Even though I still had almost 12 GB of VRAM free, the game was using around 4 GB of shared GPU memory. It causes massive stutters every time I switch to an external view. I really don’t understand how the game is handling VRAM allocation
It looks like you have Integrated Graphics (as I do). This setting is enabled in the BIOS. If you are not using your internal Intel GPU, you can disable it there.
The “Shared GPU Memory” shown under the RTX 5090 is system RAM being used by the 5090, not by the integrated GPU.
Disabling it in BIOS wouldn’t explain or fix this behavior.
But, did you test it? Windows treats the GPUs as one unit when you have Integrated Graphics enabled in the BIOS. I used to fly with my monitor plugged into the integrated GPU displayport, and my VR headset plugged into my Nvidia displayport. The sim ran fine in 2D because it was using the Nvidia GPU even though it was plugged into the onboard port. Windows is likely allocating memory for the combined GPUs as one unit. I haven’t been able to find details on how it works yet.
What you were seeing was likely caused by having the monitor connected to the integrated graphics. The discrete GPU was rendering the game, then the finished frames had to be copied over to the integrated graphics for display, which can create system/shared memory usage on the integrated graphics side. That’s a cross-GPU display path, not Windows combining both GPUs into one memory pool.
Okay, you are more knowledgeable than me on this topic. Cheers!
I think unless you have a specific reason not to, you should connect all of your monitors directly to the discrete GPU.
Thanks for the suggestion！
Yes, I didn’t really understand what I was doing. I thought I was saving the discrete GPU only for VR, and running the sim on the iGPU. I was always starting VR before the sim, so I never noticed what was happening until I just launched the sim by itself. 
In general: the GPU is never using shared VRAM. Applications do. And the memory management is done by the Operating system and the drivers, not by the application.
You are showing the overall system memory. This does not necessarily mean the simulator is using the shared VRAM.
It is possible to free dedicated VRAM by moving apps to the iGPU:
This is a performance boost for the simulator. Btw, you can pin the simulator to the dGPU.