Priority scheduling is a CPU scheduling algorithm where each process is assigned a priority and the process with the highest priority is executed first. In preemptive priority scheduling, a process that is currently running can be preempted by a process with a higher priority.
Here is a high-level overview of how the priority preemptive scheduling algorithm works:
- When a process arrives, its priority is determined (either by the system or by the user).
- The process is added to the ready queue based on its priority.
- The CPU is assigned to the process with the highest priority in the ready queue.
- The process is executed for a quantum (a fixed amount of time) or until it completes or is preempted.
- If the process completes, it is removed from the queue.
- If a new process arrives with a higher priority than the currently executing process, the currently executing process is preempted and moved back to the ready queue.
- If the current process is blocked, it is moved to a blocked queue and the CPU is assigned to the process with the highest priority in the ready queue.
- When a blocked process becomes unblocked (e.g., I/O completes), its priority is checked and it may be moved to the ready queue.
- Steps 3-8 are repeated until all processes have completed.
Priority scheduling can be implemented using a variety of data structures, such as a priority queue or a linked list sorted by priority. In preemptive priority scheduling, the priority queue is updated whenever a new process arrives or when a blocked process becomes unblocked, and also when a process with a higher priority arrives and preempts the currently executing process.