[linux-audio-dev] jack in a live situation (LinuxSampler has this problem too)

[Benno Senoner]

Paul Davis wrote:

>>A question regarding jack use in a live setting. As far as I can tell
>>the soft mode only works with non realtime jack. What should I do if I
>>    
>>
>
>it should work with realtime mode too. this was specifically added
>several (many?) months ago, for precisely the reason you gave.
>  
>
Apparently it doesn't. I'm using 
jack-audio-connection-kit-0.98.0-2.rh90.ccrma and I have
the same problems.
LinuxSampler is producing short CPU peaks when lots of voices gets fired 
up and sometimes jack still kicks
it out. Very annoying and if one wanted to use LinuxSampler live it 
would be impossible because you would
never have the guarantee that jack does not kick it out, restarting jack 
and clients on stage is not an option :)
So please jack developers check this issue and fix it in the next update 
it will be very useful for anyone wanting
to use jack clients like softsynths,samplers, FXes etc in a live situation.
A sporadic  (let's say one each 10min or so) fragment xrun would be 
barely audible and would allow the live player
to continue playing going almost unnoticed rather than telling the 
audience "I'm sorry jack kicked my synth wait a little
bit so that I restart it so you can continue enjoying my performance :) )

>but yes, write better clients anyway :)
>  
>
LinuxSampler's jack callback is completely free from mutexes and bad 
syscalls, it could be improved by eg
limiting the number of voices that can be started/released per audio 
fragment, that way the CPU spikes are
lowered but as said you would never have the guarantee that jack does 
not kick it out.
Sometimes when the OS is loaded (due to heavy disk I/O etc) it could 
delay the execution of jack clients too
causing it getting kicked because jack thinks the client took too much 
to execute.
LinuxSampler needs to work with small fragmentsizes, 64 - 128 frames so 
that you can play it lively so the
kicking problem is more severe than in ardour where you can easily go 
with higher fragmentsizes (256-1024) without
the user complaining too much.

cheers,
Benno
http://www.linuxsampler.org