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

[Ian Smith]

On Wed, 14 Apr 2010, Garrett Cooper wrote:
 > 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]

Er, shouldn't one of those say HTT on?  and/or Turbo boost on?  Else 
they're both the same test as [4] but with different results?

 > >> Turbo Boost on,  Hyper threading on: 71% (32GFlops)    [3]
 > >> Turbo Boost off, Hyper threading off: 84-89% (38-40GFlops) [4]

Clarification of all four possible test configs - 8 if you add pinning 
CPUs or not - might make this a bit clearer?

 > > 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.

Since we're talking FP, do HTT 'cores' share an FPU, or have their own?  
If contended, you'd have to expect worse (at least FP) performance, no?

 > > 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.

Waiting for the fog to lift ..

cheers, Ian