I'm building an environment for voice quality testing, and found a strange behavior. Maybe someone has an explanation. Two identical physical computers (PC Engines APU2), connected back to back (A and B). A initiates the test calls to B's SIP address, and then both ends go through a small dialplan (less than 100 lines of XML, similar to https://github.com/voxserv/fsqa/blob/master/fsqa.xml and start recording the session. The dialplan initiates session recording to a Raw file, and plays back a 20-second audio sample, and then inserts the filename into a Redis queue, then hangs up. Then the files get processed by Sevana AQuA software in lowest CPU priority, and deleted. The files are written into a tmpfs filesystem. With 2 calls per second, I get 48 simultaneous calls, and B indicates frame drops in the audio recordings, while A shows good results. The CPU average idle time is always above 50% on both computers. Why is the B side (the one receiving INVITEs) getting more frame drops than the A (the one originating the calls)? I flipped the sides, and then the other machine started to get more drops. At first I started recording in WAV format, and similar effect started to appear at a call every 2 seconds, resulting in 12 simultaneous calls. With raw recording, the performance is much better, but still it's a mistery why there are dropped frames in the audio. The CPU is always running at its highest frequency, under "performance" governor.