[linux-audio-user] alsa rme96 jack - 186 msec latency?

[Eric Dantan Rzewnicki]

On Sun, Sep 11, 2005 at 05:11:19AM +0200, Michael Rudolf wrote:
> On Friday 09 September 2005 03:10, Eric Dantan Rzewnicki wrote:  
> > On Thu, Sep 08, 2005 at 08:01:16PM -0500, Reuben Martin wrote:
> > > I've run jack with reduced number of periods without any problem.
> > > You're output lookes like you have issues elsewhere though to fix
> > > before worrying about the numer of periods you use. The xruns,
> > > messages about interrupt delays, and driver messages suggest you have
> > > problems that need fixed berfore jack will ever work well, reguardelss
> > > of the number of periods you use.
> > > I don't know enough to comment on h/w design or driver design, I just
> > > know I've been able to use JACK with this driver at very low latencies
> > > without X-Runs.
> > > If I can help in providing specific info, let me know. :)
> > And if there are any specific problems with the drivers, let me know as
> > we have so many I'm sure management could be persuaded to let us spend
> > some time testing, debugging and fixing stuff.
> This mailing list is amazing. Thanks for all the info; it sounds promising.
> This is my configuration again:
> Sound Driver:3.8.1a-980706 (ALSA v1.0.9b emulation code)
> Kernel: 2.6.12-oci2.mdk with realtime lsm module and 
> PREEMPT/PREEMPT_BKL enabled. Distribution is PCLinuxOS P91 fully updated 
> and with the Jack packages from thac 
> (jackit-0.100.1-050708.1.pclo2005.thac).
> >From what I have read by now, it seems that preempt and realtime is not 
> enough; what I'd need would be "realtime preemption", as in a single patch 
> by Ingo Molnar (http://people.redhat.com/mingo/realtime-preempt/). I also 
> seem to be missing the "chrt" tool to change interrupt priorities, and 
> don't know yet where to obtain that.
> Eric and Reuben, are you both running 2.6.13 with said patch and the chrt 
> tool?

I'm using 2.6.13 without additional patches. PREEMPT and PREEMPT_BKL
seem to be sufficient.

in debian the chrt tool is in the schedutils package. But, I only use it
to check the priorities. I haven't had a need to change them in a
properly configured system. jackd gets SCHED_FIFO just fine here without
me needing to manually adjust anything with chrt.

> Another point is shared memory; I have that enabled in the kernel, I have a 
> tmpfs mounted on /dev/shm, and Jack is compiled with support ("JACK 
> compiled with System V SHM support."), but I have read that Jack uses /tmp 
> for pipes per default, and I can in fact see some 
> in /tmp/jack-[uid]/default/. Now /tmp is a normal ext3 file system here, 
> not a tmpfs. Could that alone be the problem?

I compile jack --with-default-tmpdir=/dev/shm since /dev/shm gets
automatically mounted as tmpfs when tmpfs is enabled in the kernel
config. As Lee has said, having your jackd tmpdir on ext3 is sure to
cause you latency problems.


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Eric Dantan Rzewnicki  |  Systems Administrator
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