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

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

On Sun, Feb 08, 2009 at 10:15:50AM -0500, Steven M. Bellovin wrote:
> > 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.

Yes, of course. If there is only one physical CPU (core) the scheduler
cannot use the "wrong" logical CPU. This problem will only occur on
system with HT/SMT and multiple CPU (cores) in situation where there
are less runable contexts (processes or threads) than the machine
has logical CPUs.

        Kind regards

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