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I do some tests on a scene with various tiles size. The first time, i set the tiles size to 256x256. It rendered in about 9 minutes. The second time, i set it to 512x512.

Now because the tile size is bigger than the first, i thought it would have rendered much faster, but apparently it finished in 12 minutes. How is this so?

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    $\begingroup$ Why/how does tile size effect performance? $\endgroup$ – brockmann Mar 6 '17 at 14:09
  • $\begingroup$ I suggest you make tests to find out what is the optimal tile size for your system. Using GPU, small tile sizes (like those used when rendering on CPU) won't let you use all of the resources available and will result in longer rendering times. Likewise if the tile size is too large, it might exceed the compute capabilities of the GPU, rendering slower. Do several tests to find what tile size works best for your system. Keep in mind that for some complex scenes that optimal size might change. $\endgroup$ – user1853 Mar 6 '17 at 14:38
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You have to see the cores of your graphic card. The number of cores is the number of tiles it can process at the same time. If you have a big amount of cores make the tiles smaller. About 1/8 of the blender's default size is ok. If you have a really good graphic card try the 1/16 of the blender's default size.

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  • $\begingroup$ This is incorrect, when rendering on GPU, the number of CUDA cores is not the number of tiles blender will render. A card with 2304 CUDA cores will not render 2304 tiles, it will process only one. If you have multiple GPUs in your system, then cycles can do as many tiles as there are GPUs. Usually, optimal tile size is between 512x512 to 128x128pixels, but that should be thought of as a starting point to test. The size of the image should not be the deciding factor on the tile size. Rendering a large 8k image or a small image of just a few pixels, the tile size should still be the same. $\endgroup$ – user1853 Mar 8 '17 at 12:47
  • $\begingroup$ Even though some people recommend having a tile size that is square and in multiples of 8, you can make the optimal size match the proportions of your final image, so that you render an exact number of tiles and not waste resources rendering "fractions" of a tile. For example on my system I render images of 1920x1080 with a tile size of 240x270. That size works well on my particluar system but it might be different for yours. Moreover there are complex scenes that will only render if I reduce the tile size, and very simple ones I can push the tile size to 480x540 or larger. Do your own tests. $\endgroup$ – user1853 Mar 8 '17 at 12:59

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