I’ve faced this problem →
and solved it by seting tiles size to 34
have anybody had this problem?
It looks like your total render is only like four tiles here?
Look in your Sysem Preferences at your CUDA device. Does it have both your GPU and CPU selected?
In that case what might be happening is that it’s handing out one tile to the GPU which is finishing in 3 seconds, but then the GPU goes idle because all the other work has been handed out to slow CPU threads and there’s nothing left to work on. You can look in the Task Manager (if this is Windows) and see where the load is (GPU vs CPU).
Try disabling the CPU in the CUDA device list or just make the tiles smaller (then CPU+GPU will probably help). As you probably know, there’s no need for big tiles anymore in GPU rendering:
https://wiki.blender.org/wiki/Reference/Release_Notes/2.80/Cycles#GPU_rendering
“CUDA renders no longer need to use large tiles. In many cases rendering with a tile size of for example 32x32 will actually be faster now. When using denoising it may still be somewhat faster to user large tiles, but this comes with a high memory usage cost.”
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As i said i fixed the problem but
it shouldnt be “CUDA renders no longer need to use large tiles.”
its more like “CUDA renders cant use large tiles.”
If you are using GPU+CPU, you should still try what Pixelfox suggested. It’s often much faster to disable CPU, even with small bucket sizes because the GPU finishes on all but the last few buckets the CPU is still working on.
In this case the reason you saw a performance gain by decreasing the bucket size was not because the GPU can’t handle large buckets. It’s because the CPU was no longer taking up 3 out of only 4 buckets.
The analogy, “A chain is only as strong as the weakest link.” sort of applies here.
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if you set tiles to size taht lets cpu render them under few seconds you completly eliminate this problem and basicly achive fastest render time with cpu and gpu working together
Not always. It’s situational based on your hardware, scene, and render settings. If the CPU can render a bucket in a few seconds, the GPU may be able to render it in a few milliseconds.
So why not just TRY it? It doesn’t affect me any, but it may help you…
I can’t find the button to change the tile size in 2.8. Where is it?
thanks. I found it about 10 minutes ago and slapped my hand on my forehead. I swear I passed that button 15 times trying to find the settings and didn’t press it. 
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I always divide the final resolution by something like 40. So if you’re rendering in 2400 x 1800 pixels and divide that by 40, you get 60 x 45 tiles. This way you don’t get a thin row of tiles at the edges, resulting in more efficiency for the number of tiles that have to be initialized.