Well, reading the paper on the new FreeBSD 5.0 ULE (sp?) scheduler, it seems as 
though they're treating HT CPUs as a form of NUMA for CPU affinity purposes.

Whether that has any measurable benefit, or causes measurable problems, I don't 
know.

The paper was presented at BSDCon '03 iirc, you can find it at Usenix.org.

David Burren wrote:
> "Toru Nishimura" <locore32@gaea.ocn.ne.jp> wrote:
> 
>>His lab tested HT servers and concluded _the_ HT SMP Linux kernel
>>implementation scales actually worse, at least no better, than
>>plain SMP kernel.  It sounded HT is rather waste of transistor
>>and electricity.
> 
> 
> My understanding is that HT can be beneficial if you have a model
> where you can avoid having threads from separate processes running
> on the same physical CPU.
> 
> Given that this requires the use of multithreaded applications in
> your performance critical path to get any advantage, and a scheduler
> that distinguishes between virtual and physical CPUs, then without
> both of those yes I'd agree it's a waste of time.
> But I'm sure there's an application for it _somewhere_... ;)
> 
> Does anyone know of a scheduler (in any OS) that is HT-aware in
> this way?
> __
> David Burren