Cycles is only using about 10% of my CPU usage when rendering on CPU

Hey guys. As the title says, my CPU isn’t hardly being used when rendering, and this is when I am in CPU render mode, not GPU. One thing to note is that I have 8GB of RAM, and it’s at 99% usage when I’m rendering. Why is my CPU not being used??

Any help is much apprenticed.


Does this happen in general, when rendering any scene? Or just with that particular scene you’re working on right now?

If your system RAM is maxed out, your OS (Which one?) might start swapping data to “virtual RAM” on your hard disk. Problem is: A HDD is so slow compared to the RAM, that the CPU will have quite a bit of idle time waiting for the data to transfer.

Just guessing, of course, as we don’t have much to work with…

I concur with IS. I had a scene that glomped 48Gb memory (16ram+32swap), meaning that the CPU wasted more time waiting for the disk to swap, than really rendering the scene. If your mem is maxed, that’s probably what’s happening.

Ah, I now see that it only happens with the scene I am currently working on. I’m on Windows 10 build 10130, which I guess might be an issue…? So, what causing so much RAM to be used while rendering? I didn’t know that RAM mattered “while” rendering, I just always that thought it only affected stuff in the viewport.

I know that heavy use of hair particles can eat a lot of RAM. You can save a lot of memory by replacing their use with object instances for things like grass (though you will want to keep the hair for creature meshes).

Hard to say without seeing the scene… The usual suspects:

  • Going overboard with Subsurf levels,
  • too many “steps” for hair paths or too many hair particles,
  • very many high res textures…

Thanks guys. I guess my heavy RAM usage is just from all the particles I have in my scene. After lots of tweaking I’m now able to render on my GPU, so I’ll just try to keep it that way.