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macrumors member
Original poster
Aug 2, 2013
Hey all

I'm pretty sure the answer to this is no....but I thought I'd ask...

Is there anyway to have both the Iris Pro and the 750M running at the same time to give increased performance? I'm just wondering if there is a game that couldn't be run on ultra settings with just one of them...could you turn them both on and get a better gaming performance out of the rMBP?

Now I'm not planning on doing was more of a hypothetical.



macrumors 68000
Nov 24, 2008
I'm not sure of the details, but it's not a simple "on-off" for both chips. If "both" are being used at the same time, its most likely the processors handling respective and somewhat exclusive tasks, not really sharing of common tasks to be faster or better than a single CPU or GPU handling the task.

Looking forward to others chiming in.

Atomic Walrus

macrumors 6502a
Sep 24, 2012
Basically, multi-GPU rendering is a nightmare even with two identical GPUs. When running two cards together they have to take turns rendering frames, and even with two of the exact same GPUs on identical PCI-E connections the timings will get messed up now and then, resulting in what we call "micro stuttering." This means the frame timings get messed up, so while you may produce 60fps they might not be evenly spaced, meaning some are too late or too early and don't actually get displayed by the monitor.

The result is the appearance of dropped frames, even though you're at a stable 60fps (and if we're talking about desktop SLI it's usually 120, but I'll stick with 60 for the example). Nvidia's SLI has been getting pretty good about this (though it definitely still exists), but AMD's crossfire still produces relatively poor frame timing results on the rare occasion that this is tested.

So the short version is: You cannot effectively do dual-GPU rendering with two different cards. To even attempt it would require collaboration between Nvidia and Intel's drivers, which isn't going to happen. If you could force it to happen the results would almost certainly be worse than running either separately (and it's not even certain that each card's frames would look exactly the same due to driver differences)

In theory you could use one for compute and the other for rendering, but you might hit power limits. I also don't know how the GPU switching works at a hardware level; it's possible that the two GPUs can never be powered on at the same time for any reason.
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