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: "Barney Cordoba" <barney_cordoba at yahoo.com>
> Cc: "Mark Atkinson" <atkin901 at gmail.com>, freebsd-net at freebsd.org, jack.vogel at gmail.com
> Date: Wednesday, January 9, 2013, 9:14 AM
> Hi,
> 
> On Wed, 9 Jan 2013 05:40:13 -0800 (PST)
> Barney Cordoba <barney_cordoba at yahoo.com>
> wrote:
> 
> > --- 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. 
> > 
> this is bad luck. I know of the kernels as I have had SMP
> and single
> CPU machines since 4.x times.
> 
> > 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
> 
> The kernel does not think like this. It is a fixed program
> flow.
> 
> > "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.
> 
> I compile my own kernels and set the parameters as needed.
> 
> Erich
> 

This explanation defies the possibility of a GENERIC kernel, which 
of course is an important element of a GPOS. Its too bad that smp
support can't be done with logic rather than a kernel option. 

The big thing I see is the use of legacy interrupts vs msix. Its not
like flipping off SMP support only changes the scheduler behavior.

BC