Hmm, this is interesting'
This slide appears to be directly from Sony. It was posted by a mod on B3D so though it's probably not a public slide, it's legit. http://forum.beyond3d.com/showpost.php?p=1830730&postcount=224
Two interesting things, it shows CPU eats into GPU BW disproportionately on PS4. Too be fair, this could be the same on Xbox One.
In other words if you just use the GPu you could have 100GB/s. Now use 10 GB/s for the CPU, the GPU only has 80 GB/s remaining. Total system bandwidth is now only 90GB/s. And so on. This is what the graph is showing.
Here is a B3D poster of high technical knowledge explaining why that could be the case (it's very technical) http://forum.beyond3d.com/showpost.php?p=1830733&postcount=225
The interleaving and ordering behavior of CPU and GPU memory pages are different, so there are hits due to CPU pages not being as aggressively distributed amongst the channels.
Unlike the GPU, the CPU portion is also latency-intolerant and more strongly ordered, so the memory controllers cannot be as aggressive about combining and waiting for more optimal accesses to opportunistically fill in gaps that would result from DRAM penalties. Long runs of accesses of a specific type become harder to sustain, and the patterns for CPUs tend to be less regular.
Since many DRAM penalties are disproportionate in their impact, a relatively small amount of sub-optimal CPU traffic will have a bigger effect from dead cycles and missed scheduling opportunities.
This may mean there is room for low-level optimization for the PS4 that could claw back some of this bandwidth, if games get creative about arranging their accesses.
Second thing is it implies a "GPU only" peak of only about 135 GB/s for PS4. This is probably similar to how Xbox One ESRAM is rated at 204 GB/s, but it's architects said it's more like 140-150 GB/s in real world usage. Any synthetic maximum wont be reached in real code, and this gives us an idea where that falls on PS4.
