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

Steve Riley steve at rileyz.net
Wed Oct 8 17:25:03 UTC 2014

On 2014-10-08 09:36:03 Steve Langasek <steve.langasek at ubuntu.com> wrote:
> 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.

To my knowledge, Ubuntu is the only distribution that changes upstream's default from CFQ to deadline. I read the Launchpad bugs and the linked IRC logs; it seems that the reasons for making the change (around the time of Precise) have been largely forgotten. Perhaps it's worth revisiting the decision?

> 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.

How much testing did Ubuntu do before changing away from upstream's default? Which aspects of Ubuntu performed "pathologically" that necessitated the change?

> 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.

To clarify: Baloo _does_ use ionice. It's the lack of support for ionice in the deadline scheduler that's the root of the problem here.

...Steve Riley

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