[PlanetCCRMA] Performance degradation with kernel-rt-3.14.17-200.rt9.1.fc20.ccrma.x86_64

Michael J. Wilson mwilson at alumni.caltech.edu
Fri Sep 5 23:39:38 PDT 2014

Hello all,

I was also noticing performance issues with recent versions of the RT
kernel (CPU load reported by Jack about 12% higher than expected, xruns
under light loads).  I can't seem to set the performance scaling governor:

$ su -c 'echo performance >
bash: line 0: echo: write error: Invalid argument

even thought it appears to be available:

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
performance powersave

This is on:

$ uname -r

It works as expected with the standard kernel.

I wanted to try to debug further before posting to the list but didn't
have time to look into it; it sounds like the same or a related issue

For reference, running this:
$ su -c 'echo "100" > /sys/devices/system/cpu/intel_pstate/min_perf_pct'

does seem to give me the CPU load I'd expect when running Jack in
realtime mode, but I didn't do a very strict comparison.  If it's
helpful I can take some time and do a more formal test.  Please let me
know if there is anything else I should include.

Michael J. Wilson

On 09/06/2014 07:17 AM, Fernando Lopez-Lezcano wrote:
> On 09/05/2014 12:35 PM, Oleg Samarin wrote:
>> Hello!
> Hi!,
>> I use Fedora 20 with PlanetCCRMA realtime kernel.
>> After upgrading the kernel from kernel-rt-3.12.12-300.rt19.1.fc20.ccrma to
>> the last one - kernel-rt-3.14.17-200.rt9.1.fc20.ccrma, the huge sound
>> distortion started occurring. When I boot my station with kernel-rt-3.12.12,
>> everything seems all right.
> Did you try to confirm by, for example, looking at the output of top in 
> both cases? Can you confirm that the cpus are stuck to low frequencies? 
> What does /proc/cpuinfo report in your case?
>> The most probably reason is a bug in intel_pstate driver, that prevents CPU
>>   speeding up: https://bugzilla.kernel.org/show_bug.cgi?id=70941,  There is a
>> patch that should solve this problem.
> Is your system actually using the pstate_intel driver? If so, what 
> happens if you try to get all cpus to run as fast as possible:
>    echo "100" > /sys/devices/system/cpu/intel_pstate/min_perf_pct
> Does that make a difference?
>> Could you include this patch in the next build of kernel-rt?
> Sure, but it would be nice to get more feedback.
> -- Fernando
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