My experience is the following :

I had a serious xruns problem using both the internal sound card of my laptop and my M-Audio MobilePre USB sound card.
After trying a lot of things, the only config that worked for me was to set my rtprio at priority of the real-time clock minus 11 (so that the jack watchdog is just under the real-time clock).
These are rather low values : priority is 50 for the rtc and 39 for rtprio in my limits.conf

but since i set up this configuration, everything runs smoothly at just under 10 ms using 3 instances of yoshimi, hydrogen, seq24 ardour and eventually a gx_head with my guitar as input. No xruns at all !!

I used "ps -eo rtprio,cmd" to check the rtc value with my realtime kernel.

hope it can be of any help...

jy

2011/3/30 Uwaysi Bin Kareem <uwaysi.bin.kareem@paradoxuncreated.com>
I found that I could run 0.363 ms latency, quite easily. No dropouts, if just running renoise.
I did compile a kernel where I turned off all that I could though. What I remember offhand - maximum level of preemption (no need for rt kernel).
Preemptible RCU,
SLAB (doesn't do as much stuff as slub, and probably therefore has lower latencies)
I didn't find enabling tickless to be a problem.
250hz timer.
Turning off IOMMU's, and similar things. You don't need them on boxes that has below 4GB RAM. IOMMU adds to latency.
Disabling all debugging, hacking, securtity, and other stuff you don't need.

Also you need to make sure that realtime-threads get the highest priority.
I also had to modify rtprio to 99 and nice to -20, in /etc/security/limits.d/audio.conf on ubuntu natty.

This is my full local config.

http://pastebin.com/JCie5RRN

I'm on a core2duo, also using a nvidia gtx280, without problems. 0.363 ms latency was with renoise, jackd and konnekt24d.
The kernel is 2.6.36-zen0 http://downloads.zen-kernel.org/snapshots/zen-stable/linux-2.6-zen.v2.6.36-zen0-full-git.tar.gz

I use this because it has some low latency patches, from 38 and 37 on top of 36, which has the old firewire stack.

Peace,
Uwaysi.
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