[alsa-devel] usleep() and nanosleep() timings seem inaccurate using ALSA
tiwai at suse.de
Fri Mar 30 14:56:27 CEST 2007
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.
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