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

[Michael Poole]

Maho NAKATA writes:

> From: Michael Poole <mdpoole at troilus.org>
> Subject: Re: Only 70% of theoretical peak performance on FreeBSD 8/amd64, Corei7 920
> Date: Mon, 12 Apr 2010 10:06:55 -0400
>
>> 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.
>
> Hi Michael,
> I read http://www.intel.com/support/processors/sb/cs-023143.htm
> and TurboBoost on 920 is 2.80GHz.

Ah.  I was looking at http://ark.intel.com/Product.aspx?id=37147 .
Given a 2.80 GHz TurboBoost, the 44.8 GFLOPS theoretical performance
number makes sense.

I think the more important point is that TurboBoost on this CPU gives at
most a 10% speedup, so it cannot explain the 25% performance difference.

Michael