SMP IRQ affinity, started by Ingo Molnar <mingo@redhat.com>
  
  
  /proc/irq/IRQ#/smp_affinity specifies which target CPUs are permitted
  for a given IRQ source. It's a bitmask of allowed CPUs. It's not allowed
  to turn off all CPUs, and if an IRQ controller does not support IRQ
  affinity then the value will not change from the default 0xffffffff.
  
  Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
  the IRQ to CPU4-7 (this is an 8-CPU SMP box):
  
  [root@moon 44]# cat smp_affinity
  ffffffff
  [root@moon 44]# echo 0f > smp_affinity
  [root@moon 44]# cat smp_affinity
  0000000f
  [root@moon 44]# ping -f h
  PING hell (195.4.7.3): 56 data bytes
  ...
  --- hell ping statistics ---
  6029 packets transmitted, 6027 packets received, 0% packet loss
  round-trip min/avg/max = 0.1/0.1/0.4 ms
  [root@moon 44]# cat /proc/interrupts | grep 44:
   44:          0       1785       1785       1783       1783          1
  1          0   IO-APIC-level  eth1
  [root@moon 44]# echo f0 > smp_affinity
  [root@moon 44]# ping -f h
  PING hell (195.4.7.3): 56 data bytes
  ..
  --- hell ping statistics ---
  2779 packets transmitted, 2777 packets received, 0% packet loss
  round-trip min/avg/max = 0.1/0.5/585.4 ms
  [root@moon 44]# cat /proc/interrupts | grep 44:
   44:       1068       1785       1785       1784       1784       1069       1070       1069   IO-APIC-level  eth1
  [root@moon 44]#