Re: [LAU] Realtime latency kernel testing

On 12/12/2010 01:41 PM, Robin Gareus 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

Thanks for your excellent info.
I will need to have a closer look on specific details to get a better
understanding of the problem with doing this kind of testing.
Kernel .config is not my language yet, for instance.

 From a practical point of view for someone as ignorant about the
technical details as myself, I suppose I'm just trying to get a general
idea of what you get from different kernels.

So, I was making outlines on how to do these two things:

1. a test/script for making tests on a single machine to compare
performance on different kernels.

2. results summed up in a table that gives you a general idea of what
you get with different kernels.

There could perhaps be a number of different tables for different
processors, assuming that the processor is the main important factor
that decides the actual latency you get with a kernel.

These tests could then be published in a wiki, if deemed worthy.

- Why such a table should be made, and for who?:

Some practical uses that may require a rt kernel:
* live audio processing (requires low latency)
* monitoring (requires low latency)
* using firewire devices (As far as I understand ffado works best with
-rt kernels)

Possible/impossible?

-- 
ailo
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