Question about render times

[engineer]

Last time before I went into some holidays, I started a render job hoping to see a completed picture coming back. Unfortunately this wasn't the case, after 8 days not even the prepass was completed.
Cause smaller sized test renders had "normal" render times, I started some sort of investigation about render time increase when the picture size increases and found the following:
1261 x 394 => 1:14,32
1574 x 492 => 2:00,15
1968 x 615 => 3:11,48

For these 3 the render time increase (more or less) linear with the picture size which is matching with my expectation...
But the next size is still rendering: 2458 x 768 => ~20hrs for the prepass.
I guess that this picture will be completed after ~60hrs which is roughly a factor of 20 by increasing the size with a factor of ~1,56.

For these tests I have kept all parameters identical (except the size) to avoid any influence by other parameters.
The RAM utilization was so far below 35%.

Does anybody has an explanation why the render time "explodes" at a certain picture size?

[Oshyan]

Impossible to say without seeing an example image and/or TGD. I'm curious to see if we can track it down though.

- Oshyan

[Oshyan]

Well, I was unable to reproduce the tremendous render time difference. The 2458 x 768 render does take a bit longer than linear increase would suggest, but not much longer. The base render at 1261 x 394 was 28 minutes on my system. The number of pixels in the 2458 x 768 is just under 4x, so you'd expect about that much of a render time increase. That would be 1 hour and 50 minutes or so. My render time was exactly 2 hours, so slightly longer than expected, but not by a significant amount.

Looking at the scene the main explanation I can see for any possible non-linear render time increases would be the tremendous amount of small-scale detail you have. As your renders get larger, there is more and more visible small-scale detail that the renderer has to deal with. This can be particularly challenging for the GI algorithm.

What are the specs of the system you're rendering on?

I also note you have raytraced object rendering disabled, which we usually don't recommend. Is there a reason for this setting in your scene?

- Oshyan