[alsa-devel] usleep() and nanosleep() timings seem inaccurate using ALSA

[Takashi Iwai]

At Fri, 30 Mar 2007 09:53:33 +0800,
Carlo Florendo wrote:
> 
> Takashi Iwai wrote:
> > At Thu, 29 Mar 2007 17:46:38 +0800,
> > Carlo Florendo wrote:
> >> Good Day!
> >>
> >> After studying the intricacies of MIDI, I ended up writing an 
> >> implementation of the MIDI protocol and file format.
> >>
> >> I then studied the ALSA sequencer API to be able to control a synthesizer 
> >> keyboard and play MIDI files.  I've used aplaymidi, aconnect, arecordmidi, 
> >> and all those great ALSA utilities.  They're very good!
> >>
> >> However, I wanted to have a simple ncurses based, command line MIDI 
> >> sequencer meant for small Linux distributions such as DSL or Trustix so I 
> >> began to write a command line sequencer using ALSA.  I've encountered one 
> >> problem about using usleep() and nanosleep() especially in 2.4 kernels.
> > 
> > You cannot get a small sleep usually on user-space processes.
> > Usually, usleep() is implemented with select/poll and its timeslice is
> > defined by HZ in kernel config.  In most cases, it's HZ=100, 250 or
> > 1000 while 2.4-i386 kernel supports only HZ=100.  That is, the least
> > sleep time is 10ms no matter what value you pass to usleep().
> > 
> > This can be overcome by using a realtime schedule class and priority
> > like JACK does.
> 
> Ok. I will take that seriously.  I've heard about JACK and know it's 
> popular but I've never used it.

Well, I don't mean JACK handles the MIDI in that way but as an example
of real-time (audio) application.  Basically you have to just set
process scehduler class to SCHED_FIFO and raise the priority.  Then
your process would get invoked as soon as triggered, such as return
from poll() events.

> In any case, how does aplaymidi (or even timidity) produce the proper 
> timings using ALSA?  (In other words, how does the queue output sounds with 
> correct timings?)

The ALSA sequencer queue is implemented on kernel, so it can have and
use more accurate timer sources.


Takashi