Mac Pro (Late 2013) GPUs and Display Numbering

Discussion in 'Mac Pro' started by bax2003, Mar 25, 2015.

  1. bax2003, Mar 25, 2015
    Last edited: Mar 25, 2015

    bax2003 macrumors 6502a

    bax2003

    Joined:
    Dec 25, 2011
    Location:
    Belgrade, Serbia
    #1
    I have few questions about Late 2013 Mac Pro....and its GPU connections:

    - I have two Thunderbolt Displays connected to it, and in whichever Thunderbolt port I connect them (1 through 6), in System Information it says that both monitors are connected to AMD Fire Pro D300 (Slot-2). Is this normal ? There is never display connected to Fire Pro in Slot-1.

    [​IMG]

    - Is there any logic in OS X display numbering ? For example monitor connected to Thunderbolt Bus 1, Port 1 is Thunderbolt Display (2), and other one to Thunderbolt Bus 2, Port 2 is Thunderbolt Display (1)... which to me does not make no sense.
     
  2. mattspace macrumors 6502

    mattspace

    Joined:
    Jun 5, 2013
    Location:
    Australia
    #2
    Someone can correct me, but simplistically, while the 2013 has dual graphics cards, only one of them is ever used to drive displays under OS X. The other is (only?) for OpenCL compute. So if you're not doing that... I guess it works to rid the world of evil, evil electricity.

    Actually that's something I was wondering about - do you get double the number of possible screens when booting in Windows?
     
  3. bax2003 thread starter macrumors 6502a

    bax2003

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    Belgrade, Serbia
    #4
    Excactly, because of that diagram, there is no sense that monitor plugged in to Thunderbolt Bus 1, Port 1 is labeled Thunderbolt Display (2).

    I have not tried windows on it yet, but there is one more interesting this: TV connected to native HDMI 1.4 port displays picture but does not show under Displays -> Arrangement.
     
  4. teagls macrumors regular

    Joined:
    May 16, 2013
    #5
    You are correct. The nMP is setup so one GPU drives the displays and one GPU is an offline compute GPU. This offline GPU is not physically connected to the thunderbolt/display output controller. So it is impossible to drive a display with the offline GPU.

    The one exception is in Windows you can use crossfire at the driver level. Unfortunately, Mac OS does not have crossfire. You can however emulate it from software, but it will never be as fast.
     
  5. bax2003 thread starter macrumors 6502a

    bax2003

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    #6
    I doubt that first card does not have anything to do with video outputs....just for driving 6 Thunderbolt Displays (6 x 2560x1440) one Fire Pro D300 (Fire Pro W7000 with 2GB VRAM) can't do that.
    How do you 100% know that first GPU is not connected to Thunderbolt (DisplayPort) ? If you have reliable source, PDF or anything, please share it :)
     
  6. teagls macrumors regular

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    May 16, 2013
    #7
    My source is Apple's Developer information. They released at WWDC 2014 information for utilizing OpenGL/OpenCL on the nMP and how all the internals work. It specifically says in the information they provided the main GPU is the only one connected to the display output.

    You might need a developer acc to access the pdf.

    http://devstreaming.apple.com/video...f_the_mac_pro_with_opengl_and_opencl.pdf?dl=1
     
  7. bax2003 thread starter macrumors 6502a

    bax2003

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    Dec 25, 2011
    Location:
    Belgrade, Serbia
    #8
    Thanks for sharing. You are referring to:

    What Can You Do Slide:
    Use the primary GPU for display
    Add support for the secondary GPU
    • OpenCL
    • Off-screen rendering...

    I also understand that GPUs can be online or offline and you can find out with "Finding the Offline Renderer ID". This explains that GPU can be unused or offline, but when you connect 6 displays i dont believe that anything can idle at that time. Interesting part is that System Profiler say that card in Slot-2 has displays connected to it.
     
  8. teagls macrumors regular

    Joined:
    May 16, 2013
    #9
    Yes, Apple uses the offline gpu paradigm. You can use the offline GPU to render offscreen frames for an application, but those frames have to be copied back to the main gpu for displaying on the screen. This process is slow because you signal this from the software stack and it does not occur at the driver level like crossfire. I don't really know why Slot-2 has the displays connected to it, but it might just be the way the cards were detected. If you get Apple's OpenGL profiler tool you can very easily see what GPUs you have and their setup if they are offline or connected to displays.
     
  9. bax2003, Mar 26, 2015
    Last edited: Mar 27, 2015

    bax2003 thread starter macrumors 6502a

    bax2003

    Joined:
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    Location:
    Belgrade, Serbia
    #10

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