How does OS X handles multiple GPUs?

Discussion in 'Mac Pro' started by NickRichyRich, Jul 7, 2013.

  1. NickRichyRich macrumors member

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    #1
    Hello everyone,

    Been trying to find a answer for this but after some googling I have not been able to find anything in detail. I was wondering how OSX handles multiple GPUs, being since OSX does not support SLI or Crossfire? Does OSX some how pool the resources of both cards together? Or does it simply keep the two cards separate and they do not share resource with on another?

    Thanks!:confused:
     
  2. ScottishCaptain macrumors 6502a

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    #2
    It doesn't.

    Two cards show up as two cards. As far as the software is concerned, you have two separate GPUs. Nothing is pooled. It is up to the software to figure out how to utilize both units.

    -SC
     
  3. ElderBrE macrumors regular

    ElderBrE

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    #3
    As far as I'm aware, no, it doesn't.
     
  4. brand macrumors 601

    brand

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    #4
    No


    The video cards are currently independent of each other and do not share resources with each other. The new Mac Pro may not have the same hardware limitations that the Mac Pro 5,1 and older models have.
     
  5. NickRichyRich thread starter macrumors member

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    #5
    Thanks for the fast replies!

    My question is this then, wouldn't video professionals benefit more on the windows platform being since you can SLI and Crossfire?

    Thanks!
     
  6. Pakaku macrumors 68000

    Pakaku

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    #6
    I hear the stability of both of those are iffy, at best, and you'd be much better off buying one amazing single graphics card, for the price you'd spend on those two cards.
     
  7. ElderBrE macrumors regular

    ElderBrE

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    #7
    Those solutions are mostly for gaming, for video you either have a powerful graphics card or specific solutions for video. There are lots of workstations out there doing video with bad graphic cards and then render farms pull out the heavy work, or a heavy duty server or even third-party.
     
  8. NickRichyRich thread starter macrumors member

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    #8
    Then another question I have to ask is if the new Mac Pro is expandable through Thunderbolt and allows you to add more GPUs doesn't it make sense to let them pool together. The main thing I do not understand is when people say that the software picks how to handle the resources of the cards can it only pick one or can it utilize both of them?
     
  9. ChrisA macrumors G4

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    #9
    Yes, and I think the software is called "OpenCL".

    Any software that uses OpenCL will take advantage of whatever hardware OpenCL detects in the machine. Apple will need to update OpenCL to support new video cards but you old software (if it uses OpenCL) will then automatically use the new cards.

    If you are talking about using the two cards via the video drivers then, you can use then if you have two monitors.

    But for most things people want to run on a Mac Pro the best use of the GPU is for computation so you hope the developers read up on how to use OpenCL and then did in fact use it.
     
  10. NickRichyRich thread starter macrumors member

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    #10
    Awesome, I looked into OpenCL and that help make sense of everything! Thank you very much =D
     
  11. rabidz7 macrumors 65816

    rabidz7

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    #11
    If someone stuck an AMD 7990 in a Mac Pro, would it be not any faster than a single 7970 in games such as CIV V, or am I mistaken.
     
  12. brand macrumors 601

    brand

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    #12
    Yes a 7990 would be faster than a 7970.
     
  13. Pakaku macrumors 68000

    Pakaku

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    #13
    Total guess: The system chooses the more-powerful card and uses its resources before the less-powerful one.

    At least I'd hope so. I got a cheap GT 120 to add a third monitor, and I hope it's not being prioritized over my GTX 560 for some reason :B
     
  14. brand macrumors 601

    brand

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    #14
    I would think that the performance is specific to the monitor attached to that card.
     
  15. Umbongo macrumors 601

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    #15
    A 7990 would be faster as each GPU is faster than those in the 7970, it would only be able to use one for games though.
     
  16. rabidz7 macrumors 65816

    rabidz7

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    #16
    The GPUs are exactly the same.
     
  17. Tutor, Jul 7, 2013
    Last edited: Jul 8, 2013

    Tutor macrumors 65816

    Tutor

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    #17
    Yes, OSX handles >1 GPU for GPGPU computing; but the road is still somewhat bumpy.

    [ http://en.wikipedia.org/wiki/OpenCL ] (open standard) for ATI and Nvidia video cards and "CUDA" [ http://en.wikipedia.org/wiki/CUDA ] (proprietary) for Nvidia video cards only. Both are technologies that harness the power of video cards to perform some GPGPU [ http://en.wikipedia.org/wiki/General-purpose_computing_on_graphics_processing_units ] computation chores. Currently, ATI cards generally excel at OpenCL tasks over comparably priced Nvidia cards. But CUDA has the edge on the number of currently available applications that take advantage of it [see, e.g., http://www.nvidia.com/docs/IO/123576/nv-applications-catalog-lowres.pdf ] and CUDA drivers have been around somewhat longer in a fully developed, stable stage [ http://www.anandtech.com/show/3972/nvidia-gtc-2010-wrapup/5 ; also compare http://en.wikipedia.org/wiki/OpenCL ] with http://en.wikipedia.org/wiki/CUDA. Mac OSX, Windows and Linux OS can take advantage of multiple video compute cards, but your mileage will vary depending upon the specific application, video card(s), driver(s), OS and platforms, with Windows systems and OS leading the way.
     
  18. derbothaus macrumors 601

    derbothaus

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    #18
    It would be faster only in Windows as that OS can use the crossfire that bridges the 2 gpu's. Yes, they use xfire even when 2 gpu's are on the same pcb. In os x it would be slower than a 7970 Ghz edition but faster than stock 7970. It would benefit in opencl though.
     
  19. Umbongo macrumors 601

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    #19
  20. goMac macrumors 603

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    #20
    It does. The new Mac Pro does not pool the GPU resources together.

    Note: None of this applies to OpenCL, which allows software to pool GPU resources together if it decides to. This is part of the spec, and is fully supported on the Mac.
     
  21. rabidz7 macrumors 65816

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    #21
  22. ScottishCaptain macrumors 6502a

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    Unfortunately, that's not true.

    OpenCL is just an API, it doesn't take advantage of anything. That is up to your software. Two separate GPUs will show up as two separate OpenCL devices. Your software must be written to take advantage of that- it needs to be able to spawn whatever your OpenCL kernel is on more then one device, and handle the communication to and from each of your kernel instances.

    Sadly, not a lot of software is written to take advantage of more then one OpenCL device. Having two GPUs doesn't magically make all your OpenCL stuff run twice as fast. The software needs to handle that itself, otherwise it'll be just like running on one GPU.

    For all intents and purposes, the new Mac Pro is the same thing as the current generation with two GPUs installed. It's safe to say Apple will likely extend the Display preferences panel to include selecting which GPU powers which thunderbolt device display wise, but that's it. Everything else will function the same from a software standpoint.

    -SC
     
  23. jasonvp macrumors 6502a

    jasonvp

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    #23
    Except that Apple is updating OS X to Mavericks prior to the Mac Pro's release. And they're promising a new version of Final Cut Pro X that'll coincide with the new Mac Pro. Wanna bet that the new version of that software will be multi-GPU aware?

    Then there's Adobe's new version of Premiere Pro: already multi-GPU ready.
     
  24. goMac macrumors 603

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    #24
    My understanding is that Apple will use one GPU for OpenCL and one for OpenGL.

    So you will see an overall better experience, but they probably won't put much effort behind using two GPUs for OpenGL at the same time. It's not the point of dual GPUs. OpenCL and OpenGL fight for resources, so if they're running at the same time, best to put them on GPUs.

    So I guess in a way that's multi GPU? But also not.
     
  25. jasonvp macrumors 6502a

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    #25
    Was this published anywhere or did someone pull it out of their backside and present it as "fact?" There's no reason to believe Apple will do any such thing. Software coded against OpenCL can use multiple GPUs if the software engineer does the right thing.

    Why wouldn't Apple do that?
     

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