As
recreation and
Xavster have said, it's definitely the number of oversize textures that is causing the fallback-to-cpu issue.
For example, in the log file there are references to many many 4096x4096 or 2048x2048 images that do not appear to even be visible in the scene (like all the textures for the "Ranger" car and many textures for interior wallpaper).
1. First thing to do is make separate copies of the scene environment for each major render scene so you can edit without losing the base setup you might need for future scenes.
2. Next, in your scene specific copy, delete objects (hiding doesn't work) that are not visible, like in this case the Ranger car.
3. Third step is to get the Scene Optimiser product and use it to massively reduce the texture sizes of stuff that is not close to the camera - if your render output is full HD, and you have a single texture plane that just fills up the entire viewport, then the maximum resolution you need for the texture is 2048x2048. anything higher than that is overkill even in this artificial case. the only time you need 4096x4096 is for extreme closeups (or large objects with non-repeating textures across the entire surface).
In many cases, for objects only in the background of your render, its ok to reduce the texture size to 512x512 - which takes 1/16th of the GPU VRAM space. Texture size reduction using the tool is 100% reversible, as it makes separate copies of each texture image before reducing, and simply updates that specific scene's object's materials to use the smaller texture. You can undo it in the Scene Optimiser interface if you want, or just go back to a fresh copy of the original scene (or reload the objects) to go back to original textures.
As a test, I repeated my render from yesterday after running scene optimiser:
1. reloaded the scene from yesterday (identical settings)
2. used Scene optimiser to reduce all the textures to 512x512
3. re-ran the render
This time the GPU VRAM usage reported by task manager was 3.5GB, down from 5.8GB. So it's added a bunch of room for other figures etc...
View attachment 1215074
Visually it looks pretty much identical to me.
My log file from the reduced texture size render has this line:
2021-05-27 10:56:53.742 Iray [VERBOSE] - IRAY:RENDER :: 1.13 IRAY rend stat : Texture memory consumption: 79.421 MiB for 166 bitmaps (device 0)
Whereas previously with the default texture sizes it was:
2021-05-26 13:44:21.486 Iray [VERBOSE] - IRAY:RENDER :: 1.8 IRAY rend stat : Texture memory consumption: 2.338 GiB for 166 bitmaps (device 0)