Is rendering time really not a zero-sum game??

Support Lighting and Rendering
#1

I´m just curious… I thought that rendering two little movies at the same time would slow down each rendering process by half…but no!!
They both render with approx the same speed as they do when renering alone.
This is good news - perhaps too good?

Would anyone be able to explain this phenomenon to me?? - I have tried it before too, but I have also experienced in the past that render time is slowed down by multiple renderings.

Does anyone know the how Blenders INTERNAL rendering system/brain works because its the internatl render that I use:D

PS wondering btw if it could be harmful to my pc…(=overloaded)

#2

It could be due to BI not being able to make complete use of the CPU, or it could be that you’re using less threads than what is available in your machine.

I know with Cycles at least, a second render being started (using all threads) would mean both of them going at half the performance.

#3

Rendering movies? As in, 3D animation or video encoding? Because in the latter case, there’s more limitations because A. switching to the next frame takes a fraction of a second and B. saving to the hard drive does too (if you have an SSD and an HDD, compare video render times, I might be an outlier but I found it renders a fair bit faster on SSD). Outside of video effects (especially blur, I found), the per-frame render time is so low that you can assume it doesn’t really take that much out of the hardware, so rendering several at once doesn’t sound like something particularly taxing for a halfway up-to-speed system.

#4

Also bear in mind that the process of rendering involves two distinct operations:

  • Calculating the contents of the next frame, and …
  • Writing the frame to disk.

This second operation is “completely Input/Output.” It does not involve the CPU at all. Therefore, another thread or process could most-certainly use that time, during which the CPU (and the GPU) would otherwise be sitting completely idle.

#5

As a rule, rendering is very taxing to your machine so please make sure that it has more than enough cooling.