How exactly GPU swishing works?

[Perezx]

Hello everybody!
Just curious and thinking about this. Perhaps, someone much more advanced can explain
So, how exactly does Mac OS decide which GPU should be used? Just monitoring TDP and/or idle time of iGPU seems to be not enough as the load changes and this will mean constant switching and moving gigabytes of data between GPUs memories, uh?

 

[joevt]

Hello everybody!
Just curious and thinking about this. Perhaps, someone much more advanced can explain
So, how exactly does Mac OS decide which GPU should be used? Just monitoring TDP and/or idle time of iGPU seems to be not enough as the load changes and this will mean constant switching and moving gigabytes of data between GPUs memories, uh?

I think if an app uses certain APIs, then dGPU is used (3d stuff usually).
Also, dGPU is used when external displays are connected (I don't think Macs have an option to use iGPU for external displays when dGPU exists).

The gfxCardStatus app lets you force dGPU or iGPU (when possible).

gfxCardStatus by cody krieger