Re: [Xen-devel] Question Also regarding interrupt balancing

[harish <mvharish@xxxxxxxxx>]

Hi Keir,
I had tried the following experiment on a 4-way machine:

1) Pin the physical interrupts for the physical nic to pcpu1
2) Pin the domU to pcpu3

And ran a quick netperf test. Noticed that the cpu utilization was
around ~50% on pcpu0 although my interrupts were being pinned to pcpu2
and domU on pcpu3. That is when I noticed that vif#id.0 has a dynamic
irq which is serviced by pcpu0. Does this irq always run on pcpu0?
Considering that it is dynamic, I understand that we cannot change the
affinity and so am wondering if there some other configuration related
to it.

Any suggestions/help would be great.

Thanks,
harish

On 6/13/06, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:

On 13 Jun 2006, at 00:42, harish wrote:

> echo 2 > /proc/irq/20/smp_affinity [...works..]
> echo 4 > /proc/irq/20/smp_affinity [...works..]
> echo 8 > /proc/irq/20/smp_affinity [...works..]
>
> But, a cumulative does not work...meaning...
> echo 3>
> echo 5>
> echo f> etc.... do not work.
>
> Is that a bug or is it by design?

You should find it locks onto the first CPU in the mask that you
specify. As I said, the kernel does not load-balance IRQs so it
currently does not make sense to specify multi-cpu cpumasks. So this is
by design, for now.

  -- Keir

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