Re: [FFADO-user] Xruns in Fedora 14 and Mandriva 2010.2

[Stefan R. <st...@s5...>]

On Apr 18 Luis Pablo Gasparotto wrote:
> The strange thing is I'm able to do it with an EMU 0404 USB (which is
> supposed to be more prone to xruns than a firewire interface) with 0
> xruns.

The DMA from and to the USB device is entirely managed in the kernel.
Linux FireWire sound drivers (FFADO) on the other hand manage a good deal
of the DMAs by means of userspace threads.  These threads depend on
being run when its time for them, but there is no guarantee for that.
There is no such guarantee for in-kernel threads either, not even for
soft-IRQs/ kernel tasklets, but chances especially of soft-IRQs/ tasklets
are better than those of userspace threads for the time being.

> I get xruns in both 2 distros (Mandriva and 
> Fedora) so I guess it's an IRQ issue but I'm unable to follow this guide:
> http://subversion.ffado.org/wiki/IrqPriorities 

Threaded IRQ handlers which can be prioritized are a feature of the -rt
kernel patch.  The other kernels don't have this feature.

Starting with kernel 2.6.39, threaded IRQ handlers can optionally be
enabled in the stock kernel too.  I don't know though whether the chrt
command has any effect on 2.6.39 IRQ handler threads.

> Things get better (but not perfect) if I use the CCRMA RT kernel but I should be able 
> to do it with the regular kernel. 

Why do you think so?  Linux is not a realtime kernel.  PC hardware is not
designed for low-latency realtime applications either.  The FireWire bus
is designed for it, but whether generic consumer-grade hardware lives up
to it is another matter.

However, either a kernel with the -rt patch (which still doesn't make
Linux a realtime kernel in a stricter sense but helps) or a newer kernel
(newer than the 2.6.35 of F 14 or 2.6.33 (?) of Mdv 2010.2) may work
better for you.

I currently run mainline 2.6.38.2 here.  It seems I never get xruns with
moderately sized buffers and with Timemachine running as the only jackd
client --- not actually writing to disk, just sitting there listening and
drawing its level meters.  I do get occasional xruns when audacious2 plays
an internet radio stream through jackd...
-- 
Stefan Richter
-=====-==-== -=-- =--=-
http://arcgraph.de/sr/