How to change SMP affinity of an IRQ on Ubuntu?

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http://serverfault.com – I'd like to change IRQ SMP affinity for reasons, outlined in this question: CPU0 is swamped with eth1 interrupts But I can't — I see Input/output error when I try to write to /proc/irq/*/smp_affinity. Please point me to the HOWTO on the matter. (A formal reference on /proc/irq/*/ would be cool as well.) Gory details: $ uname -a Linux MYHOST 2.6.38-15-virtual #59-Ubuntu SMP Fri Apr 27 16:40:18 UTC 2012 i686 i686 i386 GNU/Linux $ lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 11.04 Release: 11.04 Codename: natty $ sudo cat /proc/irq/*/smp (HowTos)