[LAU] kernel 5.15 irqs

[Len Ovens]

On Thu, 24 Feb 2022, Brent Busby wrote:

> Would not having threadirqs set in a modern kernel be a poor 
> configuration for Linux audio (potentially cause xruns with Jack)?

It depends...
  1) on what latency you wish to work with
  2) on the mother board and CPU
  3) on how the various irq happen to get prioritized
  4) on how much use the core your device is using gets
  5) numerous other things

It is (remotely) possible that someone might have a setup where their 
audio device just so happens to end up prioritized over almost everything 
else. Or that the required latency is high and the computational load is 
low enough that audio is just always ready when the device is. However, in 
most cases this is not true. Remember that Motherboards, CPUs, GPUs, etc. 
are a product to be sold. The general measure of a "good" product is data 
throughput and so all these things are designed to get maximum throughput 
all other things being equal. Another thing concidered "good" is 
operational feel. This might make mouse responce seem more important than 
low latency sound for example.

I did read some of the Intel documents about HDA audio. One of the 
"features" is low latency audio.... which they define as at least 30ms 
latency. Perhaps not very good for guitar effects.

Threadirqs allows the user to change some of these priorities, to make 
sure your audio device gets taken care of before your mouse or the 
internal audio. Effectively, you are able to take a machine designed for 
one use and make it work better at another use. This means less xruns at 
very low latencies and less xruns even at higher latencies if you are 
pushing your proccessor hard. It is not magic but a good start to tuning a 
system for audio work.

While there have been many rules of thumb in the past for tuning a system 
for audio, the reality is that each system is different and requires 
different steps (with testing). The other reality is that some of the 
newer systems have fewer choices (like laptops) and the user will just 
have to get used to higher latency audio use. Of course USB 2.0 audio has 
it's own group of difficulties as well. Perhaps USB3 or thunderbolt will 
offer something that helps.



--
Len Ovens
www.ovenwerks.net