[arch-proaudio] cyclictest

[Joakim Hernberg]

On Sat, 23 Dec 2017 23:28:29 +0100
LX <gimmeapill at gmail.com> wrote:

> [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

> 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?

This has a quite simple answer, note the line that says:
# /dev/cpu_dma_latency set to 0us

This is a device that a normal user doesn't have access to, but opening
it and writing 0 to it and keeping it open tells the pstate driver that
you want the absolutely fastest response, so it effectively
prevents the CPU from entering anything else then C0 and C1 states,
which explains the lower results.  AFAIK it's available on most modern
Intel CPUs.  You can verify it by running i7z in another terminal.


-- 

   Joakim