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Using CUDA and CPU together is slower than cpu alone.
Needs Information from User, NormalPublic

Description

System Information
Operating system: windows 10 1908
Graphics card: Dual GTX 980 + 5700xt
CPU: Threadripper 3960x
RAM: 64gb 3400mhz

Blender Version
2.82a

Issue detail
When rendering with the CPU only I can render a frame in 15 min, When using the gpus (both gtx 980) alone they take about an hour when using the CPU and GPU together it takes about 35 min, which is faster than the GPU's alone but slower than the CPU alone.
At a guess this is due to running CUDA on the CPU.

Event Timeline

Cuda run on Nvidia GPU only.
I guess you use OpenCL for AMD, Nvidia and CPU which is very slow on Nvidia and CPU.
Change to Cuda and use smaller tiles, 64x64 for example.
5700XT is disabled but should be faster than CPU alone.

Mib

Aaron Carlisle (Blendify) changed the task status from Needs Triage to Needs Information from User.Fri, Mar 27, 12:01 AM

I understand that CUDA is for Nvidia.
Im using a tile size of 64x64 already.
The AMD card is unused for rendering due to a bug with it.
In the system menu I had both gtx 980 selected and the cpu and it took longer to render than just using CPU in the device menu.
If I understand correctly when a certain rendering mode is selected cycles will use just that code base IE when in openCL it uses openCL on the gpu as well as cpu same with CUDA is this correct?

Please provide a sample blend file
also clarify your GPU settings + renderer settings + time for all cases