New CELL Article Online

Article: CELL Microprocessor III
By: David Wang (dwang.delete@this.RWTexpanded.com), August 6, 2005 12:00 am
Room: Moderated Discussions
David Wang (dwang@RWTexpanded.com) on 8/3/05 wrote:
---------------------------
>Panajev2001a (g_marocchi@yahoo.com) on 8/3/05 wrote:
>---------------------------
>>David Wang (dwang@RWTexpanded.com) on 8/2/05 wrote:
>>---------------------------
>>>fastpathguru (fastpathguru@yahoo.com) on 8/2/05 wrote:
>>>---------------------------
>>>
>>>>Re: The Mystery of the Missing FLOPS, is it possible that it's solved by accounting
>>>>for the extra meat in the PPE? I didn't see that mentioned as a possibility.
>>>
>>>You need 68 flops per cycle to get 218 GFlops @ 3.2 GHz.
>>>
>>>It's easy to get 56 of those flops from 7 SPE's, using FMADD, but then you'd need
>>>12 flops per cycle out of the PPE to get to 68.
>>>
>>>Again, assume that you can crank 8 Flops per cycle out with FMADD, you still need
>>>4 flops per cycle. The PPE is multi-threaded, but the impression that I got from
>>>the IBM folks is that the scalar and vector FP units are actually shared so you
>>>really can't get more flops out of the PPE either. Regardless, even if the FP units
>>>aren't shared, you still can't get 4 flops per cycle out of the scalar side of thing.
>>>
>>>So no, the beefier PPE can't get to 12 flops per cycle even if it had independent scalar/vector FP units.
>>
>>Why not ? See Gekko ;).
>
>I've forgotten about Gekko entirely.
>
>http://www.hotchips.org/archives/hc13/2_Mon/06ibm-gekko.pdf
>
>So the 12 Flops/cycle can be attained by the PPE if the PPE's scalar FPU can do
>a 2 wide SP FMADD in addition to a separate and independent Altivec unit. That
>does make things quite curious. I'm not sure how many programmers will be able
>to make use of this capability, but I think you're right, the Gekko ISA extension
>to the DD2 PPE explanation sounds to be the most reasonable explanation that I have
>come across about how Sony was able to claim the 218 GFlops rating with only 7 SPE's @ 3.2 GHz.
>
>Perhaps you can write a rebuttal article and we can link it to the current article.
>I think the readership in general will benefit from this discussion.

I've since received two negative feedbacks on the "DD2 PPE has Gekko" speculation from people who *should* know. So I'm now back to square one. The flops are mysterious once again with no explanation as to how to account for the last 4 flops per cycle.

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                              Implausible at best, irrational most likely...David Kanter2005/08/08 06:51 PM
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