I have a test scene to render for stereo VR on YouTube. They suggest 5120x2560, and you get stereo stacked Top-Bottom. I have set the settings so it renders in 2 minutes at 25% and then also reasonable about 10 minutes at 50%. If I go to 100% the GPU does nothing and it slows to an unknown maybe 15 hours or let's argue 8 hour render, same settings. I guess my GeForce GTX 780 Ti has a limit of some kind so it dumps it out to CPU only I'm assuming. There are no textures in the test just one for Environment lighting. Around 200K Verts and Faces with Tris double that.
I also tried 75% which guesses about 2 hours but I think maybe that's per eye, so even at 3840x1920 it seems to stop being able to use the GPU efficiently. I have tried larger Tile sizes to no effect currently 256x256
I think maybe the Cycles Rendering pipeline is not able to split the work when the card can't keep up. It could maybe stitch together multiple renders automatically. Like I must have foolishly assumed tiling does. Obviously in this case we don't have a Mem Fault, which culling could be another level of rendering more efficiently. I kind of realize the irony that Eevee will do it instantly or something hopefully (but I've not investigated it since it's still not officially available).
Any insight here would be appreciated. Otherwise maybe this needs to be reported as a bug or something? I'm still confused what's wrong is there a way to check that this is going to happen (render-crawl) other then looking at estimate render time progress?
Mem is only Peaking under 700M normally 330M at 75%
Normally I render at 4000x4000 without any issues.