[Beowulf] The Walmart Compute Node?

Larry Stewart larry.stewart at sicortex.com
Fri Nov 9 09:42:54 EST 2007


Robert G. Brown wrote:

> On Thu, 8 Nov 2007, Jim Lux wrote:
>
>> In general, a N GHz processor will be poorer in a flops/Watt sense 
>> than a 2N GHz processor.
>
Well that just isn't so.  It seems pretty clear from IBMs BlueGene/L, as 
well as the SiCortex processors, that the
opposite is true.  The new Green 500 list is brand new, and there's not 
much on it yet, but the BG/L is delivering 190MF/Watt
on HPL, whereas the machines made out of Intel and AMD chips are half 
that at best.

>>
>> 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.
>
This probably IS true, but high performance cores have a lot more logic 
in them to try to achieve performance: out of order
execution, complex branch prediction, register renaming, etc. etc.  A 
slower core can be a lot simpler with the same silicon process,
so a decent lower-clock design will be more power efficient than a fast 
clock design.

>>
>> 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.
>
The flaw in this argument is that a slower clock design can use the same 
small transistors and the same current state of the art processes and it 
will use many fewer transistors to get its work done, thus using very 
much less power.  Our 1 GF core is 600 milliwatts, for example.
Even after adding all the non-core stuff - caches, memory controllers, 
interconnect, main memory, and all overhead, it is still around 3 watts 
per GF.

>
> In ADDITION to this is the fact that the processor has to live in a
> house of some sort, and the house itself adds per processor overhead.
> This overhead is significant -- typically a minimum of 10-20 W,
> sometimes as much as 30-40 (depending on how many disks you have, how

This factor does not scale this way!  With low power processors, you can 
pack them together, without the endless support chips, you
can use low power inter-chip signalling, you can use high efficiency 
power supplies with their economies of scale.  If you look inside
a PC there are two blocks doing useful work - memory and CPUs, and a 
whole board full of useless crap.  Look inside a machine designed
to be a cluster and there should be nothing there but cpus and memory.


-- 
-Larry / Sector IX

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