iMac VRAM Usage - Dual Monitors and Viewing

Discussion in 'iMac' started by VideoBeagle, Jun 23, 2015.

  1. VideoBeagle macrumors 6502a

    Joined:
    Aug 17, 2010
    Location:
    App Q&A testing by request.
    #1
    I have two questions regarding VRAM

    1) I use 2 monitors...the iMac screen and a monitor connected by HDMI thru Thunderbolt. Does driving the second monitor use up VRAM? How much of a hit is it?

    2) Is there a good utility to see VRAM usage, and if valid...clearing a VRAM Cache or something like that?


    (I'm interested as I'm currently experimenting/learning about the IRAY Render engine from NVDIDA to use with Daz Studio, which use GPU for speed...some reports say that it doesn't..empty the memory? maybe as well as it should between renders..I often can do about 5 or 6 render runs, and then I have a huge crash.)
     
  2. MartinAppleGuy macrumors 68020

    MartinAppleGuy

    Joined:
    Sep 27, 2013
    #2

    I'm pretty sure the vram usage is a few MB's worth per added monitor (can't remember the exact figure, but off the top of my head it was something like 12MB of VRAM per 1440p monitor).
     
  3. dagamer34 macrumors 65816

    dagamer34

    Joined:
    May 1, 2007
    Location:
    Houston, TX
    #3
    32bits per pixel * 1920 * 1080 / 4 bytes per bit / 1024 bytes per KB / 1024 KB per MB * 2 monitors = 31.64MB. On a device with 2GB minimum, 31.64MB is only 1.54% of total video memory.

    Translation: it's negligible.
     
  4. MartinAppleGuy macrumors 68020

    MartinAppleGuy

    Joined:
    Sep 27, 2013
    #4
    There is 8 bits to a byte. 4 bits equal a nibble ;)

    So by correcting your calculations, 2 1080p displays would use a total of 15.82MB of VRAM, and 0.77% of total VRAM. To the point it will be pretty much impossible to notice with a 2GB card or 4GB.
     
  5. dagamer34 macrumors 65816

    dagamer34

    Joined:
    May 1, 2007
    Location:
    Houston, TX
    #5
    Oops. Thanks for the catch!

    It's important to point out that it's 8MB per display for the final composited frame but of course before the GPU gets to that point it has to deal with all the assets. It's likely several tens if not hundreds of megabytes more depending on how complex of a scene is being rendered. Still not that much in the grand scheme of things (that's what VRAM is for after all) but a bit more than just the final buffer.
     

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