While it's true that there's no reason it has to swap processors for a CPU-bound thread, my experience is that the way SVR4 timesharing works is that even on an otherwise unloaded machine, the thread will move amongst all of the processors. The computer is always doing something else, so there will always be context switches. On the other hand, this is probably less true on a 12-CPU machine than on a 2-CPU machine, and AIX is about as distant from "standard" SVR4 as you can get, so my assumptions may be wrong.