[linux-audio-user] APIC is bad?

Mark Knecht mknecht at controlnet.com
Fri Jul 16 10:28:01 EDT 2004


Florin Andrei wrote:
> I rebuilt the latest Fedora 2 kernel update to enable preempt and
> IO-APIC.
> I'm not yet sure about preempt, but IO-APIC has been acting weird.
> 
> It gave me more interrupts (up to 21 instead of 15), but the devices
> were distributed suboptimal.
> 
> Without APIC, the nvidia module was alone on its own interrupt, the
> EMU10K1 was alone, the ide and eth modules were on separate interrupts,
> etc. Quite ok.
> 
> With IO-APIC, nvidia, EMU10K1 and bttv were on the same interrupt, ide2,
> ide3 and eth0 were on the same interrupt.
> Instead of messing with the kernel again, i just rebooted with the
> "noapic" parameter and now the interrupts are looking good again.
> 
> BTW, anyone has any measurements on how bad it is to put essential
> devices on the same interrupt? (in terms of xruns)
> 

Please note - The 'optimal' numerical assignment of interrupts with 
using the APIC model has nothing to do with the older, non-APIC, order. 
Please do not confuse the idea that 'interrupt #9 is best' with the 
numbers assigned on an APIC system. These are completely different models.

Personally I have done no optimization work on APIC systems and cannot 
tell you what would be best. However, TTBOMK, there is no simple way 
today to optimize APIC interrupts in a Linux system anyway, so as far as 
I can tell you basically take what you get. If you get too many xruns 
then I guess you go back to non-APIC mode.


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