Nvidia SLI attempt with 2013 15" MacBook Pro + (2X) Sonnet III-D + (2X) GTX 780 Ti

Discussion in 'MacBook Pro' started by squinks, May 15, 2014.

  1. squinks macrumors newbie

    Oct 2, 2013
  2. Quu macrumors 68030


    Apr 2, 2007
    Just so you know, NVIDIA will not allow SLI to be activated unless the motherboard supports SLI.

    The drivers have a check in them to read a flag which is embedded into the motherboards BIOS or UEFI firmware (depending on the board), only once that is read can SLI be activated from the driver control panel.

    NVIDIA has charged motherboard manufacturers to include this flag within their BIOS / UEFI Firmware since about 2009 when Intel locked NVIDIA out of the chipset game by changing from FSB to QPI buses on their CPU processors thus invalidating a prior chipset license agreement Intel held with NVIDIA for the FSB.

    So had you been able to get both cards to boot on your Retina MacBook Pro you wouldn't of been able to activate SLI mode from the NVIDIA Control Panel and they would have instead operated independently with one card doing all of the 3D Graphical stuff while the other card sat idle.

    The only exception to this SLI situation are dual-GPU cards like the GTX 590, GTX 690 and the forthcoming Titan Z - These cards use an SLI implementation for their two chips on one board and do not need a motherboard SLI flag to activate their SLI mode unless you use two of the cards (Quad-SLI essentially) then the flag does need to be present again.

    But thank you for posting your findings it's very interesting.
  3. squinks thread starter macrumors newbie

    Oct 2, 2013
    Thank You

    This is the kind of information that myself and others need to know. The insight on hardware and/or software limitations is very helpful. Of course, card detection is indeed the first obstacle, however, knowing the drivers for SLI will lock you out without a flag is very important to be aware of.

    Now the question arises, is there any possible way that the BIOS flag could be simulated or bypassed in Windows.
  4. Quu macrumors 68030


    Apr 2, 2007
    Lots of people have tried to do that so that they can get SLI mode to work on unsupported motherboards.

    A long time ago a few people did succeed but these are with very old drivers, before the GTX 500 launched. Since that time NVIDIA has lowered the license cost to a couple of bucks per board which has resulted in SLI support being added to almost every single PC motherboard released which has two full length PCIe slots.

    This has meant such efforts to emulate the flag have been abandoned. You can even get SuperMicro and Tygon server boards with SLI support these days.

    I can say however that AMD cards do not have any such licensing or restrictions on Crossfire mode and simply having both cards detected in a system is enough to enable it.

    I would suspect however that you'll face a similar detection problem at boot up when used with the Retina MacBook Pro.

    I wonder if the issue is caused by the Retina MacBook Pro only having a single Thunderbolt 2 chip? Perhaps the Mac Pro which has three separate chips to drive its six Thunderbolt 2 ports would be able to address up to three separate cards? (iFixit confirms three separate Thunderbolt 2 chips on the Mac Pro Motherboard, each providing physical two ports for a total of six).
  5. squinks thread starter macrumors newbie

    Oct 2, 2013
    I bet you're right right. With one controller per eGPU, they would likely both detect properly. However, as you've pointed out, the SLI support seems unattainable regardless. With 2 eGPUs on a Mac Pro running AMD cards in crossfire, there you would have a promising external dual-GPU configuration.

    But wow, the cost!!
  6. VanneDC macrumors 6502a


    Jun 5, 2010
    Dubai, UAE
  7. 827538 macrumors 65816

    Jul 3, 2013
    Really nice, SLI wouldn't be that great even if it could work. You would easily hit the limits of the CPU and memory bandwidth.
    I would be interested in doing something like this myself if only the enclosure wasn't so expensive.

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