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External GPU - Driving iMac Screen?

E

eonaeon

macrumors member
Original poster
Feb 15, 2008
79
22
Down Under
Hi all, I pondering two extremes of my iMac purchasing - either the base model with 512GB SSD upgrade, or the top end model with the i7 upgrade and the 512GB SSD...

My question is - with High Sierra, and an External GPU box, is it possible to drive the iMac screen using the external GPU? I'm wondering whether I should get the Radeon Pro 580 over the 570 to be a bit more future proof, to just upgrade to an eGPU some time in the future?
 
eonaeon said:
Hi all, I pondering two extremes of my iMac purchasing - either the base model with 512GB SSD upgrade, or the top end model with the i7 upgrade and the 512GB SSD...

My question is - with High Sierra, and an External GPU box, is it possible to drive the iMac screen using the external GPU? I'm wondering whether I should get the Radeon Pro 580 over the 570 to be a bit more future proof, to just upgrade to an eGPU some time in the future?
Click to expand...

I don't think anyone knows yet. I went through the same thought process and ended up getting the base with 512GB SSD. I don't want to have too much heat generated in the iMac case, so I don't have to listen to the fans running. If the eGPU enclosures can drive the internal display, I will look at that in the future to keep the iMac current longer. I'm upgrading a 2009 mini, so it will be a while before I have to worry about it feeling slow.
 
I am going to guess that it cannot be done since the iMac does not support Target Display Mode so there is likely no way to feed the signal back to the iMac's display. And even if you could, you're going to see a 10-20% drop in performance based on benchmarks of feeding the signal back to the MacBook Pro's display versus driving a separate display right from the eGPU card.
 
CWallace said:
I am going to guess that it cannot be done since the iMac does not support Target Display Mode so there is likely no way to feed the signal back to the iMac's display. And even if you could, you're going to see a 10-20% drop in performance based on benchmarks of feeding the signal back to the MacBook Pro's display versus driving a separate display right from the eGPU card.
Click to expand...


I've seen videos of thunderbolt 2 and 3 MacBook pros using eGPU to accelerate the internal screen and MacBook pros have obviously never had target display mode so I'm not sure your first point is valid. If in a few years you added an eGPU and it was say twice as powerful as the one that came with the computer originally I'm sure many would accept a 20% performance loss as part of the caveats of using eGPUs if the cost vs benefit still made sense.

I personally would buy the computer with the graphics card you want today and if eGPUs work down the line this is a bonus. Depending on them to add to the longevity of an iMac bought today would be risky but less risky than a few weeks ago before Apple announced they were supporting them going forward.
 
Well MacBook Pros feed the signal back via a hack which saps some 50% of the performance (in addition to the hit you take over the TB2 (20%) or TB3 (10%) cable). So I guess you would have to hack the iMac's display, as well, but the iMacs use custom timing controller chips to handle the display so that could be a block or perhaps another area where the performance takes a hit. So in the end, you could lose most of the performance boosts of using an eGPU by sending it back to the iMac's display.
 
I'm having a similar debate as the original poster. It would seem that it may be better to get the 580 now than gamble on a future working solution. Are we certain the additional heat is the cause of the louder fans? I'm still reading mixed reports on this forum.
 
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