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Blender 2.8 has a user preference of using GPU and CPU simultaneously during rendering. So, when a scene is getting rendered using this preference and 4 threads:

  1. Is it true that during the beginning of render, GPU renders 1 tile and CPU cores render 3 tiles?
  2. And also, suppose that near the end of render, 4 tiles remain to render. GPU starts rendering a tile and CPU cores start rendering remaining 3. As GPU would finish its tile faster than CPU cores, will it become idle while CPU cores slowly finish their tiles? Or will it take control of another tile from a CPU core to finish rendering faster?
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  • $\begingroup$ Hello :). This sounds like an interesting question. You can easily test it yourself. Just create a render with 4 tiles, and open your Task Manager (or Activity Monitor) and you'll see if your GPU becomes idle near the end. In the render window you can see how many tiles are currently rendering (they have orange corners). $\endgroup$ – Jachym Michal Dec 4 '19 at 15:02
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    $\begingroup$ Hi I tested part 2 by rendering BMW scene at 256*240 with tile size 128*180 and according to MSI Afterburner, GPU usage becomes 0% while CPU cores render remaining 3 tiles slowly. What about part 1? $\endgroup$ – hhj8i Dec 4 '19 at 15:19
  • $\begingroup$ I believe you just answered both parts yourself :). It seems all 4 tiles are rendered at once, with GPU being faster than CPU. The GPU can only render 1 tile at a time, which means 3 tiles are rendered by CPU. Post it as an answer an I'll give you +1. $\endgroup$ – Jachym Michal Dec 4 '19 at 15:26
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  1. Yes it is true the GPU will only render one tile, but each core of your CPU will render a individual tile, so if you have a 4 cored CPU it will render 4 tiles simultaneously and your GPU another one, so it will be 5 tiles at the same time and the GPU tile probably will render a lot faster.

  2. I noticed once a tile begins to be rendered with the CPU it will stay as a CPU tile till it finished the to render the tile. So if you are at the end of the render the CPU tiles will stay for a while. It is faster to render with GPU and CPU if you have a good CPU compared to your GPU. But if you have a very powerfull GPU and a very weak CPU stick to GPU render because your GPU can render all the tiles before you GPU render the last tiles, but i really think this is probable.

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  • $\begingroup$ Actually its not 5 tiles at the same time. GPU renders a single tile and 1 CPU core does not render any tile. $\endgroup$ – hhj8i Dec 4 '19 at 16:02
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I have tested this by rendering BMW scene at 256*240 (with tile size 128*180). So: 1. It is true that during the beginning of render, GPU renders 1 tile and CPU cores render 3 tiles according to MSI Afterburner, which shows 99% GPU usage. 2. Near the end of render, GPU usage becomes 0% while CPU cores render remaining 3 tiles slowly.

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