can't change value in smp_affinity

I am trying to set irq affinity on linux by changing the value in smp_affinity. When I echo the new value into the file, I don't get any error but when I read it back, the value remains unchanged. I don't have irqbalance enabled, so I am not sure what else could be preventing me from changing it. For example: > cat /proc/irq/51/smp_affinity f > echo 1 > /proc/irq/51/smp_affinity > cat /proc/irq/51/smp_affinity f