I had tested it before with a Zen2 CPU and a GCN4 Radeon GPU. I rendered a simple scene in Cycles. The CPU’s rendering time was about twice as faster than the GPU. When I used CPU+GPU, the rendering time was a lot slower than when I used CPU alone. (I made it sure that “kernel rendering time” would not be the problem by rendering it multiple times.) I thought this was a particular problem of AMD’s GPU or because the GPU was too slow.
Now, I bought a new Turing Nvidia GPU (about twice as fast as the previous Radeon GPU), and I tested rendering the same scene. Here is the result:
- GPU 11.3
- CPU 21.3
- GPU+CPU 30
As you see, using both GPU and CPU actually took longer time than using CPU or GPU alone. When I used GPU+CPU, at about 13 seconds, the scene looked all rendered, but the orange cross marks were there. I am not sure if Blender was refining the scene, but I could not see anything changing. The UI was still showing that it was rendering till about 30 seconds, and then said it was finished.
So, the same thing happens for both AMD and Nvidia, and even when the GPU is relatively powerful (for AMD, the CPU was twice as fast than the GPU, for Nvidia, the GPU is twice as fast than the CPU). Why does it take longer time to use both than using one?