[Beowulf] The Walmart Compute Node?

Jim Lux James.P.Lux at jpl.nasa.gov
Thu Nov 8 17:43:31 EST 2007


At 11:04 AM 11/8/2007, Peter St. John wrote:
>Vincent,
>That's tough for me to answer, presumably the 1.5 is cheaper per hertz
>in power than a 3 GHz, but because of the other issues it may not be
>cheaper in GFLOPS per power. No hablo EE.
>Peter
>
>On Nov 8, 2007 1:58 PM, Vincent Diepeveen <diep at xs4all.nl> wrote:
> > For a compute cluster wouldn't it be a thought to also consider the
> > cost of 3 years of nonstop electricity for the amount of gflops it
> > delivers?
> >


In general, a N GHz processor will be poorer in a flops/Watt sense 
than a 2N GHz processor.

The power draw is a combination of a fixed load plus a frequency 
dependent load, so for the SAME processor, running it at N/2 GHz 
consumes more than 50% of the power of running it at N GHz.

If you go to a faster processor design, the frequency dependent load 
gets smaller (smaller feature sizes= smaller capacitance to charge 
and discharge on each transition).  The core voltage is also usually 
smaller on higher speed processors, which also reduces the power 
dissipation (smaller number of joules to change the voltage from zero 
to one or vice versa).  So, in general, a 2N GHz processor consumes 
less than twice the power of a N GHz processor.

Complicating this all is:
a) A significant fraction of the load in a PC is all the other stuff 
that's toggling back and forth, like memory address and data 
lines.  This will be driven more by the FSB speed, which might be the 
same for the two processors.
b) You may have a lower core voltage, but the regulator making that 
voltage may or may not be as efficient.
c) Power supply efficiency can vary a LOT from model to model.  All 
the way from 50% for a really crummy design to 95% for a good design.
d) Faster processors aren't necessarily architecturally identical to 
slower processors. They may have different pipeline depths, different 
microcode, different ALU strategies, etc.  It's not just a matter of 
shrinking the masks and turning up the clock.


Jim


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