Re: irqbalance mandatory on SMP kernels?

["Theodore Ts'o" <tytso@mit.edu>]

On Mon, Apr 17, 2006 at 11:01:33AM -0400, Lee Revell wrote:
> > There is an in-kernel IRQ balancer. Redhat just choose to turn it
> > off, and do it in userspace instead. You can re-enable it if you
> > compile your own kernel.
> 
> Round-robin IRQ balancing is inefficient anyway.  You'd get better cache
> utilization letting one CPU take them all.

IIRC, Van Jacobsen at his Linux.conf.au presentation made a pretty
strong argument that irq balancing was never a good idea, describing
them as a George Bush-like policy.  "Ooh, interrupts are hurting one
CPU --- let's hurt them **all** and trash everybody's cache!"

Which brings up an interesting question --- why do we have an IRQ
balancer in the kernel at all?  Maybe the scheduler's load balancer
should take this into account so that processes that have the
misfortune of getting assigned to the wrong CPU don't get hurt too
badly (or maybe if we have enough cores/CPU's we can afford to
dedicate one or two CPU's to doing nothing but handling interrupts);
but spreading IRQ's across all of the CPU's doesn't seem like it's
ever the right answer.

						- Ted