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

[Matthias Scheler <tron%zhadum.org.uk@localhost>]

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.

        Kind regards

-- 
Matthias Scheler                                  http://zhadum.org.uk/