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]#