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What happens when I press that button?

geoff6geoff6 Posts: 250
in Technical Help (nuts n bolts)

Hi all,

I'm not writting with a problem, I have a question for all you techies out there about rendering with Iray. What actually happens to the data when I start a render, what is the flow, as it were? I refer to a still render with only the GPU selected. The reason I ask is to understand the role that the CPU plays in a GPU only render.

Thanks in advance,

G

Comments

  • fastbike1fastbike1 Posts: 4,075

    Short, simplified flow: The scene loads into the  GPU memory (that can take a minute or so). The GPU starts processing. CPU has a limited role once you start rendering. However, CPU speed and RAM play the dominant role in EVERYTHING else you do on the computer. Loading every prop or figure, every move, everyshader, every light.

    This is not a game for the faint of PC/Mac.

  • geoff6geoff6 Posts: 250

    Thanks for that answer fastbike1, the reason I asked was that I'm always suprised by how much activity there appears to be in the CPU during the render.

    Thanks again,

    G

  • SpottedKittySpottedKitty Posts: 7,232

    If  there's a lot of CPU activity... is that render unusually slow? It might be blowing past the limits of your graphics card's VRAM, so that the render falls back to CPU-only.

  • geoff6geoff6 Posts: 250

    Hi SpottedKitty,

    That's interesting as the GPU is an Nvidia 980 but the renders are a bit slow although as I've upgraded cards over time, I've also made the renders more complex so I don't think I've ever not challenged the card! My question now is; if the VRAM is overspilling as you suggest, and the CPU is getting the overflow, are the GPU and CPU sharing the load or is the majority of it being taken by the CPU? I ask that because now that you mention it, the GPU doesn't seem to be working that hard.

    Many thanks,

    G

  • fixmypcmikefixmypcmike Posts: 19,565

    If the scene won't fit on the GPU, the GPU doesn't get used at all.

  • geoff6geoff6 Posts: 250

    Gulp!

    What might be the solution to that scenario?

     

  • geoff6geoff6 Posts: 250

    Actually, on the subject of scene size, do scene elements present in the scene but not in the frame count towards the load on the GPU during a render?

  • nicsttnicstt Posts: 11,714
    fastbike1 said:

    Short, simplified flow: The scene loads into the  GPU memory (that can take a minute or so). The GPU starts processing. CPU has a limited role once you start rendering. However, CPU speed and RAM play the dominant role in EVERYTHING else you do on the computer. Loading every prop or figure, every move, everyshader, every light.

    This is not a game for the faint of PC/Mac.

    To add to the bolded part.

    Folks thinking they have a good computer are likely and often thinking it's a good gaming rig.

    Well a good gaming rig doesn't really cut it for rendering. It might be ok, but good? No.

    There is no such thing as too much computing power for rendering.

  • fixmypcmikefixmypcmike Posts: 19,565

    Usually the textures are what accounts for the most RAM usage, so one solution is to make scaled-down copies of textures that don't need as much detail -- taking a 4096x4096 jpg and making a copy at 2048x2048, and using that copy instead of the original, cuts the memory needed for that jpg by a factor of 4.

  • geoff6geoff6 Posts: 250

    Thanks for your comments, though I'm not really concerned in getting faster render times, I'm trying to get my head around how resources are used not how big or small those resources might be. For example, I'm suprised that the GPU is taken out of the loop entirely if the scene doesn't fit into it's VRAM. If that is the case, why is there an option to select the GPU only in the Iray renderer?

    Thanks in advance,

    G

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