CPU 100% when CUDA GPU

Thank you for the knowledge about VRAM!

I encountered CPU usage 100% but GPU was below 10% while rendering 360 stereo top-bottom with Cycles However my settings are set as CUDA and check GPU only.

But in two cases it will work perfectly using gpu render 1.When I uncheck Stereoscopy and even crank up Sample from 512 to 2048 (resolution as 3840x1920) It renders in GPU as expect (GPU 100% and cpu about 20%) 2.Stay render in 360 stereo top-bottom but shrink resolution from 3840x1920 to 1920x960

So as a newbie in blender my assumtion for these situation are 1.Cycles will automatically switch to CPU render when the scene is too big to fit in VRAM and this is decided before start rendering?

2.Cycles will use system memory when it runs out of VRAM. but this will only happen when it was decided to use gpu render in the firstplace?

sorry for my bad english

Jamie