Do you know why? Nvlink. And software. On hardware level of the GPUs there is nothing that would stop Kepler or Maxwell to have the same feature. But Nvlink, and CUDA software blocks it. Why? Marketing to sell new GPUs. But that is absolutely understandable. What is not understandable however is that why people think that Pascal is new architecture, when it is only improved Maxwell with gigantic boost clocks? Why NVlink makes it different? Nvidia Distortion Field
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Did I said that, or it did confirmed your bias? Polaris does not compete with GP100. 232 mm2 GPU vs 600mm2 monster. It would be funny if it would compete however...
S9300X2 competes with it in SP performance, and beats it. 13.9 TFLOP in 300W of TDP vs 10.6 300W GPU. However there is absolutely NOTHING, yet, in the market that can compete with GP100 in DP.