Real-time scheduling for OS

With real time scheduling you have a notion of dispatch latency, this is the time it takes between when a process is scheduled and when it actually is executed. The interrupt handlers can't be interrupted so they form the worst case dispatch time. If the OS is pre-emptible then it can be interrupted between any step of a system call, with some exceptions like interrupt handlers. The disadvantage of a pre-emptible OS (like Solaris) is that it has to protect all its data structures with mutex and semaphores to prevent resource conflicts. Non-preemptilbe OSs can not be interrupted in the middle of a system call so they dont have this problem.

Another option besides making the OS pre-emptible, insert preemption points in long duration system calls.