Re: [linux-audio-user] APIC is bad?

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Subject: Re: [linux-audio-user] APIC is bad?
From: Mark Knecht (
Date: Fri Jul 16 2004 - 21:15:32 EEST

Florin Andrei wrote:
> Well, the thing is, without APIC (default with Fedora single-CPU
> kernels) there was some overlap in the way the devices were assigned
> interrupts. Nothing bad (especially since the EMU10K1 got it's own IRQ),
> but still i thought there is room for improvement.
> So, since IO-APIC usually provides more interrupts, i thought, well, if
> there are more IRQs available, the kernel might find a better way to
> assign them.
> The reality is quite the opposite. With IO-APIC the IRQs suffer from
> more overlapping than without it. Which is kind of strange to me.
> BTW, the mobo is based on the NForce v1 chipset.

As per Clemens' note this is complicated. I'm not sure I understand your
feedback about 'suffers from more overlapping'. Could you post the
output of /proc/interrupts for the same machine configuration with
IO-APIC and old style interrupts?

One other 'misdirection' you may run into here is that many motherboards
will tie together the interrupt line from multiple PCI slots to a single
wire and run that line to the interrupt controller. This was very common
with the older style interrupts and would show up as two or more devices
sharing an interrupt. The only way around this with the older machines
was to move boards around.

With the newer IO-APIC machines the interrupt controllers have more
inputs and there is generally less reason for motherboard designers to
tie these together, but I have heard that they are still tied together
on certain motherboards. (I don't know if this is true...I've just heard
it.) Therefore it might still happen that someone (you?) would see
shared interrupts because fo the way the motherboard is designed. The
info is often in the motherboard manual, or at least it alway is for
Via-based motherboards. I have one non-audio nForce2 box but it's
running in traditional mode:

mark_AT_gandalf mark $ cat /proc/interrupts
   0: 112390402 XT-PIC timer
   1: 126469 XT-PIC keyboard
   2: 0 XT-PIC cascade
   5: 1336833 XT-PIC NVidia nForce2, eth0
   8: 2 XT-PIC rtc
   9: 0 XT-PIC acpi
  10: 68038461 XT-PIC ohci1394, ehci_hcd, usb-ohci,
usb-ohci, nvidia
  11: 2188451 XT-PIC ide2
  12: 10755041 XT-PIC PS/2 Mouse
  14: 3 XT-PIC ide0
NMI: 0
ERR: 0

As always, I think this is an interesting topic being that I'm a PC
hardware guy. I cannot imagine why too many musicians would want to go
through all of this though!

Good luck!

- Mark

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