Hi, Stefan,

Thanks for your input.

If you think about it, 4.0 milliseconds amounts to 3.2 million clock cycles (at 800 MHz), which amounts to somewhere in the neighborhood of (conservatively) 1 million processor instructions.  And compared to the FSB bandwidth, that's one three millionth of what's available.  Further, the total data stream of the FP10 (10 input ports at 96000 24-bit samples per second) is 23 megabits per second.  That's about one fifth of what the PCI bus can do, about 1/4 of what a firewire interface can manage, and about one two-thousandth of the memory move rate of the computer (DDR2-800).  (http://en.wikipedia.org/wiki/List_of_device_bit_rates provides a handy chart for this sort of thing.)

Given the fact that my machine has four independent cores and is running ONLY a single user and a fairly light mix at that, I just have a hard time imagining that I would ever see an XRun at all.  There is just too much muscle in a modern computer for that - even in a single core machine.

Your thought about NMIs sounds more plausible.  I have suspected that there may be a problem related to the video interrupts.  Still, this is a whole lot of hardware to not be able to handle this task fluidly.

I've filed a ticket on the development site.  I hope I can get some help with this.  I would really like to just RECORD.

Thanks again,


On Fri, Apr 22, 2011 at 3:03 AM, Stefan Richter <stefanr@s5r6.in-berlin.de> wrote:
On Apr 21 Christopher Shubert wrote:
> + /usr/bin/jackd -P70 -dfirewire -r96000 -p128 -n3

128 frames per period times 3 periods per buffer divided by 96000 samples
per second give merely 4.0 milliseconds coverage by buffers.  Try a larger
number of frames per period (or, if acceptable, a lower sample rate).

> Given my hardware, I wouldn't expect to see any XRuns at all.

You don't need lots and lots of bandwidth, you need
 - guaranteed minimum bandwidth,
 - guaranteed maximum latency.
A commodity server/ desktop OS running on commodity PC hardware with a
commodity PC BIOS doesn't give you any such guarantees.

Check the BIOS settings hat anything that might involve NMIs (non-maskable
interrupts, e.g health monitoring) is switched off.

You are using kernel 2.6.35 which should be quite good; nevertheless,
check if there is a newer kernel package available for your distribution.

You could switch to the "performance" CPU frequency scaling governor,
though I don't think the on-demand governor which you are perhaps
currently using increases latency that much.

(Maybe others can comment in detail on your ffado-diag and jack outputs;
I am just an occasional FFADO user.)
Stefan Richter
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