linux-kernel - Re: Interrupt Affinity in SMP

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Message-ID: <AANLkTinMxn8bNx8BZH-8UcB_5ULFu5uNWLrFyAoeoUzt@mail.gmail.com>
Date:	Mon, 19 Jul 2010 13:03:33 -0700
From:	Bryan Hundven <bryanhundven@...il.com>
To:	Robert Hancock <hancockrwd@...il.com>
Cc:	linux-kernel@...r.kernel.org, mchehab@...hat.com
Subject: Re: Interrupt Affinity in SMP

On Mon, Jul 19, 2010 at 12:22 PM, Robert Hancock <hancockrwd@...il.com> wrote:
> On Sat, Jul 17, 2010 at 2:02 PM, Bryan Hundven <bryanhundven@...il.com> wrote:
>> Please see the two attached examples.
>>
>> Notice on the 5410 example how we start with the affinity set to 0xff,
>> and change it to 0xf0.
>> This should spread the interrupts over the last 4 cores of this quad
>> core - dual processor system.
>>
>> Also notice on the 5645 example, with the same commands we start with
>> 0xffffff and change to 0xfff000 to spread the interrupts over the last
>> 12 cores, but only the first of the last twelve cores receive
>> interrupts.
>>
>> This is the inconsistency I was trying to explain before.
>
> As I mentioned before, I believe the behavior in this case is chipset
> dependent, and potentially not something the kernel has control over.
> In most cases distributing the same interrupt across multiple cores is
> likely a bad idea in any case, unless the interrupt load is actually
> so high that a single CPU can't handle it.

Can anyone confirm that this is how the 5520 and newer xeon chipsets
handle affinity?

I might be totally wrong, but I thought that RSS (Receive Side
Scaling, which is available on the 82576 network card in that 5645
xeon example) helps in that scenario?

--Bryan
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