To hyper-thread or not to hyper-thread

[Jussi Lehtola]

On Mon, 2009-12-07 at 09:44 -0700, Greg Woods wrote:
> On Mon, 2009-12-07 at 16:18 +0100, Joerg Bergmann wrote:
> > The problem of the Pentium 4 D: It is not really a dual core one.
> > Hyper-Threading means: There is one core with two execution paths, which
> > means some of the common CPU features, but not all, are present twice.
> 
> One feature in particular that is not present twice is some of the
> caching. This is sort of why they named it "hyperthreading". If you can
> get multiple threads of the same process, sharing the same memory, to
> run simultaneously, there is a performance boost. But if you try to run
> two completely different processes simultaneously, there will actually
> be a performance LOSS because of all the cache misses this will cause.

This may not be true - in the high performance computing community
hyperthreading is usually not used, since if you're cpu bound, then
execution is about 20% faster in without hyperthreading since no
performance is lost because of the dual core emulation.

However, in normal desktop use you don't really care about the MFLOPS;
hyperthreading makes the system more responsive.

-- 
Jussi Lehtola
Fedora Project Contributor
jussilehtola at fedoraproject.org