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From: John Baldwin <jhb@freebsd.org>
To: Sean Bruno <seanbru@yahoo-inc.com>
Date: Wed, 25 Apr 2012 15:43:08 -0400
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	(bigwig.baldwin.cx); Wed, 25 Apr 2012 15:44:09 -0400 (EDT)
Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org>,
	Jack Vogel <jfvogel@gmail.com>
Subject: Re: igb(4) Pondering a bind to cpu patch
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On Wednesday, April 25, 2012 3:30:25 pm Sean Bruno wrote:
> On Wed, 2012-04-25 at 06:32 -0700, John Baldwin wrote:
> > CPU IDs are not guaranteed to be dense.  However, you can use
> > CPU_FIRST() and 
> > CPU_NEXT() with your static global instead.
> > 
> Ah, does CPU_NEXT() reset to 0 when it reaches the end of its list of
> CPUs?

Yes.

> > OTOH, if igb were to just leave the interrupts alone instead of
> > binding them 
> > by hand, they would get round-robin assigned among available cores
> > already.  I 
> > think in this case the best approach might be to add a tunable to
> > disable 
> > igb's manual binding and instead let the default system round-robin
> > be 
> > preserved. 
> 
> also, yes.  Why *are* we binding to CPUs in the first place?  Are we
> afraid that the scheduler won't do the right thing and we're trying to
> work around some unknown performance issue ?

Well, in some cases you want to know exactly which CPUs are being used
as you might bind other resources associated with the queue to those
specific CPUs as well.

-- 
John Baldwin