot.  Removing the visual indicator wouldn't improve app responsiveness for
these users; the configurations in question would still have been sluggish
under CFQ.

That baloo is not well-modeled by the performance testing that was done
before, is clear.  But changing the scheduler for the whole system is a very
big hammer, not to be used lightly.  We've agreed that the kubuntu-settings
change is acceptable for an SRU in spite of reservations; but it's in
everyone's interest - including that of Kubuntu users who run VMs on their
desktop, or other workloads that suffer under CFQ - to find a long-term
solution that doesn't require such a heavy-handed change.

If someone were to demonstrate that CFQ is actually better than deadline
with recent kernels across all relevant workloads, then it's a no-brainer to
switch back.  But without that, something like the proposed cgroup handling
is probably in order.

-- 
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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