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BlueFalcon on 22 January 2013
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zarx said:
Your comparisons are pretty off the GT640 is going from 28.5 GB/s for DDR3 to 80GB/s for GDDR5 for example, there are no DDR3 cards on the market that offer anywhere near 68GB/s and even if they did the comparison still wouldn't work as the Durango has the ESRAM and also aparently some new memory controlers that allow the GPU to access both pools at the same time.  

If MS wants more system memory, it might have been way too costly to use 8GB of GDDR5. A compromise could be  a custom 256-bit bus over DDR3-2133? That gives us exactly the rumored 68.256GB/sec (256 bit x 2133mhz / 8 bits per byte)

zarx said:

Also keep in mind Orbis will likely be limited to a 128-bit Bus (wider Buses get exponentially more expensive and can limit die shinks in the future) so is probably looking at ~86GB/s (highest I have seen for GDDR5 on a 128-bit bus) it's self so will likely not be some bandwidth monster ether.

You make a good point. But even if HD7970M in PS4 has half of the memory bandwidth it might have dedicated GDDR5 for the GPU. You still have not addressed the other issue I talked about regarding sharing DDR3 through the northbridge = a large latency penalty. 

HD5450 and E-350 (6310) have the same number of shader processors but the HD5450 has a dedicated 1.6GHz memory bus to feed it.

Look at the huge performance penalty incurred by a GPU with shared DDR3 memory over the NorthBridge (the Xbox 720 approach):

http://images.anandtech.com/graphs/graph4023/34111.png

http://images.anandtech.com/graphs/graph4023/34112.png

http://images.anandtech.com/graphs/graph4023/34113.png

"Compared to the Radeon HD 5450 the 6310 offers between 66 - 69% of its performance in our GPU bound tests. The performance reduction is entirely due to the 6310's limited memory bandwidth being shared with the dual Bobcat cores on-die."

From what I am seeing, the memory bandwidth limitation and the additional latencies due to GPU having to go through the NorthBridge to communicate with the system memory will produce at least a 30% performance hit, and that's assuming MS is able to maintain the memory bandwidth at HD7770Ghz level to begin with. So you have a double-whammy: the additional middleman NorthBridge latency + reduction in memory bandwidth on the Xbox 720 due to lack of GDDR5. 


zarx said:

As long as you can fit the framebuffer in the eSRAM the main memory bandwidth shouldn't impact ROPs too much, especially as the GPU can access both pools at the same time. And with a modern tile based renderer you can swap out tiles as needed so the don't even need to fit all the render targets in eSRAM at once, just like what devs do with the X360s 10MB framebuffer today. It's still a bottleneck in many cases but not one that couldn't be worked around with the eSRAM even if it means dropping the native resolution a bit.

If eSRAM was a better approach to having a dedicated GDDR5 memory, then GPU makers would solder dedicated eSRAM/eDRAM chips next to the GPU die on the same package and save $ on not having to use the more expensive GDDR5/wider bus widths feeding the GPU. If you look at Wii U's crippled memory sub-system, it actually goes with a similar cost savings route by neutering the GPU with DDR3 and trying to make up for it with eDRAM on the die. MS promised us nearly 'free 4xAA' with Xbox 360 due to its embedded RAM and we got nothing of the sort. Sorry, but I am smelling more marketing BS from them on this one. Xbox 1 was the most powerful console in its generation and it was the closest to a PC design. It appears PS4's GPU subsystem mimics the approaches used on the PC a lot closer as well. I am going to lean towards the PC approach being superior for performance and efficiencies because that's the approach used for $500-1000 PC gaming GPUs. Anything else smells like cost cutting.

Ironically, eDRAM allows larger amounts of memory to be installed on smaller chips compared to eSRAM (about a 3x area savings vs. eSRAM). If MS is using eSRAM instead of the more expensive eDRAM, this only continues to highlight all these areas they are trying to save $$$ in my eyes. I hope I am wrong.

Of course all these are just rumors but MS's console seems to be pushing marketing to the average consumer ("Oh it has 8GB of system memory, it must be awesome!!!!"), and some "magic sauce" modules. For all we know the Video Codecs and Data Move Engines could be used for recording TV shows directly onto the Xbox 720 and have nothing to do with aiding graphics. This "secret sauce" hidden deep within Xbox 720 is not inspiring much confidence at the moment.