HyperThreading makes worse to me (was Re: How to reproduce: Re: Only 70% of theoretical peak performance on FreeBSD 8/amd64, Corei7 920)

[Garrett Cooper]

On Wed, Apr 14, 2010 at 5:46 PM, Maho NAKATA <chat95 at mac.com> wrote:
> Hi Andry and Adam
>
> My test again. No desktop, etc. I just run dgemm.
> Contrary to Adam's result, Hyper Threading makes the performance worse.
> all tests are done on Core i7 920 @ 2.67GHz. (TurboBoost @2.8GHz)
>
> Turbo Boost off, Hyper threading off: 82% (35GFlops)    [1]
> Turbo Boost off, Hyper threading off: 72% (30.5GFlops)  [2]
>
> Turbo Boost on,  Hyper threading on: 71% (32GFlops)    [3]
> Turbo Boost off, Hyper threading off: 84-89% (38-40GFlops) [4]

Doesn't this make sense? Hyperthreaded cores in Intel procs still
provide an incomplete set of registers as they're logical processors,
so I would expect for things to be slower if they're automatically run
on the SMT cores instead of the physical ones.

Is there a weighting scheme to SCHED_ULE where logical processors
(like the SMT variety) get a lower score than real processors do, and
thus get scheduled for less intensive interrupting tasks, or maybe
just don't get scheduled in high use scenarios like it would if it was
a physical processor?

Thanks,
-Garrett