ATHLON vs XEON: number crunching

[Bill Broadley]

> What I found is that Xeon 2.2 GHz is 1.5 times faster! than any athlons I
> tested. But the most strange thing is all these athlons: MP 1200, MP 1900,
> MP 2100 give approximately the same timing within 5%. This is completely
> above my comprehension.

A few benchmarking rules/tips:

#1 You can't use Mhz to compare across architectures.
#2 Within architectures often performance does not scale linearly with clock
   speed.
#3 Any comparable cpu's can be shown to be faster OR slower then another 
   if you pick the right benchmark.
#4 Because a given benchmark shows X is faster than Y, that does NOT
   mean X will be faster then Y for your application.
#5 Bottlenecks outside the cpu often do not scale AT ALL with clock speed.
   So if it's memory bandwidth a 2.0 Ghz can be 0% faster then a 1.5 Ghz
   cpu.

The golden rule of benchmarking:
#1 Use the application that justifies the purchase of the machine to
   compare price/performance.  Only then can you be assured of getting
   the most performance for your money.

> Did someone encounter such a strange pattern and what can be a source of
> this behavior?

For things with certain memory access patterns P4's enjoy a significant 
advantage in memory bandwidth.  On the other hand the athlon enjoys
a performance lead on many other types of scientific codes.

Take specfp2000, a collection of 14 scientific applications (NOT 
microbenchmarks).  The 2.0 Ghz P4 gets 669, the MP2000 gets 642.  So
they are very similar right?  Nope, the reality is that at some parts
of the specfp2000 suite the p4 is 1.64 times faster then the Athlon.
At other benchmarks the Athlon is 1.56 times faster then the P4.

Which of the 14 parts of the speccpu 2000 suite is your application?
Hard to see, see the golden rule #1.

-- 
Bill Broadley
Mathematics/Institute of Theoretical Dynamics
UC Davis