Thunderbolt SLI possibilities?

Discussion in 'iMac' started by russofris, Jun 18, 2012.

  1. russofris macrumors regular

    Mar 20, 2012
    Hi there,

    Just a bit of out-loud speculation. The Nvidia 680m and ATI 7970m both appear to be SLI/Crossfire capable chips. With prior generations of iMacs (with the exception of the 2011 model), the key factor that prevented the user from outward-scaling their GPU was the lack of an exposed PCI-Express slot.

    Thunderbolt 'might' change this as it multiplexes PCI Express and display port on a single bus. One could reason that you could build an external enclosure containing a TBolt to PCI Express transformer and place a GPU in it for outward scaling.

    While the 10G/sec of TBolt's bandwidth roughly equates to that of a PCI-Express ver2 8-lane slot, the one thing that will have to be overcome is the lack of the Jumper/Ribbon cable that attaches one card to another.

    I'm curoius to see what others think. Do you think the development of a cheap external TBolt SLI box would be something that would fill the need of a large enough market to recuperate its R&D/production costs? Keep in mind that this would likely work for any Tbolt enabled system (including desktop PCs).

  2. orangezorki macrumors 6502a

    Aug 30, 2006
    So let me get this right - you're thinking of essentially having the internal GPU rendering half of the image, being sent to the monitor via the display port part of the signal, then the data required to render the other half being sent as data to an external GPU?

    It's really a nifty idea, but very complex and expensive to do, even if the Thunderbolt specification could allow it. Firstly, you usually need an identical GPU to do SLI, and as all TB equipped Macs use mobile GPUs, they are not usually available as expansion cards. If an integrated version of the box was made, a separate model would need to be produced for every different GPU used in Apple's lineup - not Apple at all. Then you have the problem when the internal GPU outperforms the bandwidth of TB - if it hasn't done so yet.

  3. russofris thread starter macrumors regular

    Mar 20, 2012
    This had crossed my mind. You would basically build a box that turned a single TBold connection into an 8 lane PCIE slot and included a power supply (much like the one I linked). From there, you could insert the card of your choosing. I am certain that one could have cards based on mobile GPUs manufactured or sourced from a VAR for a small premium.

    Two thunderbolt ports would enable 3way SLI, etc.

    The idea seems sound, with the exception of the SLI jumper cable. The cost of sourcing mobile cards may be the expensive part. In addition, the existing TBolt to PCIE boxes seem to be over $500 at the moment.

    I'll do some digging and back-of-envelope calculations and see what I can come up with.

  4. forty2j macrumors 68030


    Jul 11, 2008
    Assume the 680M for a moment. That's an underclocked 670, right? I wonder if you could get a 670 to put in an enclosure (and underclock it yourself..)
  5. MagicBoy macrumors 68040


    May 28, 2006
    Manchester, UK
    Thunderbolt is the equivalent of a 4x PCIe lane. Probably not the best idea for SLI.
  6. russofris, Jun 18, 2012
    Last edited: Jun 18, 2012

    russofris thread starter macrumors regular

    Mar 20, 2012
    @4x, you're probably correct in that performance would be poor. The wikipedia entry shows 4X PCIE (under revision 2) as being 2GB/sec as opposed to TBolts 10GB/sec. They also seem to indicate that it should work in non-SLI configurations
    Theoretically, external PCIe could give a notebook the graphics power of a desktop, by connecting a notebook with any PCIe desktop video card (enclosed in its own external housing, with strong power supply and cooling); This is possible with an ExpressCard interface, which provides single lane v1.1 performance.
    A user would (externally) leverage desktop graphics card (a GTX 690 for example) which feed the video back into the iMac screen via the DP over Tbolt. Gamers would be able to purchase a lower GPU spec'd iMac for less money if they know that they would be able to use a power desktop style GPU. Physics or other workloads could still be handled by the integrated GPU (i Believe that nVidia already has physics offloading to non-sliable cards.

    If you believe I'm confused, have misunderstood something, or am simply arithmetically challenged, please call out and set me straight.

    Ivy bridge has PCIE 3.0, doubling the PCIE 2.0 bandwidth. Hmmm.

    The diagram below supports your 4-lane PCIE statement, but includes "Other Configurations Available". I'm not sure if that will help me though.
  7. JaHull macrumors member

    Apr 18, 2009
    Check this page for the bandwidths of PCIE2 and Thunderbolt.

    If you look PCIE2.0 (4 Lanes) is 16Gbit/s (2GB/s) and Thunderbolt is 10Gbit/s (1.25GB/s) though Thunderbolt does have 2 channels which could be used to double its speeds to 20Gbit/s (2.50GB/s)
    Using both lanes this would make it equivalent to PCIE2.0 (5 Lanes)

    As for running a Nvidia 690 GTX or AMD 7990 they would want the speed of PCIE2.0 (16 Lanes) or at minimum (8 Lanes) which are a fair bit faster than Thunderbolt.

    I wouldn't get to exited about high end GPUs over Thunderbolt until a major speed boost comes up or they switch to PCIE3.0
  8. bogatyr macrumors 65816

    Mar 13, 2012
  9. russofris thread starter macrumors regular

    Mar 20, 2012
    Great link. They even include a chart with TBolt, and it looks like it will indeed be feasible to deliver the necessary bandwidth with Ivy Bridge / PCIE 3.0.

    The big question for me ATM is whether Ivy Bridge currently uses PCIE 3.0 over TBolt, or if it is still using PCIE 2.0.

    This dated article indicates that they are "planning on it" but does not confirm it.

  10. theSeb macrumors 604


    Aug 10, 2010
    Poole, England
    You've mixed up giga bits and giga bytes.

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