shared irqs and performance

Sandy Rutherford sandy at
Tue Sep 14 22:32:31 PDT 2004

>>>>> On Wed, 15 Sep 2004 10:21:52 +0530, 
>>>>> Subhro <subhro.kar at> said:

 > The card won't be facing any stability problems. And there is no
 > reason why you should force the card to another IRQ because modern
 > motherboards DOES allow IRQ sharing and there is no performance hits
 > for it.

That's supposed to be the case and is usually the case.  However, when
left to its own devices, my Supermicro P6DBE board would always share
an IRQ between a triple channel Mylex extremeRAID 1100 controller and
one of the channels of a dual channel ZX348 ethernet card.  When this
happened, I would always see about 20% dropped packets on the ethernet
channel sharing the IRQ.  Manually overriding the IRQ assignment so
that the RAID controller didn't share an IRQ with anything solved the

None-the-less, if there is no obvious evidence of odd behaviour, IRQ
sharing is perfectly fine.  However, if things are clearly not
performing as they should, investigating potential IRQ sharing
problems is not a bad idea.


 > Regards
 > S.

 > On Tue, 14 Sep 2004 18:56:18 -0600, Gary Aitken <garya at> wrote:
 >> On my 4.10 system, I added in a third ethernet device.
 >> However, I see from dmesg that the irq assigned is shared with my
 >> scsi controller:
 >> de1: <Digital 21041 Ethernet> port 0xf880-0xf8ff mem
 >> 0xfdfffc00-0xfdfffc7f irq 9 at device 16.0 on pci0
 >> bt0: <Buslogic Multi-Master SCSI Host Adapter> port 0xfcec-0xfcef
 >> irq 9 at device 17.0 on pci0
 >> Since the driver was already present, things went ok, in the sense
 >> that the system booted and scsi devices still work.
 >> 1.  Will this work reliably (I haven't actually hooked anything
 >> to the new network card yet)?
 >> 2.  Even if it does work, I assume it would be better to force
 >> it to an unused irq for performance reasons?
 >> Thanks,
 >> Gary
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 > -- 
 > Subhro Sankha Kar
 > School of Information Technology
 > Block AQ-13/1 Sector V
 > ZIP 700091
 > India
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