I am using a 2019 Macbook Pro which has a:
- AMD Radeon Pro 5300M 4 GB, and also a
- Intel UHD Graphics 630 1536 MB
When I connect an external display, what graphics card drives this display? How can I find out to be sure?
Yes, from what I can gather, MBPs switch to the dedicated GPU (if present) to run external displays.
One method of telling whether the discrete GPU is in use is documented here:
(Activity Monitor shows a "Requires High Perf GPU" column in Energy panel when the dGPU is active.)
It's possible there are other ways. There is an undocumented
gpuswitch option for
pmset. I don't know if
pmset -g or
pmset -g gpuswitch will show which gpu is in use.
You can also find out from the Apple menu (top left) > About this Mac.
Details can be found after clicking "System Report" and looking up "Graphics/Displays" in the sidebar.
The discrete GPU part will usually mention your external display - officially an external display will always run on that GPU but I'm not sure what to make of the DisplayLink driver software (for connecting a DP monitor via USB) telling me it's running the monitor on the integrated GPU.
At some point I came across an app called gSwitch which supposedly allows you to tell what GPU is running and switch it manually but for me it was unreliable to the point of being useless. The DisplayLink driver didn't work for me either, it had a rather substantial appetite for resources and I had crashes and other stability issues multiple times a day. But maybe it's great software that just didn't get along with my configuration.