[ardour-users] catching xruns that are caused by disk transfers

[chris simpson]

thanks to helped me with my previous post (running FIFO as root), i've fixed
both problems listed, which were related funnily enough. the FIFO problem
was solved by editing /etc/security/limits.conf (thanks Vegard).

anyway, i've come across a solution (that while may not be ideal, still
works), with catching some xruns. i know my system can handle a fairly heavy
load during recording, but the disk transfers do create some xruns that can
be cause at the end of recording, or opening / importing. unfortunately this
also affects mixing down. on the ardour.org forums, paul mentioned that
running a kernel with RT doesn't affect the scheduling of ardour.

this lead me to realise that while jack has abstracted all audio for
playback and recording (that is, ardour does not talk to a soundcard
directly, ever), jack doesn't abstract the loading or saving of the audio
files themselves. so in that case, ardour talks to the VFS, which in turn
talks to the kernel or whatever and does the disk transfers. the problem is,
if for whatever reason the drivers don't run particularily well or you have
old archaic disks but have fast memory, recording (and possibly playback)
will be a breeze, but mixdown (if jack is set to realtime) can cause alot of
xruns.

my way around this was to compile jack with my ramfs/shmfs to a fairly
common place - "/media/songs_working", and then mixdown everything to here
first. this ignores disk transfers completely, and my experience (so far)
has been no xruns, where there were previously lots with an otherwise
identical setup. compiling a RT thus will also help with some playback
xruns, as while it might not change anything with the audio handling layer,
it might schedule disk transfers better (which again so far has been my
experience).

the other benefit of this, is often i find myself having to mixdown a number
of times to get the file i need (ie, master tracks weren't selected,
connections went directly to hardware instead of ardour master etc); doing
it this way writes directly to memory so it saves much quicker (which is
great if you have alot of effects or hi resolution), and then previews
quicker (in xmms etc).

anyway, that was long winded, but i thought it might be useful to some...
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