[linux-audio-user] APIC is bad?

Mark Knecht mknecht at controlnet.com
Fri Jul 16 13:08:24 EDT 2004

Clemens Ladisch wrote:
> What I tried to explain was that the priority is not determined by the
> interrupt number but by the interrupt vector number.

Ah, right - vector number not interrupt number.


> When the I/O-APIC is enabled, interrupt priority is determined by the
> interrupt vector number (lower vectors numbers have higher priority).

So, I was backwards, as usual! ;-) Thanks!

> (My patch to change vector numbers doesn't work with current kernels.)
Bummer, but I must admit I wasn't aware that there was ever a patch to 
effect this. Maybe one day it will work again?

On the issue where I thought you and I never really finished before, in 
the older style interrupt processing there seems to be a measureable 
difference in the number of xruns when my cards are placed on IRQ9 (best 
user level interrupt) vs. IRQ7. (worst user level interrupt) This is not 
Linux specific at all. I wa sort of surprised when I came to Linux that 
more people weren't aware of this sort of optimization. It was very 
common for us optimizing Windows machines for audio to mess with this 
sort of thing.

However, on the IO-APIC machines it was my impression that you were 
saying that it didn't matter what interrupt vector hardware was placed 
on. Was I incorrect about that? Were you just saying that it didn't 
matter what interrupt number it was placed on since you had a patch to 
make any interrupt number be the highest priority vector?

In my mind there is no cange in the IRQ service routines no matter which 
sort of machine I work on. If I place a sound card on the worst 
interrupt vector (highest number) and, let's say, I put a bthersome 
Ethernet NIC on the highest priority vector (lowest number) then 
logically I would guess that the Ethernet interrupts will get in the way 
of the sound card doing it's work and we'll get more xruns.

Do I have this right in your mind, or am I still off base on IO-APIC 
machine operation?


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