GPU upgrade: understanding PCIe vs. Thunderbolt

Discussion in 'iMac' started by jcurri, Oct 6, 2013.

  1. jcurri macrumors newbie

    Oct 6, 2013
    I have a ton of questions regarding upgrade possibilities with my current GPU. I've done a little bit of research on the thunderbolt to PCIe bridges like the ones offered by Sonnet:
    I'm not quite clear, however, on the bandwidth offered by PCIe and thunderbolt.

    Question 1: Does anyone know what bridges the current video card to the motherboard of a late 2012 iMac, and what the bandwidth of that bridge is?

    Question 2: If I wanted to by the above PCI:thunderbolt bridge, would a higher end video card like say, the NVidia GTX 780, max out the bandwidth offered by thunderbolt if I was playing a game like battlefield 4?

    Question 3: Is there anywhere online where I could buy a 680MX with 2 GB of dedicated ram (or a similar card that is compatible with the late 2012 iMac) and simply install it myself?

    Alternatively, what do you think my chances are of going to an apple store and offering to pay a decent amount of $$ for an upgrade?

    Thanks all...
  2. jcurri thread starter macrumors newbie

    Oct 6, 2013
  3. Krevnik macrumors 68040


    Sep 8, 2003
    It is PCIe, but I'm not sure how many lanes are wired up, and how much bandwidth of that provided it actually uses. Although graphics cards usually get 8 or 16 lanes. PCIe 2 offers ~4Gbps per lane. Thunderbolt offers 10Gbps per channel and offers two channels.

    5 lane PCIe is as fast as dual-channel thunderbolt. And graphics cards use more lanes than that. (This does ignore 3.0 which makes PCIe double the speed of 2.x, but is just starting to appear)

    Going by the numbers above, very likely. I am not sure a GPU over thunderbolt will get to use both channels, but even then it is basically bandwidth starved compared to how it was designed to operate. And running out of VRAM becomes expensive due to added latency to swap textures from RAM over thunderbolt.

    Nope, the GPU is soldered on the logic board with the 2012. You'd need a whole new logic board salvaged from the model that has the GPU you want.

    The odds are pretty terrible. Apple doesn't offer such a service.
  4. leman macrumors G3

    Oct 14, 2008
    It is quite clear actually. TB is a wrapper around 4x PCI-E 2.0. This makes TB equivalent of a 2x PCI-E 3.0 link. Current high-end GPUs are usually connected via 16x PCI-E 3.0.

    Well, its soldered on the mainboard. The interface is either 8x or 16x PCI-E 3.0 (most likely 8x though) My iMac is at home so I can't look right now, but its possible that the interface can be seen in the system profiler.

    You can see the impact of available bandwidth on gaming performance here:

    Basically, the difference between 8x and 16x is virtually non-existent while 2x does exhibit performance degradation. Modern games tend to be shader/fill limited though - in other words, actual rendering takes lots of time, so the speed at which data is supplied to the GPU plays a less significant role.

    TL;DR: while the 2x PCI will certainly hinder the GTX 780 from reaching its full potential you will still be able to pretty much max everything and get very decent framerate.

    No. Its soldered.

  5. X-Ravin macrumors regular

    Nov 30, 2008
    Sorry to hijack a little, but would the external GPUs be able to use the iMac's screen? I know someone figured out a trick to get a Mac laptop to use an external GPU for the internal screen, but I don't know if it would work for iMacs.
  6. Mac32 Suspended

    Nov 20, 2010
    External high end GPU for gaming = poor performance

    End off story. I saw benchmarks for thunderbolt 1 with a GTX680, and it lost up to about 40% performance in real life. Obviously the performance loss would be even greater with GTX780 or similar cards. We have yet to see benchmarks for thunderbolt 2, but I'm very sceptical.. I'm seeing some people promising great results with external thunderbolt GPUs, but they are failing to deliver the proof.

    Sorry, just trying to keep it real.
  7. yjchua95 macrumors 604

    Apr 23, 2011
    GVA, KUL, MEL (current), ZQN
    Proof's here mate:

    Note that it's just Boot Camp. Good enough for most purposes. But you do have a point. That PCIe-Thunderbolt adapter will limit the link just to 5 Gbps. So no point getting a GTX Titan. Just stick with the GTX 5xx-6xx series if you intend to wire up an eGPU via Thunderbolt.
  8. Mac32 Suspended

    Nov 20, 2010
    ?? I don't see any benchmarks. I know it can be done practically, but the performance just isn't good enough to warrent the expense when compared to other solutions. You would not be able to improve on the performance of ...say a 680MX, at least not with thunderbolt 1.
    Believe me I had great hopes for this myself, but then I saw the benchmarks..not so good.
  9. yjchua95 macrumors 604

    Apr 23, 2011
    GVA, KUL, MEL (current), ZQN
    My bad. But at least the video's enough of a testament that a PCIe-Thunderbolt based eGPU is the next best thing to having a discrete one. It's fast enough to play games at high settings.
  10. jblagden macrumors 65816


    Aug 16, 2013
    How does it limit the link to 5 gigabits per second? It’s a 4 lane PCIe slot, which has 20 gigabits of total throughput. Or are you talking about the per-lane bandwidth?
  11. jblagden macrumors 65816


    Aug 16, 2013
  12. desmond2046 macrumors regular


    Jun 2, 2015
    Columbus, OH, United States
    Don't buy an external graphic card (~$400) if you just want to improve the gaming performance. It is a complete waste of money. You can use that money to
    1. Buy a PS4.
    2. Build a PC with maybe Athlon 860K ($70), 2nd hand Radeon 7970 ($130), MB RAM SSD Case etc. ($250)
  13. Samuelsan2001 macrumors 604

    Oct 24, 2013
    And you'd need a PCIe enclosure for that graphics card which is another $400, at which point you could just buy a good gaming rig.
  14. jblagden macrumors 65816


    Aug 16, 2013
    I’ve considered those options the past, but I don’t really care for consoles(I prefer I mouse and keyboard), I don’t want to run Windows, I don’t want a Hackintosh and I don’t want another computer(i just want to stick with one computer). For me, an eGPU is the best option.
  15. old-wiz macrumors G3

    Mar 26, 2008
    West Suburban Boston Ma
    The linked part is for a Mac Mini. The external GPUs have no access to the laptop screen.
    It is really easier to just buy a PS/4 or Xbox and a monitor if you are really into gaming.

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