Question 100% usage but only 3gb out of 6gb used on my GPU

Aeacus

Titan
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It is possible due to the fact that VRAM utilization is game dependent, and not GPU dependent.

Meaning that when game is coded to use 3 GB of VRAM, it won't ever use more, even if you have loads of VRAM, e.g 24 GB. Game still uses up to 3 GB.

Now, if you'd be short of VRAM, e.g game needs 8 GB of VRAM but your GPU only has 6 GB of VRAM, then you'll take performance hit when playing a game. Either lowered in-game graphics or lower FPS.
 
Vram holds the video code you need to run the game.
The game manages it and probably does not need more .
A more worrying stat is the 100% gpu utilization.
It indicates that your game is limited in performance by the gpu.
Or, that the gpu has a problem.

If your game performance is good, do not worry.
If you think it is lacking, tell us more about the game and all of your pc specs.