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CosmicScale

macrumors newbie
Original poster
Dec 10, 2017
17
1
London, UK
I’ve got the stock Nvidia GT 120 and an AMD RX 560 instated in my Mac Pro 4,1 (5,1 firmware) running High Sierra. I’ve got one screen plugged into the GT 120 and another plugged into the RX 560. Interestingly when I go to “About This Mac” the “Displays” tab shows both screen as connected to the “AMD R9 xxx”.

I know with a MacBook Pro you can switch between your integrated and discrete GPU. I was wondering if it is possible to swap which GPU is active on a Mac Pro with two graphics cards?
 

Fl0r!an

macrumors 6502a
Aug 14, 2007
909
530
OS X will always use the GPU, which your display is connected to. MBPs with two GPUs use a muxer to physically change the routing of the outputs, Mac Pro's can't do that.
 

CosmicScale

macrumors newbie
Original poster
Dec 10, 2017
17
1
London, UK
OS X will always use the GPU, which your display is connected to. MBPs with two GPUs use a muxer to physically change the routing of the outputs, Mac Pro's can't do that.

But that can't be 100% true. I can run Cinebench on the screen connected to the GT 120 and I get a high frame per seccond. If I just run it on my machine without the RX 650 installed it chugs along at 12fps.
 

h9826790

macrumors P6
Apr 3, 2014
16,370
8,348
Hong Kong
But that can't be 100% true. I can run Cinebench on the screen connected to the GT 120 and I get a high frame per seccond. If I just run it on my machine without the RX 650 installed it chugs along at 12fps.

Did you start the CineBench on the screen that connected to GT120? Or start it on a screen that connect to the RX560 and then drag it to another screen?

I ran dual card some time ago, from memory, if I start an apps on one screen, and then drag it to the other, the graphics are still rendered by the original GPU. It won't switch GPU when the window goes into another display. Which actually make sense. Otherwise, think about if you start CineBench, then drag the window and put it at the middle of two displays (each screen shows half of the windows), what will happen? The GPU that already "assigned" to handle the graphics will continue to do the job, rather than only handle half of the graphics and let the other GPU to handle the other half (split frame rendering).
 
Last edited:

CosmicScale

macrumors newbie
Original poster
Dec 10, 2017
17
1
London, UK
Did you start the CineBench on the screen that connected to GT120? Or start it on a screen that connect to the RX560 and then drag it to another screen?

I ran dual card some time ago, from memory, if I start an apps on one screen, and then draw it to the other, the graphics are still rendered by the original GPU. It won't switch GPU when the window goes into another display. Which actually make sense. Otherwise, think about if you start CineBench, then drag the window and put it at the middle of two displays (each screen shows half of the windows), what will happen? The GPU that already "assigned" to handle the graphics will continue to do the job, rather than only handle half of the graphics and let the other GPU to handle the other half (split frame rendering).

Ah, so that's how it works! The RX 650 I've got gets a little noisy with its fan when on a Skype video call whereas the GT 120 never did. I'll just launch Skype on my other monitor and drag it over then!
 
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