PS3 vs X360: Apples to Apples high level comparison...

I think people are getting really mixed up over everything. Between 32GB/s external bus throughput to 256Gb/s external bus throughput to 256GB/s internal bus throughput. People end up mixing up terms and buses. I think most people were of the opinion that the 256GB/s external bus width was probably fictional, but anything on the edram chip internally could talk much faster. Granted, I don't know if people expected the edram "processor" to be able to do blending/aa/etc.

Nite_Hawk

 

Jaws:

Microsoft claimed 'more than 1 TFLOP'. The X360 GPU probably rates well over 1 TFLOP by itself by counting its fixed functionality as floats in the same way nVidia did. It appears Microsoft was counting this way and just rounded to the nice 1 TFLOP spec, and Sony outdid them in their announcement by not rounding down.

 

How in the world does the Nvidia rate at 1.8 Teraflops? Nomatter what I've read it just doesnt add up.

 

Same way Xenos rates at 900 GFLOPS... it's called misleading the consumer. For instance, RSQ could be counted as 1 FLOP, but not in marketing-land. Instead, we'll count the lookup as one FLOP, and count all the FLOPs used in the NR refinement, and then you'd get something like 15-odd flops in a single shader instruction. Or perhaps you can imagine that it does SIN/COS using the first 4/5 terms of the Maclaurin Series and geometrically mirroring the results. That would amount to... what... 30 FLOPs per instruction? So all you have to do is consider how much computing power the GPU would have if you did nothing but SIN and/or COS and/or RSQ for every single instruction you'll ever execute. There's a few TFLOPs for you.

 

"ATI: The 2-terabit (256GB/sec) number comes from within the EDRAM, that’s the kind of bandwidth inside that RAM, inside the chip, the daughter die. But between the parent and daughter die there’s a 236Gbit connection on a bus that’s running in excess of 2GHz. It has more than one bit obviously between them."

http://firingsquad.com/features/xbox_360_interview/page3.asp


also, old diagram:
http://www.xbitlabs.com/misc/picture/?src=/images/news/2004-04/xbox2_scheme_bg.gif&1=1

 

FiringSquad: What types of operations do the EDRAMs 192 processors perform?

ATI: Well they do z-compares, they do alpha blends, they do blends of samples to make a pixel. That kind of thing. They do stencil operations also. And this is the first time memory has access to something like this, right in the memory, so it never leaves the memory die. The memory and the logic is all built into one die. And it’s also a power savings by the way.

http://firingsquad.com/features/xbox_360_interview/page3.asp

 

Jaws:

I don't think the total "targeted" FLOPS "power" of the Xenos graphics chipset has ever been disclosed. The PR rough guideline for total system performance is too vague to consider it an absolute quantity useful in deriving 885 GFLOPS for the GPUs. Considering the NV40 was already rated around 1 TFLOP by similar nVidia accounting, I suspect X360's next generation graphics chipset probably delivers something comparable and more.

Microsoft probably felt claiming the magical TFLOP barrier would be spoiling enough, and Sony was left in the position to be more exact in order to show that there would still be some improvement in power for their system.

 

NVidia claims 360 Gflops for NV40, counting PS, VS, texturing and blending. That figure is a bit on the high side, but probably not too far off.
If we take the 136 to 53 shader ops comparison as RSX being "2.57 times NV40", we arrive at 920 Gflops. And btw, it could very well mean RSX has 28 of 32 pixel pipelines (a parallel to Cell )

 

which is 880gflops less than they claim

 

Do you remember where you read those numbers? some official nvidia document?
BTW, you have a PM

 

That's where they counted the other parts: triangle setup, the whole Z subsystem, LOD calculation, interpolators, whatever.
Given the emphasis on HDR, they have probably doubled the capabilities of the TMUs handling FP textures, so sampling a FP16 texture is very likely single clock. And texturing is more than 40% of that NV40 figure.

 

http://developer.nvidia.com/object/xdc_2005_presentations.html
It's in the slides pdf.
But I'm not sure these numbers are entirely correct. The counting for texture and blend flops seems a bit off.

 

Anybody know where and when MS gave out the 115.2 GFLOPS number? It isn't in any of their official documents. :?

 

Jaws your integer numbers are all over the place and basically off on some points.

Among other things, SPEs are dual issue - so if you want to make sweeping generalizations about performance you need to count them as 2 integer instructions per clock (scalar or vector for that matter ).

 

The things that consume the most integer execution time are generally not the integer math ops, but things like store, fetch, and branch.

Comparing the # of instructions per second peak doesn't give you a meaningful number at all.

 

Of course not, but if you do go writing it out at least you should make it accurate.

For that matter the idea that dual-issue will double your instruction throughput couldn't be farther from the truth on in-order CPUs either. Especially in any kind of general purpose code.

Actually the places where dual issue makes the most difference is what SPEs tend to be optimized for.

 

Should be 96GFlops unless they've got a sneaky instruction that adds another 19Gflops...

 

Some people have speculated that XCPU FPU could possibly have Gekko-esque 2-way SIMD mode in single precision, adding 2 more flops/cycle to peak numbers.

Personally I would find it ironic if that's the case, given how little use that would have outside specsheets and how they harp on Sony all the time about pushing peak numbers.