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 7:49 PM, Garrett Cooper <yanefbsd at gmail.com> wrote:
> 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?

Err... wait. Didn't see that the turbo boost results didn't scale
linearly or align with one another until just a sec ago. Nevermind my
previous comment.
-Garrett