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Re: Hyperthreading/SMT and NetBSD; how well does it work?
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
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.
Matthias Scheler http://zhadum.org.uk/
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