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Full use GPU instead of CPU

Kyan001Kyan001 Posts: 75
in Technical Help (nuts n bolts)

Hello folks. 

I got an i5 7000X series, and two RTX3090. 

The real problem is that CPU makes a great bottleneck (which doesn't happen switching it with an i9). 

Now my question is, if I've selected in the render tab to use ONLY the two GPUs. Why on the earth CPU use goes to 90\100%?

Plus, I'm using only iRay assets. 

Help in advanced\unknown config settings is appreciated. 

Thanks in advance.

Comments

  • Richard HaseltineRichard Haseltine Posts: 100,728

    If the CPU is being used at 100% then it sounds as if the GPUs hacve dropped out or not been used at all (check your driver version). Each GPU will need one thread of the CPU for management, so if that's a four core CPU with no hyperthreading it might be used up to about 50%

  • richardandtracyrichardandtracy Posts: 5,660
    edited March 2021

    There is another possibility that could be the cause of the high CPU load factor. I have an old Xeon CPU and a GTX 1060, and 25% of CPU is used feeding data to the GPU during rendering. I can well imagine that going much higher with a higher spec GPU. Then putting in two GPUs that are about as good as you can get.. Well, I'd be surprised if that didn't take a huge amount of data from the CPU and thus CPU work to keep up with the flood of rendering coming from the GPUs. The way I understand it is (and I may be wrong) is that while the textures and geometry are fed into the GPU at the start, the CPU still tells each cuda core which pixel to render, which facet is visible etc so while the GPU takes most of the load, the CPU keeps global control of what goes on, and the more that's going on, the higher the CPU load.

    To test my speculation, you could try switching one of the GPUs off, and check CPU load. I suspect it would be lower. If it is, the CPU power is close to being a bottleneck

    Post edited by richardandtracy on
  • PerttiAPerttiA Posts: 10,024

    What does you log say?

  • Kyan001Kyan001 Posts: 75

    richardandtracy said:

    There is another possibility that could be the cause of the high CPU load factor. I have an old Xeon CPU and a GTX 1060, and 25% of CPU is used feeding data to the GPU during rendering. I can well imagine that going much higher with a higher spec GPU. Then putting in two GPUs that are about as good as you can get.. Well, I'd be surprised if that didn't take a huge amount of data from the CPU and thus CPU work to keep up with the flood of rendering coming from the GPUs. The way I understand it is (and I may be wrong) is that while the textures and geometry are fed into the GPU at the start, the CPU still tells each cuda core which pixel to render, which facet is visible etc so while the GPU takes most of the load, the CPU keeps global control of what goes on, and the more that's going on, the higher the CPU load.

    To test my speculation, you could try switching one of the GPUs off, and check CPU load. I suspect it would be lower. If it is, the CPU power is close to being a bottleneck

    This was the problem. :) 

  • richardandtracyrichardandtracy Posts: 5,660

    In which case.. A higher spec CPU is needed. Possibly. Unless you can live with CPU at nearly 100% for the few seconds a render would take with those mighty GPU's. You are obviously very close to the point where the CPU is the limiting factor in your setup.

    Regards,

    Richard.

     

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