The Lack Of HBM2 Is No Big Deal
Given AMD’s use of HBM on its Fiji-based cards, hardware enthusiasts are quick to question Nvidia’s lack of HBM2 on the Titan X. The company instead chose to give this card a massive 12 GB of GDDR5X with a nominal memory bandwidth of 480 GB/s. That's a 42% increase over the previous-generation Titan X that offered 337 GB/s.
While it is true that HBM2 facilitates a bandwidth advantage over GDDR5X, the benefits of more throughput would only become apparent in situations where on-die resources aren't being fed fast enough. The concept is akin to PCIe 3.0/2.0/1.1 links between CPU and GPU, and you can already run any modern card with four lanes of third-gen PCIe with little or no performance degradation. Bumping up that bandwidth with eight or 16 lanes yields a 1% to 2% performance increase on average, and anything beyond confers negligible gains.
Below you can see a short run of The Witcher 3 running on two Titan Xes in SLI at 5K (5120x2880p).
Notice that the bottleneck (as you would expect) is on the GPU’s front end. The memory controllers are busy 50-60% of the time, indicating plenty of available headroom with these GDDR5X modules. Would this card be faster if equipped with HBM2 instead of GDDR5X? We down-clocked the memory by -20% and observed a -4% impact on performance in 3DMark. So, more bandwidth might be good for another percentage point or two, at best.
