Whenever the CPU becomes idle, the operating system must select one of the processes in the ready queue to be executed. The selection process is carried out by the short-term scheduler, or CPU scheduler. The scheduler selects a process from the processes in memory that are ready to execute and allocates the CPU to that process
Preemptive Scheduling
CPU-scheduling decisions may take place under the following four circumstances
- When a process switches from the running state to the waiting state (for example, as the result of an I/O request or an invocation of wait() for the termination of a child process)
- When a process switches from the running state to the ready state (for example, when an interrupt occurs)
- When a process switches from the waiting state to the ready state (for example, at completion of I/O)
- When a process terminates
Dispatcher
Another component involved in the CPU-scheduling function is the dispatcher. The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. This function involves the following
- Switching context
- Switching to user mode
- Jumping to the proper location in the user program to restart that program
The dispatcher should be as fast as possible, since it is invoked during every process switch. The time it takes for the dispatcher to stop one process and start another running is known as the dispatch latency