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Iray Scene Rendered Much Faster With CPU Only

LenioTGLenioTG Posts: 2,118
in Technical Help (nuts n bolts)

I've noticed that sometimes the same scene, with all Iray materials, is rendered much faster by the CPU alone...does this make sense?

For example, to reach 70 iterations it took my GPU (1060 3Gb) 40 minutes, and the PC was slowed down a lot.

Now I'm rendering it with my CPU (Ryzen 5 1600), and it's at 100 iterations in just 15 minutes: these are 3,8x more iterations per minute!!! And the PC is much more usable.

Do you know why? Am I doing something wrong? I'll attach the render made by my GPU.

103.png
2560 x 1440 - 7M

Comments

  • SimonJMSimonJM Posts: 5,945

    That does sound counter-intuitive. If your CPU is being used for the render process you woudl expect that to make the machine more sluggish as reders tend to make use of all available cores, thus leaving less CPU for other stuff.  That said, usinf the GPU (assuming sole GPU) will make the machine seem sluggish as screen updates will lag badly as most of the GPUs capabilty is being used up doing the render. Check the DS logs (Help> Troubleshooting> View log file ...) to check each render.  It is also possible that the second render goes a bit faster as soem 'stuff' is already cached and ready for Iray use.  Not knowing exact details I'd suspect that your render is starting off on GPU then 'falling back' to use the CPU - if so, that shoudl be in the log ... somewhere!

  • LenioTGLenioTG Posts: 2,118
    SimonJM said:

    That does sound counter-intuitive. If your CPU is being used for the render process you woudl expect that to make the machine more sluggish as reders tend to make use of all available cores, thus leaving less CPU for other stuff.  That said, usinf the GPU (assuming sole GPU) will make the machine seem sluggish as screen updates will lag badly as most of the GPUs capabilty is being used up doing the render. Check the DS logs (Help> Troubleshooting> View log file ...) to check each render.  It is also possible that the second render goes a bit faster as soem 'stuff' is already cached and ready for Iray use.  Not knowing exact details I'd suspect that your render is starting off on GPU then 'falling back' to use the CPU - if so, that shoudl be in the log ... somewhere!

    It starts with the CPU already, because the scene is bigger than 3Gb! The first time I reduced the texture by 4 to make it fit, then I restored everything, so CPU only!

  • SimonJMSimonJM Posts: 5,945

    If it starts with CPU only then I am confused by "it took my GPU (1060 3Gb) 40 minutes"

  • LenioTGLenioTG Posts: 2,118
    SimonJM said:

    If it starts with CPU only then I am confused by "it took my GPU (1060 3Gb) 40 minutes"

    It was the first render, the second one started already on CPU only mode! :)

  • kenshaw011267kenshaw011267 Posts: 3,805

    They both were on CPU. The first one spent time trying to setup the GPU render.  There may have been some residual overhead running on the CPU intended to support the GPU render that doesn't get properly cleared when the render drops to the CPU that never even gets loaded when you explicitly start a CPU only render.

  • SimonJMSimonJM Posts: 5,945
    kenshaw011267 said:

    They both were on CPU. The first one spent time trying to setup the GPU render.  There may have been some residual overhead running on the CPU intended to support the GPU render that doesn't get properly cleared when the render drops to the CPU that never even gets loaded when you explicitly start a CPU only render.

    Pretty much my thinking.

  • LenioTGLenioTG Posts: 2,118

    Ah, okay, thanks! :D

    It happened to me other times too, sometimes I just go with the CPU to have less problems!

    And yes, it slows down the PC, but it was even more slowed down during the first render :)

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