NVIDIA Linux OpenCL Performance vs. Radeon ROCm / AMDGPU-PRO
With Rodinia's OpenCL Heartwall test, the GTX 1070/1080 were running it too quickly to even accurately record the measurement, while the GTX 1050 series and GTX 1060 were faster than all of the tested Radeon Linux combinations.
The Radeon GPUs on their Linux drivers are at least competitive with Darktable.