iMac with dGPU, does it use the Intel GPU?

[Darmok N Jalad]

So I have a 2017 5K iMac, which of course includes a dedicated AMD GPU, the Radeon Pro 570. I know the CPU is the i5-7500, which includes the HD 630 graphics. All this time, I assumed that the system didn't use the Intel GPU for anything, but I came across some interesting findings that suggest that maybe it does after all. I had a video stream playing (via flash on a website), and I noticed Mac's Fan Control reporting my CPU PECI temp as 50C (10C higher than usual idle), but none of the CPU cores were registering above 45C. That got me wondering, so I opened Intel Power Gadget, and to my surprise, it's showing the Intel graphics (GT), running at 1.10GHz:

As soon as I shut down the video stream, the GT drops to 0:

My guess is that the iGPU of the i5-7500 is still being used to handle video decode. Possibly certain kinds of HD streams, as when I resumed my stream, it started out with the GT bouncing from 0-100-300MHz. Then an HD ad loaded, and now it's locked in to 1.10GHz again.

Any thoughts? I'm not worried, just curious.

 

[sputnikBA]

Quick Sync (Wikipedia)

Intel Quick Sync (Intel)

 

[jerwin]

quicksync.

 

[Darmok N Jalad]

That's pretty much what I thought. I haven't seen anything official on what's going on, but I have seen people suggest that the GPU is disabled entirely on the iMac (since it does not show in System Profiler). It would appear that while the GT may not drive the display, the shader hardware is still available for tasks like these. That's actually pretty cool that the hardware still gets used when it can do the job. Does this happen on Windows machines when a dGPU is used as well?