Re: irqbalance mandatory on SMP kernels?

[Nick Piggin]

Arjan van de Ven wrote:
> On Wed, 2006-04-19 at 10:38 -0400, Theodore Ts'o wrote:
>
> > On Wed, Apr 19, 2006 at 04:23:14PM +0200, Arjan van de Ven wrote:
> >
> > > as long as the irqs are spread the apaches will (on average) follow your
> > > irq to the right cpu. Only if you put both irqs on the same cpu you have
> > > an issue
> >
> > Maybe I'm being stupid but I don't see how the Apache's will follow
> > the IRQ's to the right CPU.  I agree this would be a good thing to do,
> > but how does the scheduler accomplish this?
>
>
> iirc this part of the kernel uses wake_up_sync() and such, which tend to
> pull the apache to the cpu (if it's idle) in the long term
> (or it ought to; at one point it did)

Yeah it has "affine wakeups" now, which will do that for all
types of wakeups, and not just to idle CPUs either (sync
wakeups just get pulled a little more strongly).

IIRC SGI reported something like a factor 8 improvement in
CPU efficiency on a database IO simulation on a smallish
system (16-way maybe).

--
SUSE Labs, Novell Inc.
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