Blender slows down drastically on a specific frame when rendering animation

Support Lighting and Rendering
1

I dont know why but Cycles slows down drastically on some frames (especially frames 10, 20, 30, 40 etc. sometimes eg 19, 29, 39…). I use 2x GTX 780 TI. Rest frames are rendered quickly. I dont know where is the issue. Its very important to me to solve this problem because of project deadline. Daily build don’t solve the problem :frowning:

2

Could it be because of something going on in your animation? Is there any scaling going on in the animation? are your gpu’s getting too hot?

we run a cluster of GPUs… have never experienced this problem before.

3

Animation is pretty simple (also without scaling) about 1,2 mln tris & memory usage around 400 mb. I have custom system of fans and GPU’s are around 49-55 in full stress so i don’t think it’s too hot - rather cold i guess. I know its strange. For now I just avoid those frames :confused:

4

If your render one of the ‘fast’ frames on CPU, and then a ‘Slow’ frame on CPU, does the difference in time still occur?

5

Oh are you rendering to an image sequence or to a movie file? it could be ffmpeg creating keyframes for the movie file?

6

Well I can’t check it right now but i guess it’s only GPU side issue.

Render as still or animation -> same results. 19 frames fast and 20 frame slow like the other 19 :smiley: weird…

7

I notice the same issue (on 2.83.10) and for me it seems that motion blur is the cause.

I have an NLA animation consisting of several repetitions of a walk cycle plus the movement of the armature. At the beginning of the walk-cycle my armature moves backward quite a bit to compensate for the forward movement of the walk cycle. So at the start frame of each walk cycle segment my render times go through the roof (from 1 minute to around 3 hours). If I disable motion blur the render times on those start frames are about the same.

I am animating a very furry long haired animal so I can imagine motion blur eats resources, but on the other frames there is just as much movement. The motion of the armature might not be smooth, but the overall animation (and the movement of the simulated hairs) is smooth, so motion blur is not much different from frame to frame …

I’ll have to investigate a bit more to make it easily reproducible and I haven’t checked the bug tracker yet, but this feels like a bug …