Only 70% of theoretical peak performance on FreeBSD 8/amd64, Corei7 920

[Michael Poole]

Antony Mawer writes:

> This may well be the same sort of issue that was discussed in this thread here:
>
>     http://lists.freebsd.org/pipermail/freebsd-hackers/2010-March/031004.html
>
> In short, the Core i7 CPUs have a feature called "TurboBoost" where
> the clock speed of one or more cores is boosted when other cores are
> idle and in a C2 or C3 sleep status ... if the appropriate power
> saving mode isn't active on the system (which I don't think FreeBSD
> does by default?), the idle cores are never put into the appropriate
> power saving state, and as a result TurboBoost never kicks in...
>
> It _may_ be that Ubuntu configures this correctly whereas FreeBSD does
> not (out of the box)?
>
> Of course it may be something else entirely, but worth checking out...

Nakata-san's theoretical performance numbers assume 4 to 4.2 operations
per core per cycle at the nominal (2.66 GHz, non-TurboBoost) clock rate.
(DGEMM is double precision, but I am not familiar enough with scientific
computing or with the Nehalem implementation of SSE to know why it is
four operations per cycle rather than two -- is it because double
precision counts as two FLOPs or is it because of multiple issue?)
TurboBoost runs up to 2.93 GHz on this CPU, so it doesn't fit either the
theoretical peak performance or the performance discrepancy very well.

Michael Poole