does OpenGL handle multi-threading in the GPU for you when drawing?
The GPU is it's own hardware that runs independently of the main CPU, so yes, kindof. It's more then just "a different thread" though, since it's entirely different hardware, and your shaders will most likely be run as not only one, but a bunch of threads in parallell.
also is there any big difference in performance if you use multi threading for games, say for your update functions ?
The gain you get with multithreading depends on the problem you try to solve.
If the tasks you do in parallell has no dependencies on each other, you might get close to NumberOfThreads times the performance, but a game engine states is by its nature quite dependent on each other, having lots of requirements on what needs to be done before what, and is very tricky to multithread in an efficient way.
Short Answer: its impossible to tell in general.
Edited by Olof Hedman, 26 February 2013 - 06:20 AM.