When interviewing in tech jobs I had :
- build an asynchronous communication queue from scratch (which implies building semaphores)
- optimize the drawing of a sphere on a screen
the rest was more regular (I didn't apply to super elite jobs, and both the companies with these two hard tests failed, so it's no indication neither about them, neither about me)
It lasted two hours. Now the way it worked was that I was doing on my own but I had to constantly explain to a "peer" my line of reasoning. So I was not alone in front of a white paper :-) This was an extremely interesting wy to interview. I felt I had the opportunity to demonstrate my skills on a difficult problem (which I didn't solve). I didn't remember the details of Dijkstra's algorithm but I had the opportunity to show that I can approach a complex problem in a logical and efficient way (well, that day, that kind of problem; don't ask me to work on helicoidal tomography :-))
the rest was more regular (I didn't apply to super elite jobs, and both the companies with these two hard tests failed, so it's no indication neither about them, neither about me)