6. Operating System - Process Scheduling

Definition: Process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy.

Process Scheduling Queues (Job Queue, Ready Queue, Device Queue):

Job queue - This queue keeps all the processes in the system.
Ready queue - This queue keeps a set of all processes residing in the main memory, ready and waiting to execute. A new process is always put in this queue.
Device queues - The processes which are blocked due to the unavailability of an I/O device constitute this queue.

Two-State Process Model (Running, Not Running):

Running: When a new process is created, it enters into the system as in the running state.
Not Running: Processes that are not running are kept in a queue, waiting for their turn to execute.

Scheduler (Long-Term Scheduler, Short-Term Scheduler, Medium-Term Scheduler):

Schedulers are special system software that handles process scheduling in various ways. Their main task is to select the jobs to be submitted into the system and to decide which process to run.

Long-Term Scheduler: It is also called a job scheduler. A long-term scheduler determines which programs are admitted to the system for processing. It selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling. The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound.

Short-Term Scheduler: It is also called a CPU scheduler. Its main objective is to increase system performance in accordance with the chosen set of criteria. Short-term schedulers are faster than long-term schedulers.

Medium-Term Scheduler: Medium-term scheduling is a part of swapping. It removes the processes from the memory.

Context Switch: A context switch is a mechanism to store and restore the state or context of a CPU in the Process Control block so that a process execution can be resumed from the same point at a later time.

Scheduling Algorithm:

First Come, First Serve (FCFS): Jobs are executed on a first-come, first-served basis.
Shortest Job Next (SJN): Best approach to minimize waiting time.
Priority Based Scheduling: Each process is assigned a priority. A process with the highest priority is to be executed first and so on.
Shortest Remaining Time: The processor is allocated to the job closest to completion but it can be preempted by a newer ready job with a shorter time to completion.
Round Robin Schedule: Each process is provided a fixed time to execute; it is called a quantum.

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