[alsa-devel] Playback underrun recovery

Paul D. DeRocco pderocco at ix.netcom.com
Sat Apr 21 23:12:20 CEST 2012

I've got a synth program outputting directly to the main hw PCM device, with
no intervening plugins. It uses about 75-80% of the time on one core, so it
normally doesn't underrun. Once in a while, other burdens on the system
trigger an underrun, and my program never recovers. My buffer contains two
periods of 256 stereo frames (with 32-bit samples), which at 96KHz comes out
to a little over 5.3ms total. When it fails, its CPU usage goes up to 100%
on that core, and it spits out bursts of 5.3ms of audio (two full periods,
not one), with about 4ms of silence between them. My top level code consists
of a loop that generates one period at a time:

    int buf[512];

    while (true) {
        generate_samples(buf, 256); // generate 256 stereo frames
        if (snd_pcm_writei(h, buf, 256) < 0)

The funny thing is that if I break to the debugger and then restart, I get
exactly one underrun as you would expect, and then it recovers. What could
cause it to get into a state where it fails every other time around the
loop, and then sucks up milliseconds of CPU time before it gets going again,
thus guaranteeing another underrun?


Ciao,               Paul D. DeRocco
Paul                mailto:pderocco at ix.netcom.com 

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