# Eevee is using my CPU to render

I rendered a short animation using Eevee engine, but my cpu was used 100% of the time. The temperature and fan of the cpu went crazy. How is that possible? Eevee is supposed to use OpenGL and has nothing to do with the settings in preferences, correct me if I'm wrong. I do have Cycles on GPU Compute and Cuda selected in Preferences with no CPU, but that shouldn't matter anyway. I have my both monitors plugged in the GPU display ports. It's a RTX 2070. I rendered the animation 3 times and checked the hardware with CAM and Task Manager. Why is Eevee using my CPU instead of GPU???

• Please refer to this question here: blender.stackexchange.com/questions/162264/… – stphnl329 Jan 5 at 18:52
• I have the Cycles engine setup just like that with GPU Compute and Cuda in Preferences. But it shouldn't matter anyway. I'm using Eevee for animation rendering and it uses my CPU 100%. How could that be? By the way, I just rendered it with Cycles and it's all good using my GPU completely. – Daniel Jan 5 at 18:55
• Ah sorry, wrong link. I'll post the full answer – stphnl329 Jan 5 at 18:58
• Simulations? Particles? Complex nodes? Number of Subdivisions? Compositing?There are quite a number of operations that are done in CPU regardless. Please describe your scene in detail. – user1853 Jan 5 at 19:33
• Guess what. Because you mentioned the Compositing piece I checked it out. I had an Alpha Over, an RGB and a Denoise Filter. I removed the denoise filter and problem solved. The CPU doesn't go nuts anymore when rendering with Eevee. If I keep the denoise on with Cycles there is no problem with the cpu though. – Daniel Jan 5 at 20:04

If anybody runs into the same problem in Eevee where the cpu is overloaded while rendering check your Compositing mode and make sure you don't have a Denoise filter on. It doesn't create a problem when you render in Cycles but it does put stress on the cpu when rendering with Eevee.

• Good to know, thanks :). – Jachym Michal Jan 5 at 21:35
• Why on earth would denoising ever cause issues like this? It's either using a neural network to denoise (much faster on the GPU), or it is using a wavelet transform (again, much faster on the GPU). This is a bug in Eevee if it is trying to do denoising on the CPU. – whn Jan 6 at 15:08
• @whn s/bug/missing feature/? – rubenvb Jan 6 at 15:59

From the Blender Manual:

CPU Rendering

Being a rasterization engine, Eevee only uses the power of the GPU to render. There is no plan to support CPU (software) rendering as it would be very inefficient. CPU power is still needed to handle high scene complexity as the geometry must be prepared by the CPU before rendering each frame.

In short: CPU is still needed to prep the scene, GPU is used to render it.

• There is no complexity in my scene. There is no need for the CPU to kick in. The GPU is not even loaded. I wish I could send you a vid with the CAM graph to see it. It didn't happen before. It's just something new out of whack. Not sure what on planet earth is happening. – Daniel Jan 5 at 19:03