Render the last 1% takes up to one third of the render time

[seq]

Our current scenes (animations, BF + LC) renders in about 5 minutes/frame. Strange thing is that the first 99.3% or 99.67% gets rendered in about 1 min 30 sec. Then the render takes another 40 sec for the final 0,7 or 0,33%. Why this? There are no bad polys in the scene and also no heavy material.

We use DR with 5 machines with total of 32 cores. The render region division setting was 64, not we reduced it to 32. Images size is 960 x 610 pixel.

We read that the smaller the render region division size is the more the master computer has to stich together and that would lead to longer render times. Is there a way to calculate the optimal setting?

Thanks,
Lars

[vladimir.nedev]

May be the last couple of buckets are rendered by some really slow DR host ? Are all the hosts roughly with the same performance (CPU/memory) ? I think smaller buckets will have a negative effect on render time only if you have a relatively large filter size. In that case V-Ray will need to trace additional rays around each bucket in order to compute the proper final image. A bucket size of 16x16 will probably make the rendering faster (despite the added overhead) with that much processors working on a relatively small image (960x610).

Greetings,
Vladimir Nedev

[seq]

Thanks, Vladimir. All machines are roughly the same performance - only one is slower (Q6600 vs Core i7/Xeons).

Just test your suggestion and it seems that with a bucket size of 16 we gain 1 minute each frame. Thats great! Or do you think it makes more sense to render one scene on half of the machines and another scene on the other half?

[vladimir.nedev]

It depends, if the two scenes render in roughly the same time, there might be some benefit. But if one of the scenes renders faster, its half of the machines will stay idle while the other half still works on the slower scene. So I'm not sure, it's probably best to test this on your particular scenes.

Greetings,
Vladimir Nedev

[seq]

Okay.

One more thing: We just realize that the machines renders several frames in 3 minutes, then suddenly it took over 20 minutes for one frame. The next images again renders in 3 minutes. Here again it is the last 1% of the frame that took that long and the machine that renders this last bucket is the fastest one.

This seems to happen every 50th frame. Another strange thing is that all the render times of the frames are alternate greatly: Frame 49 render in 4 minutes and frame 50 in 9 minutes (no big change in the scene).

Still a lot to learn. How much easier it is to render high res still images!

[vladimir.nedev]

I'm not sure what the reason for the slowdown on certain frames might be, if they're really pretty much the same. Does it happen on the same frame if you run the rendering twice ?

Greetings,
Vladimir Nedev

[seq]

With reducing the bucket size the phenomenon disappear. At least untill now. We will render the scene once again with the new settings. Let's see if it happen again.