@obe
You could also try:
opencl_scheduling_profile=default
opencl_device_priority=+0/+0/+0/+0/+0
Use the ID of the card to use in place of 0. This will also direct all processing to the specified card. The ‘priority’ setting is only used if the scheduling profile is set to default.
If your GPU is processing a module for a long time, and darktable has another pipeline to run, it will first wait (if the priority says using the GPU is mandatory), and then give up and fall back to the CPU anyway. For me, this usually meant lost time, because if darktable waited longer, the GPU could still have finished the task faster. To extend the timeout, you can set a large value here:
opencl_mandatory_timeout=20000
Details here: How cheap a GPU will still speed up darktable? - #62 by kofa.
If in doubt, just follow the guidance from Hanno. He knows what he’s talking about. For NVidia, in general: no tuning; and don’t try to use unlimited memory.