So the other day I was running through some numbers on certain specifications regarding both systems, and I decided to try calculating the system memory bandwidth per system to their physical memory amounts, and I think when looking at it this way I can understand MS's decision to go with two pools.
See, the PS5 has 448 GB/s of bandwidth to 16 GB of physical memory. Each of those memory modules are 2 GB and provide 56 GB/s. While the BW to TF ratio breaks down to 43.6 GB per TF, if you break down the BW to physical memory ratio, you get 28 GB/s per 1 GB. This is particularly of importance when considering that, more often than not, you're going to want your RAM to be occupied with unique assets and shave down on duplicates, so please keep that in mind when reading further (tho TBF, I would need more time to elaborate on how this would fully shake out in practice).
Going back to bandwidth-per-TF for a moment, for XSX, you get about 36.88 GB/s per TF if you average out the bandwidths between both pools and divide that by the TF amount, though a more realistic way of specifying the ratio in XSX's case is 73 .77 GB/s per TF on a TF ratio matching the physical memory amount optimized for the GPU (10 GB physical memory, 7.591 TF), and 73.76 GB/s per TF on a TF ratio matching the physical memory amount optimized to the CPU/OS etc. (6 GB physical memory, 4.555 TF).
But then there is also the bandwidth per physical GB ratio that factors into the picture; with PS4 having a unified pool at 448 GB/s, you get 28 GB/s per 1 GB physical memory, or exactly 1/2 the 56 GB/s bandwidth of a single GDDR6 14Gbps module. Once again, for XSX you could technically just do an average of the two pools, but it's clear that MS went with a "split" (not split in traditional sense like DDR4/GDDR on PC or in older consoles like PS3, GC, or even XBO's ESRAM/DDR3 split memory pools) to stress maximization of bandwidth to the system processors. 560 GB/s divided by 10 GB of physical memory gives you... 56 GB/s per 1 GB. And 336 GB/s divided by 6 GB of physical memory also gives you 56 GB/s per 1 GB. In other words, you have a 1:1 ratio between bandwidth memory per physical GB and actual chip bandwidth per GDDR6 module.
While at it, we can also calculate the TF per physical GB ratio as well. Once again for XSX we'd have to "split" the calculation due to its memory setup, but it leaves you with 759.1 GFLOPs per physical GB (TF amount partition to 10 GB, 560 GB/s pool), and the same 759.1 GFLOPs per physical GB on the 6 GB, 336 GB/s pool. Again, a 1:1 ratio of balance. With PS5, we end up with 642.1 GFLOPs per physical GB, thanks to its memory setup.
This is a pretty notable difference between the two systems and it will be interesting to see how things play out in this regard. However, we should also not forget to take into consideration that with the above numbers, you are more realistically looking at total GPU-bound bandwidth figures of 560 GB/s @ 10 physical GB on XSX and, assuming non GPU-bound tasks of physical GB ratio is same between both systems (i.e both systems using 6 GB physical memory for non-GPU orientated data, including some reserved to the OS) 280 GB/s @ 10 physical GB on PS5. AGAIN, this is taking into account GPU TF per physical GB numbers, nothing else. This is why I pontificate PS5 might have a bandwidth problem, but admittedly this is before looking into the SSDs.
(**To the above paragraph, we can also adjust the PS5's figure of bandwidth-to-physical-memory ratio to reflect an equal amount of physical memory to GPU-bound tasks as on XSX, aka 10 GB, which puts the PS5's number closer to 44.8 GB/s per physical GB. However this assumes good-faith in PS5's memory setup (which is most likely the case) and also factoring into account asymmetrical access, i.e the full system bandwidth bus would be accessing 8 chips at 44.8 GB/s each or 356.8 GB/s altogether on the first past, and would then need to access the other 2 GB at 89.6 GB/s on a second pass.
Given PS5's bandwidth is over a unified pool of 16 GB memory this is not likely to be the case because it would also facilitate accessing 8 GBs @ 448 GB/s and then the 2nd 8 GBs @ 448 GB/s which would essentially be analogous to XSX's setup yet not physically possible since there is no asymmetric mix of modules at different capacities thus no presence of potential issues in bus access that would require this type of implementation (and no physical state that would allow it to be implemented, either)**)
With the SSDs, you're looking at a maximum RAM reoccupy rate of 2.9 seconds on PS5 if streaming in 5.5 GB of raw data per second, whereas on XSX you are looking closer at 6.6 seconds. That's a 2.27x advantage for PS5's SSD, so it can technically saturate its RAM with unique assets more quickly. I stress technically however, because there are quite a few aspects on the system's design we do not know about in terms of asking if this really cuts down on a bandwidth deficit compared to XSX. For example, we know the XSX has a very strong emphasis on ML and DLSS-style upscaling techniques, so the possibility is very much there for developers to populate the RAM with lower-quality (and therefore, much smaller) texture and data assets which can then be upscaled through dedicated hardware in the GPU for such tasks. While of course there is going to be a small latency penalty incurred (any sort of processing incurs a bit of a timing penalty, some more than others), it would seem safe to say the memory bandwidth alone helps to mitigate a lot of this.
While PS5 will likely feature its own implementation of such techniques, it stands to wonder if they'll be of as big a focus on the platform's architectural design. It could also be a case where a lot of this type of work is being handled in the I/O complex before being sent to system RAM, though from what's been shown of the design so far, I personally don't think this is the case as much of the customization in the PS5's I/O and flash memory controller are designed around efficient flow, access, and organization of the data.
Anyways, I just wanted to make this post because I had thought on it all a couple days ago when writing something else, and found it very interesting to come upon. It really does go to show that both platform holders have made very calculated and deliberate design decisions to their respective systems, and what's most interesting is to see how the games on these systems will make use of that hardware, both 3rd-party and 1st-party. And thankfully, we won't have to wait too much longer to start seeing some next-gen gameplay to drive all of this home