Hi folks. I'm relatively new to linux and hope someone can steer me a bit.

Can someone help interprete the output from /proc/irq/xx/smp_affinity and how to set the irq affinity? Why I'm stumped is when I cat my 8 way opteron machine - I get a list of values (masks?).

> cat /proc/irq/201/smp_affinity
00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000002

The the last value bounces around, so it doesn't appear to indicate the CPU mask - perhaps it's the last CPU used?

However, all the documentation I can find indicates that smp_affinity returns or is set with a single mask. The docs appear to be out of date or something - even the
/usr/src/linux2.6.18/Documentation/IRQaffinity.txt file for my kernel. The kernel happens to be 2.6.18-1.2798.fc6 - x86_64.

Thanks much,

Kris

Received an out of band answer. It appears that somewhere along the way the kernel was updated to support a larger mask (256 bits wide). The "," is just included to improve readability. The correlation between my 8 processors and the 8 x 32 bit values is just coincidence. Other kernel version appears to only be 128 bytes wide.

The last value bouncing around actually is the current CPU mask at that time... I have a feeling you also have a service called irqbalance running as well, which attempts to move around irq's to different processors to balance out interrupt loading.