OK, I'm a 10 year+ user and about to upgrade GPU. Having tested my previous builds I know the following:

  • the CPU can bottleneck the GPU rendering at some point If I Overclock my CPU my gtx970s gets a slight boost and if I underclock it the GPUs dont even throttle to maximum frequency.

  • I know each gpu needs a single thread while rendering, at least that process is single threaded.

However I have no idea what the CPU does exactly and how much a certain GPU power is bottlenecked by a certain CPU power. It would be nice to have a study that relates CPUs to GPU power, at least in terms of modern CPU frequency and teraflops.

Anyone done something similar or thinking about it?



I don't think the GPU power is bottlenecked by CPU in any way. You either render using CPU or GPU. If you're using a CPU you need to set the tile size to some small number like 16 x 16 or 20 x 20 so that each CPU thread will be responsible for one of these tiles and the process becomes more efficient this way. When using a GPU for rendering you need to set the tile size to a higher number such as 224 x 224 and so on so that you can benefit from using a GPU.

Blender OpenData might give you some ideas on what GPUs/CPUs are better for rendering.

  • $\begingroup$ Yes, I use 240x240 and that was my Idea too, until I moved a card from one PC to another which were identical except for the CPUs and I noticed slightly different times on the same frames... My next experiment was to drop the CPU from 3.9 GHz max to 3.0 ( about 25% less) and noticed GPU renders drop about 20%. Next overclocked the CPU to 4.2 max. and there was 0-3% improvement,... this time not that much, but enough to understand it was at the limit of bottlenecking. Another thing I noticed was that some scenes were more affected than others,.. but I feel more has to be understood. $\endgroup$ – M25 Oct 5 '18 at 20:24
  • $\begingroup$ Where exactly does one set the tile sizes? $\endgroup$ – R-800 Apr 11 at 20:38
  • $\begingroup$ There's more than one part of rendering, though-- there's figuring out all the modifiers for this frame (mostly CPU), building BVH for Cycles, etc. How much of an impact various bits have on the final render time is going to depend on how you build your scenes, what engine you use, what materials and rendering settings you use, etc. $\endgroup$ – Nathan Apr 11 at 21:52
  • $\begingroup$ Use a lot of video textures in an otherwise simple scene and your drive's read speed will be the bottleneck. $\endgroup$ – Markus von Broady Apr 12 at 8:24

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