====== How to optimize JACK (Audio Server) ======

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ripped from Linux-audio-tuning@lists.linuxaudio.org (FIX)
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2008/8/9 Robin Gareus <robin@gareus.org>:
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> trece ocho wrote:
>> Hi there!,
>
>> I want to optimize JACK capabilities in running Linux PCs.
> Welcome Trece ;)
>
>> What JACK configuration parameters do I have to vary (and in what way) to
>> achieve that?
>> (by "that" I mean: minimum latency, zero noise and zero xruns).
>
> read `man jackd`  - you want to set "-p <num> -n <num>"
>
>  -n: usually 2 or 3 - mostly depends on your sound-card
>  -p: 32,64,128,... 4096  "the period" defines buffer-size and latency
>
> There's no "best" latency settings.
>
> For recording you probably want to use a small period size for low
> latency monitoring. But when you master, mix or edit: raise it up to
>>=1024 (many postprocessing effects are very CPU intense, and you do not
> need low latency to listen).  Most versions of JACK allow the period to
> be changed during runtime: `man jack_bufsize`
>
> Latency and Xruns are related: The higher the latency the less chance of
> xruns.  To get a 'zero-xrun' system you want to run jackd with
> /realtime-priority/ on a realtime-patched linux kernel and lock the it's
> pages into memory (no swap):
>
>  1. get or compile a realtime kernel
>  2. set up IRQs
>  3. permit audio users to use realtime capabilities
>  4. start jackd as user with realtime capabilities
>
>
> For (1) I suggest to get a binary for your gnu/Linux distribution.
>  64studio, musix, ubuntu-studio, gentoo-pro-audio-overlay, CCRMA..
>
> (2) 64studio offers a debian package rtirq for a great script from Rui:
>  [[http://www.linuxaudio.org/mailarchive/lau/2006/6/26/127914]]
>  There's probably other packages as well.
>
> (3) that's the /etc/security/limits.conf part
>  [[http://tapas.affenbande.org/wordpress/?page_id=73]]
>
> (4) /usr/bin/jackd -R -P 70 -T -d alsa -n 3 -r 48000 -p 128 -d hw:1,0
>   or use the "Realtime" checkbox in qjackctl's Setup dialog.
>
> zero noise: N/A -  check your cables ;)
>
>> I'm looking for something like a "standard procedure"...
> use qjackctl.
>
>
>>...but sometimes the audio crackles, has xruns, can't get it to work
>> even with high latency, etc... and I don't have a clue about what to
>> vary to fix that.
>
> aah, although this sound can be described as /noisy/, x-runs are no
> *noise* ;) and are not only correlated to latency.
>
> They're normal:  Unix is //fair// to all users and sometimes does not
> see a reason why it should keep playing your sound or even keep it in
> memory UNLESS you tell it to do so. (steps 1-4 above)
> There's a lot of background-jobs on a default gnu/Linux; fi. `updatedb`
> may get in your way..
>
>> I'm working..
> [..]
> That sounds interesting. - I think you should post that to the
> Linux-Audio-Users mailing list (or if you have questions about
> program-code to  linux-audio-dev). Here it is a little off-topic and you
> may get more feedback there.
>
> HTH,
> robin
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