Paul D. DeRocco wrote:

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.

When a PCM device is (re)started, the buffer is initially empty, so you need more CPU to fill it.

What is your start threshold?

My top level code consists of a loop that generates one period at a time:

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

Is this the actual code? You should check if snd_pcm_writei() returns that less than 256 frames have been written, and you should use snd_pcm_recover(), and you should that one's return value, too.

Regards, Clemens