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) snd_pcm_prepare(h); }
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?