David Wang (dwang@RWTexpanded.com) on 8/2/05 wrote:
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>mas () on 8/2/05 wrote:
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>
>> So with 7 SPEs we can say for every cycle the SPEs provide
>>2 DP FMADD and the PPE 1 DP FMADD giving 6 DP flops/cycle. 3.2 Ghz gives 19.2 peak
>>DP GFlops which means for reference that I2/P5 could match this peak at 4.8 Ghz
>>although how much a Cell could sustain in practice is not certain.
>
>It'll be harder to maintain peak DP FP, especially since you have to use separate
>tool chains for the SPE and the PPE. So you have to do a clean-sheet re-think about
>how to utilize the asymmetric threading model and keep the functional units busy.
>I think that you'll probably have to make your choice to run with "legacy" PPC ISA
>and get the FP performance there, or go with the SPE only.
>

Fair enough, that makes Xenon only look better then.

>OTOH, if you looked at the photo micrographs of the SPE and the PPE closely, you'll
>see that the PPE itself isn't that much larger than the SPE, and if there's a market
>for this sort of thing, IBM could spin a DD3 where the 7 cycle penalty for DP FMADD
>goes away. Such a chip could crank out enormous amounts of (peak) DP FP, but the
>flop to BW ratio would be heavily skewed to the flop side of things, even if IBM goes with 8 Gb/s/pin-pair XDR2.
>
>> Coincidentally
>>by having 3 PPEs the Xenon in Xbox 360 could match that DP figure with its SP rate
>>being 76.8 Gflops vs Cell's 204.8 so it doesn't look so outclassed as one would
>>think given Cell's hype.

If the PPE really is 12 SP Flops/cycle, then that comparison above becomes 115.2 to 218 Gflops.

>> I have also heard development Cell kits are only at 2.4
>>Ghz. Anyway they both sound like great distributed computing clients I hope Sony
>>and Microsoft allow them to be used in that ancillary fashion.
>
>If they're going to heavily subsidize the consoles, they may not like the idea
>of people buying them for compute farms.
>

MS look like they are with a $299 price but Sony are not in any sort of mood to do so with $450-499 being bandied about. At that differential the Xenon looks the better deal and it also would probably make the better general-purpose computer too with 3 conventional PPC-type cores.