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

[Carlo Florendo]

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.

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?)

I've tried my best to read the alsa-lib and alsa-driver code, but I am not 
able to trace, with my very limited skills, how the timings are done. 
Could anyone point out where I should start?

Thank you very much and more power to the ALSA-dev team :)

Best Regards,

Carlo

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Carlo Florendo
Softare Engineer/Network Co-Administrator
Astra Philippines Inc.
UP-Ayala Technopark, Diliman 1101, Quezon City
Philippines
http://www.astra.ph

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