9 Oct
2003
9 Oct
'03
11:04 a.m.
Robert Jonsson wrote:
Thursday 09 October 2003 12.26 skrev wes schreiner:
I'd like to see what you read. In my experience it either works or it
doesn't. One can't avoid using APIC interrupts in a dual-processor
system.
jordan muscott wrote:
Hi,
I'm pretty sure I've read that APIC is a no no and should be avoided.
Ok to be honest I'm not gonna switch
distros...... but are you saying
that Redhat offers you extra software that allows you to change the
IRQs that your pci cards are on?
There is no such software on any distro. Your motherboard's BIOS decides
which PCI slots get which IRQs. In a few motherboards the BIOS lets one
select which IRQs get assigned to certain slots, but most don't. So with
most motherboards all one can do is move cards around to different slots
and then see what IRQ gets assigned. If your sound card and your
ethernet card are sharing an IRQ, that's because those PCI slots used
both have the same IRQ assignment. If you swap just those two cards
slot-for-slot they will end up with the same IRQ again. Try moving just
one of the cards to another slot. If all of your slots are full then
move multiple cards.
On some motherboards with some processors you can turn on Local APIC
support in your kernel config and get more IRQs to work with. Dual
processor motherboards, even if they have only one CPU installed, can do
this to get more IRQs. If you have dual CPUs you should already be
running a SMP kernel and you probably don't have IRQ assignment
problems. If you do, it's back to juggling cards.
wes
Here's a link to some interesting info about
"normal" IRQs in a PC. There is
more to it than just having an "OWN" IRQ, they have different priorities.
http://myweb.cableone.net/eviltwin69/Arcana.html#IRQs
See also (the source)
http://www.djcj.org/LAU/guide/Low_latency-Mini-HOWTO.php3
/Robert
Yes, that HOWTO has good info, as far as it goes. One thing I would add
though, is that when using non-APIC interrupts the interrupt priority is
not fixed in stone (or silicon, as it were). Using the program
"irqtune" one can change interrupt priorities (see
http://www.best.com/~cae/irqtune ). I havn't used irqtune in a while,
not since kernel 2.2 days, so I'm not altogether sure that it works
correctly with 2.4.x kernels. On Debian it is in the "hwtools"
package. Hmm, I just tried it on a box with a 2.4.21+lowlatency kernel
and it seems that it worked, though it gave some warnings. I can't
really test if the IRQ priorities have changed though. YMMV.
wes