Re: Realtime latency kernel testing

[allcoms <allcoms@xxxxxxxxx>]

Thanks Robin for your massively useful tips there on how to (stress) test JACK - I think what you just said, in an edited form, should be included on the JACK FAQ page please Paul!

On Sun, Dec 12, 2010 at 12:41 PM, Robin Gareus <robin@xxxxxxxxxx> wrote:

On 12/12/2010 01:03 PM, ailo wrote:
> I've been looking around for any tests made comparing the different
> kernels, -rt, generic, or any other type of realtime enchanced kernel.
> I haven't found any test results yet, at least none audio related. I did
> find some testing tools at rt.wiki.kernel.org, but don't know if and how
> they could be made relevant to audio low latency testing.
>
> I suppose the most interesting results would come from testing different
> kernels with jack/alsa and jack/ffado.
>
> Has anyone done such tests?

It is not trivial to perform such tests and AFAIK there's no benchmark
suite to automate the process.

There are a few tools to test JACK's realtime performance:

 - the ardour-source includes `tools/jacktest.c` checks for the max DSP
  load at which an x-run occurs.

 - http://wiki.linuxaudio.org/wiki/jack_latency_tests
  git://rg42.org/latentor is a tool to automate measuring round-trip
  audio latency iterating all JACKd -n/-p/-S parameters
  However latentor is a pretty recent development and does not yet
  report x-runs. We watch qjackctl's icon for now.

AFAICT there's no recipe. It's a matter of knowing some internals about
RT-linux to come up with a proper kernel .config and doing real-life
tests. I think it is impossible to assign a number "suitability for
pro-audio" to a kernel.

For testing performance of the 64studio RT kernel: I do run a couple of
heavy-sessions (e.g. 16 jconvolvers in a 16 track ardour session + jamin
which procudes quite some DSP, system and IO load). If there's no x-run
at 32fpp*2p/48kHz after 24 h while I to surf the web and read email and
compile another kernel in the meantime I bless the build OK :) There's a
few additional things: wifi, suspend/resume, freq scaling, etc on the
checklist, too.

2c,
robin

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