Here’s a question thats more curiosity than anything else.
In the usual “4 easy ways to speed up cycles” guide that gets used a lot, it shows a chart of CPU vs GPU tile sizes, with the CPU preferring small tiles and the GPU preferring large ones.
I did my own test today, as I have recently added a second GPU to my system and I was just playing around with options…
32x32 took 14m40s
256x256 took 17m20s
This is all pure GPUs, no CPU being used.
Seems totally backwards, which confused me. Have things changed? Or do I have some odd settings that I’m not using correctly? I’m pretty new to rendering in blender so its very possible I’m doing something inefficiently.
Interesting, will the Auto Tile Size cause problems if I run both GPU & CPU at the same time? I usually just use my GPUs so its probably not an issue, just curious.
That’s not “so” simple, keep in mind that if you want a real accurate measurement, you should do it on multiple differents scenes.
Even if GPU use to be faster on big tiles, there are so geometrical reasons why sometimes big tiles are worse. I just made a Schema on Gimp to explain :
Let say you have a scene with sky background (blue) and very complex ground (brown) :
Basically, what happens is, Sky is VERY FAST, and ground is VERY SLOW (let’s say high density, complex shaders)
If you use this tilling :
What happens is on EACH tile (100%), there is some heavy ground compute and final render is quite long
Whereas if you use just a tinier tilling :
You will have 3 tiles (33.3%) on Sky being computed instantly, meaning you will be basically almost 33% faster
I’m not saying that you are always in this case Sky / Ground of course. I’m just pointing this fact that on the image there are some complexe parts, and other less complex (as you can see all tiles compute at different speed) meaning that tile size is important for this geometrical compute aspect