Sounddevice wired playback stream intermittently stops

Question

I have a code for input-ouput wired real-time streaming playback on Rpi 3B+ with Sounddevice/python 2.7.

It does work well, but when I do bit heavier calculation simultaneously, the streams become mute. (for instance, do pitch-shift, fft calcuation..etc) Sometimes it's really gone coma but usually I can close sd.Stream object and make it again, then it works.

Though speaker become mute, I can see the USB soundcard is still activated (LED blinking)

First I have no idea why it happens. Though I can expect sort of unexpected latency could happen but I expected the stream continues after the computation. Obviously not.

Is there any way to diagnosis my sd.Stream "practically" works or not? Or any parameters could improve this situation?