[Beowulf] Larrabee - Mark Hahn's personal attack

Prentice Bisbal prentice at ias.edu
Fri Jan 27 08:34:41 PST 2012


On 01/27/2012 11:12 AM, Vincent Diepeveen wrote:
> And both are seem failures from user viewpoint, maybe not from intels  
> income viewpoint,
> but from intels aim to replace and/or create a new long lasting  
> architecture
> that can even *remotely* compete with other manufacturers,
> not to mention far too high pricepoints for such cpu's.

This argument is ridiculous. Just because two completely different
technologies (architectures) both fail, doesn't make them similar.

That's like saying a Ford Edsel and Pontiac Aztek are similar cars.

> Assuming you're not completely born stupid, i assume you will realize  
> that IN ORDER to run

Calling someone "completely born stupid" is unacceptable behavior.
> most existing x64 codes, it needs to have cache coherency, and that  
> it always has been
> presented as having exactly that.
> Which is one of reasons why the architecture doesn't scale of course.

Cache-coherent systems don't scale well? Really? SGI Origins were ccNUMA
systems, and they scaled well.

> Well you can forget about them running your x64 fortran codes on it  
> at any fast speed.
>
> You need to total rewrite your code to be able to use vectors of  
> doubles,
> and in contradiction to GPU's where you can indirectly with arrays  
> see each PE or each 'compute core'
> (which is 4 PE's of in case of AMD-ATI that can execute 1 double a  

This argument makes no sense in the context of this discussion.  You
need to do a significant rewrite of your code to take advantage of GPUs,
too, so how are GPUs better?

> cycle),
>
> Such lookups are a disaster at larrabee - having a cost of 7 cycles  
> for indirect lookups,
> so you really need to use vectors.
>
> Now i bet majority of your oldie x64 code doesn't use such huge vectors,
> so to even get some remote performance out of it, a total rewrite of  
> most code is needed,
> if it can work at all.
>
> We can then also see the insight that GPU's are total superior to  
> larrabee at most terrains and
> most importantly at multiplicative codes.
>
> As you might know GPU's are worldchampion in doing multiplications  
> and CPU's are not.
>
> Multiplication happens to be something that is of major importance  
> for the majority of HPC codes.
> Majority i really mean - approaching 90% at the public supercomputers.

I'm at a loss for words...


Prentice


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