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Is there a way to make rendering in Cycles use less GPU resources? Like some console parameters for rendering engine or settings within Blender? I tried render scene with the console (no UI), following Blender wiki example, but it still eats all the GPU and makes it really hot to 70' Celsius. I don't mind if such settings increase render time from minutes to hours, that's kinda exactly what I need to put rendering task do the job over night and keep GPU usage around 50-60% and prevent overheating.

I'm using Linux.

70 degrees is not too hot and it's completely normal for a working GPU. Something beyond 85 degrees would start getting concerning. Anyways, you may set the tile size to a low value (16 x 16 for instance) to get what you want.

However, from a more environmental perspective ( :) ) I don't recommend you doing this since you will have to leave your computer on for a much longer time to get the final rendering. In the meanwhile, your computer is using way more electricity than it would if you use the GPU efficiently by setting the tile size to a higher number (e.g. 224 x 224). Normally, a mid-range GPU uses around 200 watts or less working with the highest load. When idle, it uses around 30-50 watts. The rest of the hardware for your computer also use at least around 70-100 watts in idle and maybe around 200-300 watts when they are under descent load. Assuming that you don't do anything else overnight (8 hours) and just leave your computer on for rendering your computer will be using something around 8 x 200 = 1600 watts. The number 200 represents 100 watts for your [almost] idle computer and 100 watts for your GPU which is not under heavy load due to small tile size. However, if you set the tile size to a higher number and the rendering takes 2 hours your computer would use something around 2 x 300 = 600 watts. 300=100(everything except GPU) + 200 (the GPU under heavy load).

Note that all of these numbers are just a rough approximation and could widely vary depending on your hardware but one thing that won't change is that leaving your computer on for longer will potentially use more watts than finishing your rendering faster and turning your machine off :)