e1000: Question about polling

[Badalian Vyacheslav <slavon@bigtelecom.ru>]

Hello all.

Interesting think:

Have PC that do NAT. Bandwidth about 600 mbs.

Have  4 CPU (2xCoRe 2 DUO "HT OFF" 3.2 HZ).

irqbalance in kernel is off.

nat2 ~ # cat /proc/irq/217/smp_affinity
00000001
nat2 ~ # cat /proc/irq/218/smp_affinity
00000003

Load SI on CPU0 and CPU1 is about 90%

Good... try do
echo ffffffff > /proc/irq/217/smp_affinity
echo ffffffff > /proc/irq/218/smp_affinity

Get 100% SI at CPU0

Question Why?

I listen that if use IRQ from 1 netdevice to 1 CPU i can get 30% 
perfomance... but i have 4 CPU... i must get more perfomance if i cat 
"ffffffff"  to smp_affinity.

picture looks liks this:
0-3 CPU get over 50% SI.... bandwith up.... 55% SI... bandwith up... 
100% SI on CPU0....

I remember patch to fix problem like it... patched function 
e1000_clean...  kernel on pc have this patch (2.6.24-rc7-git2)... e1000 
driver work much better (i up to 1.5-2x bandwidth before i get 100% SI), 
but i think that it not get 100% that it can =)

Thanks for answers and sorry for my English