As for the memory segmentation, there are 3 basic scenarios to consider, only one of which has the potential to impact the GTX 970 in particular. In all cases with less than 3.5GB of memory allocated the GTX 970 behaves just as if it had a single segment, with no corner cases to be concerned about. Meanwhile in cases with more than 4GB of memory allocation the GTX 970 will still spill over to PCIe, just as the GTX 980 does, typically crushing performance in both cases. This leaves the last case as the only real concern, which is memory allocations between 3.5GB and 4GB.
In the case of memory allocations between 3.5GB and 4GB, what happens is unfortunately less-than-deterministic. The use of heuristics to determine which resources to allocate to which memory segment, though the correct solution in this case, means that the real world performance impact is going to vary on a game-by-game basis.
If NVIDIA’s heuristics and driver team do their job correctly, then the performance impact versus a theoretical single-segment 4GB card should only be a few percent. Even in cases where the entire 4GB space is filled with in-use resources, picking resources that don’t need to be accessed frequently can sufficiently hide the lack of bandwidth from the 512MB segment. This is after all just a permutation on basic caching principles.
The worst case scenario on the other hand would be to have the NVIDIA heuristics fail, or alternatively ending up with a workload where no great solution exists, and over 3.5GB of resources must be repeatedly and heavily accessed. In this case there is certainly the potential for performance to crumple, especially if accessing resources in the slow segment is a blocking action. And in this case the GTX 970 would still perform better than a true 3.5GB card since the slow segment is still much faster than system memory, but it’s nonetheless significantly slower than the 3.5GB segment as well.
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In the end while I am disappointed that these details haven’t come out until now, I am satisfied that we now finally have enough information in hand to truly understand what’s going on with the GTX 970 and what its strengths and weaknesses are as a result of memory segmentation. Meanwhile for real world performance, right now this is an ongoing test with the GTX 970. As the highest-profile card to use memory segmentation it’s the first time NVIDIA has been under the microscope like this, but it’s far from the first time they’ve used this technology. But so far with this new information we have been unable to break the GTX 970, which means NVIDIA is likely on the right track and the GTX 970 should still be considered as great a card now as it was at launch. In which case what has ultimately changed today is not the GTX 970, but rather our perception of it
