Re: [FFADO-devel] The origin of the recommendation to use "-n 3" with ffado

[Jonathan W. <jw...@ju...>]

On Tue, Apr 03, 2012 at 03:52:31PM +0200, Clemens Ladisch wrote:
> Jonathan Woithe wrote:
> > I've been wondering what the rationale is for recommending (in the README
> > file and elsewhere) that users of the "firewire" jack backend use "-n 3"
> > (that is, 3 buffers) rather than 4 (or some other number). ...
>
> "-n 2" would be optimal (if you want low latency) in theory, but FFADO
> has the problem that interrupts are generated after a fixed number of
> packets, which is not necessarily immediately after a period boundary.
> 
> It might be possible to get two periods to work reliably by increasing
> min_interrupts_per_period to some value larger than 2.
> 
> An alternative would be to change FFADO to use its own timer to wait for
> the end of the period and then to explicitly flush packets from the
> kernel (possible only with the old stack or with Juju in kernel 3.4 or
> later).

Ok, so 3 is just the smallest value of "-n" which can be expected to
work.  One could validly use higher values and still expect things to work,
but for lowest latencies 3 is it.

At this stage I see no compelling reason to alter FFADO at this point in
time in order to reduce this minimum "-n" value to 2.  I posed the original
question because I was curious as to where the value of 3 came from; some
comments I've seen around tend to suggest the "3" is *the* value to use and
that nothing else is expected to work.  Your explanation clears that up -
thanks!

Regards
  jonathan