[arch-proaudio] cyclictest

LX gimmeapill at gmail.com
Sat Dec 23 22:28:29 UTC 2017

Aye, you're right, the results of cyclictest are completely different 
when running as root, here's what I get as a normal user with 

[gimmeapill at pill-mobile4 ~]$ cyclictest -S -m -p98
WARN: open /dev/cpu_dma_latency: Permission denied
policy: fifo: loadavg: 0.45 0.51 0.23 1/407 1048

T: 0 ( 1044) P:98 I:1000 C:  21816 Min:      2 Act:    9 Avg:   13 
Max:      52
T: 1 ( 1045) P:98 I:1500 C:  14541 Min:      3 Act:   24 Avg:   14 
Max:      58
T: 2 ( 1046) P:98 I:2000 C:  10903 Min:      3 Act:   15 Avg:   15 
Max:      76
T: 3 ( 1047) P:98 I:2500 C:   8721 Min:      3 Act:    4 Avg:   15 
Max:      65

and as root:

[root at pill-mobile4 gimmeapill]# cyclictest -S -m -p98
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 0.35 0.49 0.22 1/407 1053

T: 0 ( 1050) P:98 I:1000 C:  12415 Min:      2 Act:    4 Avg:    3 
Max:      13
T: 1 ( 1051) P:98 I:1500 C:   8272 Min:      2 Act:    4 Avg:    3 
Max:      12
T: 2 ( 1052) P:98 I:2000 C:   6201 Min:      3 Act:    4 Avg:    3 
Max:      13
T: 3 ( 1053) P:98 I:2500 C:   4958 Min:      3 Act:    4 Avg:    4 
Max:      11

My previous tests where meant to compare the performance between RT 
kernel versions (I just happened to have a root terminal open at that 
time and didn't think further).
For that purpose it worked: I could see a slight degradation since 
4.9.x., but yeah, that doesn't help to compare between different 
systems, better run cyclictest as the audio user.

That difference when running as root is nevertheless puzzling, if these 
results ever translate to real world audio performance, this is really bad.
I thought we were done with running audio applications as root for a 
good 10 years, no?

Regarding the CPU difference: it's a notebook processor from 2012 with 
turbo boost disabled and hyperthreading enabled, set to performance mode.
I wouldn't expect it to be fast by today's standards, or to be able to 
compete core per core with a desktop class cpu of the same era:

The single thread performance between those two is also really 
different, so I wouldn't draw any conclusion regarding latency:



> PS: What does it all mean? A cheap Intel(R) Celeron(R) CPU G1840 @
> 2.80GHz has got less latency, than a much more expensive Intel(R)
> Core(TM) i5-3317U CPU @ 1.70GHz? How does it affect audio work? What is
> the relevance of cyclictest? Is the result somehow related to audio
> latency and/or MIDI jitter?

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