[Techtalk] Who Knows IRQ

Erin Raasch eraasch at chorus.net
Tue Aug 20 12:02:52 EST 2002

8/20/02 11:21:55 AM, mc <mcgonzalez at att.net> wrote:

>On Tue, 2002-08-20 at 09:18, Andrew wrote:
>> Hi People,
>> 	Just a quick question or three about IRQ.
>> 	What is it?
>> 	Should *four* ON-board devices have the same IRQ? IRQ11,= 
>> ethernet,usb,usb, multimedia.
>IRQ is an Interrupt ReQuest line.  Basically, you used to only have one
>device on each one, and that was its main way to talk to the CPU.  If
>your machine had lots of devices, you ended up in IRQ hell, where
>devices all wanted the same IRQ and some would not work.
>By no means a hardware expert, but friends who are tell me that modern
>machines have "PCI steering" so they can handle a bunch of devices on
>one IRQ.  Of course, these folks only run Varieties of Windows, so I do
>not know for sure if that carries over to the Linux world :)

Though IRQ sharing is possible with modern machines, it is not necessarily desirable. 
There is definitely a potential for conflicts when more than one device use the same IRQ.
Even if it's not immediately evident, it may well cause performance issues in very demanding apps.
E.g., in high-end gaming, having the sound and video on the same IRQ has the potential 
to cause problems ranging from jerkiness and slow play at one end to hangs and crashes at the other end.

Sharing an IRQ between USB and ethernet probably won't hurt, but I'd definitely want my multimedia devices
on separate IRQs.


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