Karmic I/O scheduler issues

Tim Gardner tim.gardner at canonical.com
Tue Sep 29 23:39:23 UTC 2009

Eugene Gorodinsky wrote:
> 2009/9/29 Tim Gardner <tim.gardner at canonical.com>:
>> I've been messing with I/O scheduler settings in the last couple of
>> uploads. We found that CFQ has some serious issues that contributed to
>> delayed boot times, so I changed the default to Deadline in
>> 2.6.31-10.35. Scott Remnant reported that changing to Deadline shaved 40
>> seconds off his boot.
> Umm... FORTY???

His Freenode IRC nick is keybuk if you'd like to ask him yourself.

>> Since then I found this info: http://bugs.launchpad.net/bugs/436342
>> I've been running a kernel with this patch and CFQ enabled. Perhaps some
>> other folks could compare and contrast Deadline v.s. CFQ using their
>> workloads. I find that CFQ is working a bit better then Deadline. I can
>> run a 'make -j8' kernel build and still watch video while also playing
>> music and reading email (without _too_ much stalling). Deadline is
>> noticeably less smooth when the load average approaches 8.
> The fair sleepers issue is in CFS, not in CFQ. Did you compare
> 2.6.31+patch+Deadline vs 2.6.31+patch+CFQ or 2.6.31+deadline vs
> 2.6.31+patch+CFQ?

You are correct in that NEW_FAIR_SLEEPERS is a Completely Fair Scheduler
(CFS) scheduler input. However, getting the CPU scheduler to run
correctly also has a large impact on I/O scheduling. It could be that
the I/O scheduler never really had a problem, though I suspect the CPU
scheduler and I/O scheduler have _some_ interaction. My tests used the
-11.37 kernel with only the 'sched: Disable NEW_FAIR_SLEEPERS for now'
patch. All I did was to swizzle the I/O scheduler settings.

>> Adventurous types can try this kernel from
>> http://kernel.ubuntu.com/~rtg/2.6.31-11-NEW_FAIR_SLEEPERS
>> (when its done copying)
>> You can swizzle your I/O scheduler settings thusly:
>> cat /sys/block/$YOUR_BLOCK_DEVICE/queue/schedule
>> echo cfq | sudo tee /sys/block/$YOUR_BLOCK_DEVICE/queue/scheduler
>> rtg
>> --
>> Tim Gardner tim.gardner at canonical.com
>> --
>> kernel-team mailing list
>> kernel-team at lists.ubuntu.com
>> https://lists.ubuntu.com/mailman/listinfo/kernel-team

Tim Gardner tim.gardner at canonical.com

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