Re: Hyperthreading/SMT and NetBSD; how well does it work?

["Steven M. Bellovin" <smb%cs.columbia.edu@localhost>]

On Sun, 8 Feb 2009 09:37:12 +0000
Matthias Scheler <tron%zhadum.org.uk@localhost> wrote:

> On Thu, Feb 05, 2009 at 11:34:12AM -0500, Andreas Wrede wrote:
> > Atom 330 cpu and 2GB of memory running 5.0_RC1/amd64
> > 
> > build.sh -j 8 kernel=GENERIC run times:
> > 
> > With all 4 cpus:      8 min 41 sec
> > With 0 and 1 online: 11 min 07 sec
> > With 0 and 2 online: 16 min 23 sec
> > With only 0 online:  21 min 39 sec
> > 
> > Looks like HT gets us a 25% reduction in runtime.
> 
> I'm sorry but that is not a good test. A good test would be a build
> with "-j 2" (yes, "two") with HT enabled and disabled.
> 
> As far as I know NetBSD's kernel doesn't know anything about HT/SMT.
> So if you have a machine with four logical CPUs backed by two physical
> CPUs (or CPU cores) there is a risk that the scheduler will use two
> logical CPUs on the same physical CPU to handle to two jobs. And in
> that case HT/SMT will slow down your system.
> 
I saw reductions (not quite as large) with -j4 on a machine with only
HT.

                --Steve Bellovin, http://www.cs.columbia.edu/~smb