[Beowulf] The Walmart Compute Node?
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:
>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.
>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.
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