Why so excited about eGPU when performance drop on TB3 may be over 30%

Darajavahus

macrumors member
Original poster
Aug 8, 2015
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Over the week on this forum I noticed couple times people saying that for some there's no need to get the iMac with best GPU, that better get the lowest and in the future, if needed, get external Thunderbolt 3 GPU.

I suspected there may be performance drop but first google result with bunch of bencharks shows it's worse than I could think, and the more powerful the card the bigger drop, 30% right know, but probably even worse with lot more powerful cards in the future. https://egpu.io/forums/mac-setup/pcie-slot-dgpu-vs-thunderbolt-3-egpu-internal-display-test/

I read posts people suggesting getting iMac with 570 (3.6TFLOPS) and if you later needed power of 580(5.5)(and cards with that power will be cheaper in the future) to get eGPU, but with the 30% drop, the 580 kind of card would be almost the same to the 570, 3.8TFLOPS.

So eGPU may be exciting and recommended for MacBooks but not for iMacs.
Thunderbolt 4 will be better so will be TB5 but also GPUs like Vega and Volta and succesors.
So not soon will Thunderbolt(whatever number in the future) be equally good to PCI Express(whatever number in the future).

So I would not count on TB3 eGPU for 27" iMac,
definitely yes fro MacBooks and 21" iMacs but not 27" iMacs right now
or I have this all wrong? I will admit I made this conclusion from just first google result ;)
 
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dlewis23

macrumors 6502a
Oct 23, 2007
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From things I have read before the drop in performance of 30% is really when using the internal display. When you a external display the drop is not at much. Coming back to the internal display uses a lot of bandwidth of thunderbolt, skipping that and going out from the eGPU to a external display and the drop in performance isn't as much.

Some more info here: https://egpu.io/performance-internal-vs-external-display/

From that post they say...

Using an eGPU setup with an external display in macOS yields at least 30% higher performance than feeding that signal back into the internal display of your Mac.
 

Darajavahus

macrumors member
Original poster
Aug 8, 2015
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From things I have read before the drop in performance of 30% is really when using the internal display. When you a external display the drop is not at much. Coming back to the internal display uses a lot of bandwidth of thunderbolt, skipping that and going out from the eGPU to a external display and the drop in performance isn't as much.
Yes, the same thing is in my first link, and that is what we are talking about, who's gonna buy another display for 27" iMac for a little bit of more GPU power (obviously besides few that like multiple displays), that's why I said it's good for MacBooks and better if with external display and I'm thinking not recommendable just for 27" iMacs right now, especially if you don't want additional display
 

dlewis23

macrumors 6502a
Oct 23, 2007
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Yes, the same thing is in my first link, and that is what we are talking about, who's gonna buy another display for 27" iMac for a little bit of more GPU power (obviously besides few that like multiple displays), that's why I said it's good for MacBooks and better if with external display and I'm thinking not recommendable just for 27" iMacs right now, especially if you don't want additional display
I think you have to buy a second display. I think I read somewhere that the eGPU apple has won't work back to the internal display. But I would have to look it up again.
 

richinaus

macrumors 65816
Oct 26, 2014
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Yes, the same thing is in my first link, and that is what we are talking about, who's gonna buy another display for 27" iMac for a little bit of more GPU power (obviously besides few that like multiple displays), that's why I said it's good for MacBooks and better if with external display and I'm thinking not recommendable just for 27" iMacs right now, especially if you don't want additional display
I agree - makes more sense for a laptop than a desktop.
However I will just get an external monitor if I need more Gpu power and go for an eGPU. Not a big deal really and I think it will be substantially more than 'a little bit of more GPU power' as you state. There are a lot more powerful GPU's out there than the ones in the new iMac.
 

Puonti

macrumors 6502a
Mar 14, 2011
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...who's gonna buy another display for 27" iMac for a little bit of more GPU power (obviously besides few that like multiple displays), that's why I said it's good for MacBooks and better if with external display and I'm thinking not recommendable just for 27" iMacs right now, especially if you don't want additional display
Projectors all the way. Doesn't matter what's at the other end of that HDMI cable. And even if you prefer a TV, if right now it's an iMac that will support eGPUs in the future, then adding that box in-between the two is a no-brainer.
 

mcnallym

macrumors 6502a
Oct 28, 2008
714
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Personally looking more at the egpu in terms of compute. My main thing is fcp x so wouldn't be using to drive the display but more for in the same way that the nMP uses the second GPU's. drop off is much less in that way. The GPU in the iMac then is left purely for driving the screen.
 

AdamSeen

macrumors regular
Jun 5, 2013
162
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The sensible option would be to have the option of using dual thunderbolt ports. One for data, the other for display. But for the smaller laptops it should mostly be external displays