Loading...
> >

Iray CPU faster than GPU

lasersayspewlasersayspew Posts: 71
in Technical Help (nuts n bolts)

Hello! I'm completely new to anything connected to rendering and working with 3d models. So I'm really sorry if I ask a dumb question or something that was answered over and over. Tried to google with no success. 

Here's the thing. Several times I saw people saying something like "I always turn off CPU rendering in Iray", "Don't forget to turn off CPU" etc.

What if my CPU actually renders faster than my GPU?  Should I use it then?

I tested and CPU is actually x1.5 faster than my toaster GPU. What are the drawbacks in my situation? Are CPUs the last resort measure only and can't handle rendering for too long and tend to die fast? Should I use it or should I avoid using CPU at any costs?

Thanks in advance and once again sorry if my questions are dumb. 

Comments

  • Richard HaseltineRichard Haseltine Posts: 107,987

    I think the advice is good for some systems, where having the CPU free to manage the resource may be more useful than having it try to contribute to the render. In your case it certainly wouldn't apply.

  • fastbike1fastbike1 Posts: 4,078

    If your gpu is in a toaster, then the data path is the choke point that makes the gpu render slower.

  • ebergerlyebergerly Posts: 3,255
    lasersayspew said:

    What are the drawbacks in my situation? Are CPUs the last resort measure only and can't handle rendering for too long and tend to die fast? Should I use it or should I avoid using CPU at any costs?

    I'm guessing you call it your "toaster GPU" because it runs hot? I'm kinda surprised your CPU is faster than your GPU, though you don't give specifics on exactly which GPU you have, and how you measured the speed difference. 

    As far as CPU's (or GPU's) not handling rendering for too long because they can't handle the heat, that's a popular myth that keeps coming up. CPU's and GPU's are designed to operate in a certain temperature range, and they have BIOS/driver systems that regulate the cooling fan to maintain the temps below the danger point. In fact they maintain the temps usually FAR below the danger point. So unless your system has some special issues (overclocking, or your fan cooling is blocked, or you have hardware/driver/BIOS issues), your CPU and GPU will be fine. A concern is that if your CPU does the rendering it basically locks up your computer and you can't do other stuff. And usually it is MUCH slower than any halfway decent GPU.

    Some like that CPU rendering is a great backup if you have HUGE scenes that cause the GPU to run out of VRAM. And that can be true, but for example on my system I can load a monster scene that takes 30+ GB of my 64 GB system RAM, and it will still run on my 11GB GTX-1080ti. And IMO, it's a huge pain to have my computer locked up doing rendering for hours at a time, so I prefer to, when absolutely necessary, break up my scenes into smaller ones and merge the renders later. 

    Anyway, in your case I'd be interested in hearing exactly how you determined your CPU is faster. Maybe take a look at Task Manager and/or GPU-Z to see what's actually going on.

     

  • fastbike1fastbike1 Posts: 4,078

    The device used for external gpus is often called a toaster. 

  • ToborTobor Posts: 2,300
    fastbike1 said:

    The device used for external gpus is often called a toaster. 

    Exactly. Was the term used before the Amiga?

    Anyway, some older cards may still be supported in certain versions of D|S and Iray, but the cards don't have many CUDA cores. I think on my oldest GTX card, which is not used for rendering, there are 97 cores in it.

    It might indeed be better to switch off the GPU, but you can always do speed tests to verify which is best. Try the same scene CPU only, GPU only, and the combination of the two. Be sure to close all render windows between each test. You may be surprised that the CPU/GPU combination doesn't save you any time. This is because Iray load balances the active renderers to spread out the work, and the processing is throttled by the slowest device. (The option to control this may be in the professional implementations, but I've never seen an option to deal with this in D|S).

     

     

  • ebergerlyebergerly Posts: 3,255
    fastbike1 said:

    If your gpu is in a toaster, then the data path is the choke point that makes the gpu render slower.

    Oh, you mean one of them Thunderbolt things? 

    Yeah, I'm not sure that it will necessarily slow renders. GPU's are generally designed and programmed to minimize CPU to GPU transfers, and GPU's have memory and just about all they need on board. So the Iray software transfers a huge chunk of data/calculations to the GPU (pretty much the entire scene I believe) and it's off on its own doing the render calcs. 

    You may be right that it has an impact, but I'd be suprised if it was real significant. I think the far bigger impact is the thousands of cores and zillions of concurrent threads in a GPU vs. the handful of cores in the CPU. 

     

  • lasersayspewlasersayspew Posts: 71

    I called my GPU a toaster because it has approximately the same computing power.

    GPU: Geforce gt630 

    CPU: i5 3470

    I tried rendering the same scene with CPU only, GPU only and CPU+GPU. CPU is faster by x1.5 than GPU. CPU+GPU is about 20% faster than CPU only. 

    Thank you for your answers!

  • ebergerlyebergerly Posts: 3,255
    edited July 2018
    Ahhhhh...okay. now it makes more sense. My old GTX-750 or whatever rendered a benchmark scene in like 22 minutes, and my 8 core Ryzen CPU did it in 20 minutes. So i guess your 96 (?) core GT630 might be super slow compared to, say, a 3500+ core GTX-1080ti or your CPU.
    Post edited by ebergerly on
  • lasersayspewlasersayspew Posts: 71

    Yeah. 96 cores, 2gb VRAM. Octabench score is 4.7 xD 1080ti score is around 180

    Guess, I'll have to stick to CPU rendering for now. Good to know it won't just die on me from overload, thank you for your help!

Sign In or Register to comment.