I think we have different definitions of "preemptive multitasking". There is no GPU I know of that can be preempted once given a command to draw. Once it starts if that drawing command takes 30 seconds there's no preempting it. This is why Windows has a timeout that resets the GPU if it doesn't respond. (I believe other OSes have added that feature but I'm not 100% sure). Anyway, I've yet to use a GPU or an OS that supports preempting the GPU. I'd be happy to be proven wrong. I can also give you samples to test. It doesn't require fancy shaders. All it requires is lots of large polygons in one draw call.