In a response to The Tech Report, NVIDIA confirmed the issue but claims it was done by design. The green team explains the card's memory is segmented into a 3.5GB section and a 0.5GB section and that the latter part of the memory is only used when the game requires it because this 0.5GB section has lower performance. Why this is baked into the design of the GeForce GTX 970 remains a mystery but NVIDIA claims the impact on gaming performance is minimal:
The GeForce GTX 970 is equipped with 4GB of dedicated graphics memory. However the 970 has a different configuration of SMs than the 980, and fewer crossbar resources to the memory system. To optimally manage memory traffic in this configuration, we segment graphics memory into a 3.5GB section and a 0.5GB section. The GPU has higher priority access to the 3.5GB section. When a game needs less than 3.5GB of video memory per draw command then it will only access the first partition, and 3rd party applications that measure memory usage will report 3.5GB of memory in use on GTX 970, but may report more for GTX 980 if there is more memory used by other commands. When a game requires more than 3.5GB of memory then we use both segments.In the examples from NVIDIA above, the company claims the theoretical performance loss is between 1% and 3% in typical gaming scenarios. We're not sure what to make of this. On one hand it's not a big deal because there's no real loss of performance, just some performance that was never there to begin with, but on the other hand is seems NVIDIA was misleading consumers by marketing the GeForce GTX 970 as a 4GB card when in reality the last 0.5GB of the card performs very differently from the other 3.5GB.
We understand there have been some questions about how the GTX 970 will perform when it accesses the 0.5GB memory segment. The best way to test that is to look at game performance. Compare a GTX 980 to a 970 on a game that uses less than 3.5GB. Then turn up the settings so the game needs more than 3.5GB and compare 980 and 970 performance again.
Here’s an example of some performance data of <3.5GB vs >3.5GB:
On GTX 980, Shadows of Mordor drops about 24% on GTX 980 and 25% on GTX 970, a 1% difference. On Battlefield 4, the drop is 47% on GTX 980 and 50% on GTX 970, a 3% difference. On CoD: AW, the drop is 41% on GTX 980 and 44% on GTX 970, a 3% difference. As you can see, there is very little change in the performance of the GTX 970 relative to GTX 980 on these games when it is using the 0.5GB segment.
We hope NVIDIA is crafting a more satisfying message to further clarify the issue.