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

Oleg Samarin osamarin68 at gmail.com
Sat Sep 6 05:28:22 PDT 2014

В Пт, 05/09/2014 в 15:17 -0700, Fernando Lopez-Lezcano пишет:
> 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? 

I use Jack audio stack and can see the DSP load with qjackctl. 
Every time I can hear audio distortion, qjackctl shows 100% DSP.

When I just start GrandOrgue, the DSP load is about 15% and top shows
that GO utilises 19% of CPU. 

When I start playing with GO under kernel-rt 3.14.17, qjackctl shows
100% of DSP load and top shows that GO uses 121 % of CPU and the
distortion begins. At the same time i7z indicates, that CPU frequency is
between 1600-1800 MHz.

The same under kernel-rt 3.12.12 brings about 40-50 % in qjackctl and
about 60% CPU of GO in top. i7z indicates CPU frequency 4500-4800 MHz.

> What does /proc/cpuinfo report in your case?

It is difficult to play GO and capture /proc/cpuinfo at the same time.

> > 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?

Yes, it looks better, but the CPU freq (3800-4000 Mhz) is still less
than under 3.12.12.

> > 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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