Revisiting the kernel's preemption models (part 1)

Posted Sep 22, 2023 13:56 UTC (Fri) by farnz (subscriber, #17727)
In reply to: Revisiting the kernel's preemption models (part 1) by kazer
Parent article: Revisiting the kernel's preemption models (part 1)

RT isn't only of importance in safety-critical systems; in audio systems, 3 ms of latency is roughly equivalent to 1 m distance between the output device and the listener. For simple listening cases, this is a non-issue; you just have a big enough buffer that any plausible latency is covered by the buffer (e.g. a 3,000 ms buffer will cover most likely stalls). But for telephony, live effects and other such cases where there's feedback between the microphone and the output, you need to bound latency to a much smaller value.

The usual distinction in the RT world is between hard RT and soft RT; a system is soft RT if it recovers from a missed deadline as soon as it starts meeting deadlines again, and hard RT if further action has to be taken to recover or if recovery from missed deadlines is not possible. In this setup, audio is soft RT - if you miss a deadline, the audio system is in a fault condition, but as soon as you start hitting deadlines again, it recovers automatically; a fly-by-wire system is hard RT, since a failure to meet deadlines can result in the plane needing maintenance work.