Metal_Spirit
Regular
Doing some math will indicate that @ 853 Mhz at 12 CUs, the required bandwidth to remove the bandwidth bottleneck for CUs will be approximately ~240 GB/s. This happens to be approximately the total complete bandwidth of Xbox One (192 GB/s + 67GB/s). When looking at PS4 we have a max theoretical of 176GB/s, and as such, bandwidth will become a bottleneck for all 18CUs.
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- eSRAM max theoretical speed is not 192 GB/s!
ESRAM is capable of simultaneous reading and writing. But at every 8 cycles it can only do one of them (either reading or writing)
This allows for 15 cycles read+write+1 cycle either reading or writing.
On a 1024 bits wide bus we get: 1024*856~=109 GB/s. This is a guaranteed theoretical bandwidth on the eSRAM.
But theoretically we can double this on every 15 out of 16 cycles.
109*2*15/16~=204 GB/s. This is the max theoretical speed of eSRAM. Not 192 GB/s.
Real life scenarios achieved so far a maximum of 141 GB/s. And this on very specific calculations. (alpha transparency blending operations (FP16 x4))
192 GB/s, with real life scenarios speeds of 133 GB/s were values before the overclock.
- 700 GB/s seems a theoretical scenario that covers all cases. I should assume this would include heavy texture streaming too. How do you plan on get 204+68 GB/s on the Xbox One with only 32 MBytes of fast RAM? Main memory is only 68 GB/s so no data contained on it can be read at faster speeds! And it´s 8 GB of it!
- I just had a very, very, very fast look at the document you linked. And in page 4, as you state, they do refer 700 GB/s is the speed they would require for no bottlenecks.
Since I had a quick look, I might have saw it wrong, but that point seems to refer to a baseline heterogeneous system as shown on the document Figure 1.
And Figure 1 show a GPU accessing the RAM on a non coherent bus. Not the PS4 case!
Then all the document seems to go on to speak about the advantages of heterogeneous system coherence (like in the PS4 case).
The conclusion states that this kind of system not only gets performance enhancements, but also a bandwidth reduction of 95 to 99% to the directory.
PS4 has full heterogeneous system coherence. On the other hand, as far as I know, the Xbox One CPU cannot access the ESRAM.
So, it seems to me, that the document you refer points that the PS4 is better suited than Xbox One. And not the other way around!
Please feel free to correct me. I just had an extremely fast look at the document.