To SMP or not to SMP

[Barney Cordoba]

--- On Wed, 1/9/13, Erich Dollansky <erichsfreebsdlist at alogt.com> wrote:

> From: Erich Dollansky <erichsfreebsdlist at alogt.com>
> Subject: Re: To SMP or not to SMP
> To: "Mark Atkinson" <atkin901 at gmail.com>
> Cc: freebsd-net at freebsd.org
> Date: Wednesday, January 9, 2013, 1:01 AM
> Hi,
> 
> On Tue, 08 Jan 2013 08:29:51 -0800
> Mark Atkinson <atkin901 at gmail.com>
> wrote:
> 
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> > 
> > On 01/07/2013 18:25, Barney Cordoba wrote:
> > > I have a situation where I have to run 9.1 on an
> old single core
> > > box. Does anyone have a handle on whether it's
> better to build a
> > > non SMP kernel or to just use a standard SMP build
> with just the
> > > one core? Thanks.
> > 
> > You can build a SMP kernel, but you'll get better
> performance (in my
> > experience) with SCHED_4BSD on single cpu than with
> ULE.
> > 
> I would not say so. The machine behaves different with the
> two
> schedulers. It depends mostly what you want to do with the
> machine. I
> forgot which scheduler I finally left in the single CPU
> kernel.
> 
> Erich

4BSD runs pretty well with an SMP kernel. I can test ULE and compare
easily. A no SMP kernel is problematic as the igb driver doesn't seem
to work and my onboard NICs are, sadly, igb. 

Rather than say "depends what you want to do", perhaps an explanation of
which cases you might choose one or the other would be helpful.

So can anyone in the know confirm that the kernel really isn't smart enough
to know there there's only 1 core so that most of the SMP "overhead" is 
avoided? It seems to me that SMP scheduling should only be enabled if there
is more than 1 core as part of the scheduler initialization. Its arrogant
indeed to assume that just because SMP support is compiled in that there
are multiple cores.

BC