Why does Nvidia's "maximum pre-rendered frames" option work only in fullscreen?

Discussion in '3D Hardware, Software & Output Devices' started by Frontino, Apr 4, 2011.

  1. Frontino

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    With my 9800 GX2, if I set the option to "1", fullscreen rendering caps both GPUs at 50% while windowed mode lets them load up to 70% whatever is the number of frames set.
    100% load is achieved only in fullscreen with "3" pre-rendered frames for SLI while "1" is enough to fully load a single GPU.
    Why is that?
     
  2. Davros

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    Is it capping or is it a case of there is less workload if only 1 frame at a time is rendered ?
     
  3. ERP

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    If you set it to , basically it forces a GPU/CPU sync at the end of every frame.
    So you end with less GPU usage and lower frame rate in GPU limited games.
    If it's set to more than one, then the CPU is allowed to start rendering subsequent frames without waiting for the current one to be presented, so the GPU doesn't stall waiting for data, and you get more GPU load and better performance at the cost of some potentially introduced latency.

    As to why/if it works in windowed mode, I have no idea. Given it's at the driver level, I'd assume that it does function in all modes, however it's possible that windows introduces additional buffering that has a similar effect in windows mode.
     
  4. Frontino

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    I'll make more examples to clarify.
    I will use the OCCT GPU Test as best scenario, but it happened with Crysis 2 also, although OCCT puts a constant load on GPUs, while a game like Crysis 2 has too much variables.

    Here are some results I took only with shader complexity 1 at 1024x768, because 1280x1024 is the maximum resolution available in windowed mode, but my monitor is only 900 pixels high.
    Anyway 1024x768 can already fully load my 9800 GX2:

    Single G92 - fullscreen - pre-rendered frames "0" = 95%
    Single G92 - fullscreen - pre-rendered frames "1" = 99%
    Single G92 - windowed - pre-rendered frames "0-8" = 97-98%
    Dual G92 - fullscreen - pre-rendered frames "0" = inevitable system crash
    Dual G92 - fullscreen - pre-rendered frames "1" = 44-49% per GPU
    Dual G92 - fullscreen - pre-rendered frames "2" = 96% per GPU
    Dual G92 - fullscreen - pre-rendered frames "3" = 99% per GPU
    Dual G92 - windowed - pre-renderd frames "0 - 8" = 70-90% per GPU
     
    #4 Frontino, Apr 4, 2011
    Last edited by a moderator: Apr 5, 2011
  5. Grall

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    Wut, it actually crashes your PC (completely?) if you set it to "0"? Wow. How's that for sanity checking input parameters, huh! :lol:
     
  6. Frontino

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    Well, actually it crashes only in fullscreen, as the pre-rendered frames option works only in fullscreen indeed.
    Apparently Nvidia has never spotted this problem.
    Launching a 3D application in fullscreen with "0" pre-rendered frames on systems with SLI enabled causes 100% system crash, whatever the API used is.

    I especially noticed that in fullscreen, after some seconds, the load on the GPUs becomes constant and FPS stabilize. In window the load is always inconsistent with variables up to 20% of difference.

    Also, it looks like, for fullscreen rendering, to put maximum load on the GPUs, the pre-rendered frames must be set the following way:

    1 GPU - 1 Pre-rendered Frame
    2 GPUs - 3 PRFs

    and so I'd expect...

    3 GPUs to need 5 PRFs
    4 GPUs - 7 PRFs

    Oh, I've forgot to mention that this happens only in Windows XP.
    With Windows 7, the pre-rendered frames option has no effect in anyway.
     
    #6 Frontino, Apr 5, 2011
    Last edited by a moderator: Apr 5, 2011
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