[Bf-cycles] Tiles sizes - impact on RAM and speed

Ole Jakob Skjelten olesk at pvv.org
Sun Feb 10 17:52:20 CET 2013

Thanks a lot for that Brecht! My understanding is that *reducing* tile sizes only has a positive effect with regards to reducing the size of the output buffer, which is in most cases is a rather small part of the total RAM eaten by Cycles, and thus, unless you you're in a desperate I-only-need-10MB-more-RAM-to-render-on-my-GPU scenario, increasing the tile size for speed benefits is the more likely scenario. Would you agree?

It's also my understanding that, using CUDA, keeping your tiles to multiples of 16 pixels helps speed (or rather prevent wasting GPU cycles)? And finally, does it help to operate with tiles that are multiples of the full render resolution, or should one use same values for x and y direction (square tiles) instead?


Ole Jakob Skjelten
olesk at pvv.org
+41 79 832 2976

On 10. feb. 2013, at 17:26, Brecht Van Lommel <brechtvanlommel at pandora.be> wrote:

> Hi,
> I don't have a simple answer for this either, all I can suggest is to
> just try it out. Basically you have to balance a number of things and
> it's difficult to make predictions because it depends on the GPU and
> scene being rendered.
> Bigger tiles gives more memory usage especially if you have many
> render passes enabled, since it has to fit the entire tile in memory.
> The other disadvantage is that if you have multiple GPU's, you might
> end up with one GPU working on a tile while the other is doing
> nothing.
> With smaller tiles the disadvantage is that the GPU might not be fully
> utilized, each pixel is handled by one core, and GPU's have hundreds
> of cores. Further GPU's tend to work more efficiently when they have
> multiple threads per core. The other disadvantage with smaller tiles
> is that this means more GPU/CPU communication which can also lead to
> the GPU being underutilized.
> I think generally the advice is to use tiles sizes of 256x256 and up
> for GPU's, but I don't have a single formula that I can recommend.
> Brecht.
> On Sat, Feb 9, 2013 at 7:13 PM, Ole Jakob Skjelten <olesk at pvv.org> wrote:
>> Hi!
>> I've spent quite a bit of time recently setting up a GPU hardware FAQ for
>> Cycles, based mostly on the recurring questions from the support forum at
>> BlenderArtist. I feel I got most of it covered, but one type of questions
>> keeps coming up that I do not have a good answer for, and that is the impact
>> Cycles tile sizes has on total VRAM and rendering speed. I've seen a lot of
>> empirical testing, but it's hard to transfer this into some solid general
>> advice.
>> I suppose I'm trying to answer the questions "which tile size gives optimal
>> speed given my scene and hardware?" and "how does tile size impact VRAM
>> requirements?" (which also answers the questions "I get an out of memory
>> error, can I do something about that?").
>> I was hoping there was some sort of formula I could use to understand the
>> relationship between tile size and VRAM usage, and the impact on number of
>> calculations required for a full scene. I've asked around everywhere I could
>> think of, and though people have a lot of anecdotal evidence one way or
>> another, I've yet to get any closer to the underlying technical details. But
>> perhaps there is someone on this list that could settle this once and for
>> all? (I promise to distribute the answer broad and wide ;)
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