Re: 2.6.1 and irq balancing

[Joe Korty]

On Sun, Jan 11, 2004 at 10:51:22AM +0100, Arjan van de Ven wrote:
> On Sun, 2004-01-11 at 00:14, Ethan Weinstein wrote:
> > Greetings all,
> > 
> > I upgraded my server to 2.6.1, and I'm finding I'm saddled with only 
> > interrupting on CPU0 again. 2.6.0 does this as well. This is the 
> > Supermicro X5DPL-iGM-O (E7501 chipset), 2 Xeons@xxxxxx HT enabled. 
> > /proc/cpuinfo is normal as per HT, displaying 4 cpus.
> 
> you should run the userspace irq balance daemon:
> http://people.redhat.com/arjanv/irqbalance/

I have long wondered what is so evil about most interrupts going to
CPU 0 that we felt we had to have a pair of irqdaemons in 2.6.  From my
(admittedly imperfect) experience, the APIC will route an interrupt to
CPU 1 if CPU 0 is busy with another interrupt, to CPU 2 if 0 and 1 are
so occupied, and so on.  I see no harm in this other than the strangely
lopsided /proc/interrupt displays, which I can live with.

Earlier APICs had a variation where the search for where each new
interrupt was to go started with first cpu after the one that got the
last interrupt.  If we call this 'round-robin' allocation, then today's
technique could be described as 'first fit'.

If I am wrong about this, I would dearly love to be corrected :)
Joe
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