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Main concern of a scheduler is the distribution of CPU bursts.
Scheduler decisions may occur when a process:
When decisions are made at 1. or 4., the scheduler is non-preemptive, otherwise it is preemptive.
A non-preemptive scheduler allows a process to keep control of the CPU until it releases, either by terminating or switching to the waiting state. A preemptive scheduler can switch a process out of the running state without termination.
A Race condition is where the output of a program is dependent on the timing or sequence of execution. Preemptive scheduling can lead to race conditions, where there is shared data between process / threads. One process may change data when the 2nd is preempted into the running state. The 2nd process then reads from inconsistent data.
The Dispatcher controls when process are switched into the CPU. It controls:
Dispatch latency is the time it takes the dispatcher to stop one process and start another.