Cycles GPU Render, Low GPU and CPU Utilization

I’ve noticed since 2.90 and now 2.90.1, that Blender renders extremely slowly on my dual Xeon E5-2667 workstation with 128 GB RAM and GTX1080i.
My other machine, an i7-9700K with 64GB RAM and GTX1060, seems more responsive.
Today, I observed the Task Manager during a Cycles render and was shocked to discover that only one of my two CPUs was being used at all, and total CPU use at 17%.
GPU utilization for 3D was 7%.
Is Blender not multithreaded? It seems the 32 thread dual Xeon should run circles around the 8 thread i7 machine. I know it does in Maya 2019. So I’m surprised that Blender is barely utilizing the system resources of the dual Xeon workstation.

I’ll let others try to determine why the 1060 would be more responsive than the 1080 because that doesn’t make sense to me.

I’ll just drop the following in passing:

When set to use the GPU in the render settings, make sure you’re viewing the “CUDA” or similar engine in Task manager:

When set to CPU in the render settings, it is very well threaded:

Unless otherwise specified the CPU will not be used during GPU rendering unless you enable it explicitly. However, that is a very hit or miss configuration in terms of actual performance gains (due to tile size / content differences) which is why it’s not any sort of default.

My CUDA and CPU usage levels are way below this on my workstation. In your case, you have the expected results, with CUDA and CPUs spiking to 100% utilization. Mine are always in the teens and my second processor is at zero. Only the first 16 threads show any activity at all. The threads 17-32 sit idle (NUMA logical CPU #2)
I would expect even brief spikes of 100% use, but I never get that and rendering goes relatively slowly on this platform, compared to the cheaper single CPU box that I use for general purpose computing. I’m thinking there’s some odd incompatibility between this version of Blender and multiprocessor motherboards like the Supermicro X10DRi.
Meanwhile, Maya absolutely rips through renders on this machine faster than the single processor machine. Does Blender not play well with multiple physical CPUs?

How to enable CPU to be used during GPU rendering?

In my case in “Cycles Render Devices”, I have selected “CUDA” and enabled both the “Geforce GTX xxxx” and “Intel Core i7 …”.
Does it mean that it is already enabled?

Yes, if both are checked, both will be used.
But be very carefull!! You need to test a lot with your particular scene and tile size to get a little bit of performance gain. Most times it is not worth it.

Thank you for the advices.

I have to set the tile size to 32 x 32 to get the shortest rendering time in Cycles. Increasing the tile size tend to increase rendering time in my case. It is contrary to the common suggestion that increasing tile size will improve rendering performance.

When ONLY GPU is enabled a tile size of around 256 should work the best. As soon as you also enable CPU you need a low tile size. And make sure you selected GPU Compute in the Render Settings.

Could you share a blend file so I could check if it is a settings related problem. (Although I don’t think it is).
And try to set it to GPU Compute and make sure only your GPU is selected in the system preferences as either CUDA or Optix and use a GPU monitoring tool to see your GPU usage (such as GPU-Z)

Here’s my Task Manager activity during a render of a scene using Cycles.
Just a few simple objects with some refraction and caustics.

CUDA is selected in the System Preferences and so is the CPU. But Blender does not seem to know there are two physical CPUs. It uses only one of them, and intermittently.

So the problem is the CPU, as you didn’t select the GPU to render on this example. If I understand correctly. There should be somwhere a setting to set the number of used threads. The default is to let blender decide. Try to manually set this to 64. And try to set it to CPU Rendering in the render settings, so it will ignore the GPU settings (CUDS, Optix)

Because you have a machine that uses NUMA blender might be using the CPU that the GPU is plugged in to (Idk why it would do that)

Give us screenshots of your blender settings, your preferences, your tile options, and your performance options