[LAD] ALSA latency and MMAP

[Louis Gorenfeld]

Hi,
  I have a program which, at 2-periods per buffer with 128 samples per
period size and 48khz sampling rate, takes 9ms to pass input through
to its outputs.  However, with the same hardware and settings, I
notice that JACK+Ardour2 take only 6ms (this is through the program,
not through a direct monitoring mode-- effects can be applied to the
input).  The amount of extra latency I see in my program increases
with the buffer size:  it is one buffer extra in our program.  I have
a few questions about this on which I haven't been able to find
sufficient material with Google.
  One difference between JACK and our software I notice is that JACK
is using ALSA's mmap mode.  I am unclear on the advantages of mmap's
begin/commit access vs. using the plain vanilla readi/writei.  If mmap
does offer a latency advantage and not just a CPU advantage, how does
the timing of that work as compared to read/write?  Where's my extra
buffer worth of latency coming from?

Cheers,
Louis