CPU scheduling is a process which allows one process to use the CPU while the execution of another process is on hold(in waiting state) due to unavailability of any resource like I/O etc, thereby making full use of CPU. The aim of CPU scheduling is to make the system efficient, fast and fair.
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 from among the processes in memory that are ready to execute, and allocates the CPU to one of them.
Types of CPU 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 I/O request or invocation of wait for the termination of one of the child processes).
- 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, completion of I/O).
- When a process terminates.
CPU Scheduling: 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: