Process Scheduling in Operating System

Process scheduling is one of the most important OS topics because it decides how CPU time is shared among processes.

What is Process Scheduling?

Process scheduling is the method used by the operating system to choose which ready process gets CPU execution next.

Why scheduling is needed

• CPU should stay busy
• processes should get fair chance
• waiting time should be reduced
• response time should improve

Scheduling Criteria

Common criteria:

• CPU utilization
• throughput
• turnaround time
• waiting time
• response time

FCFS

FCFS stands for First Come First Serve.

The process that arrives first gets CPU first.

Advantages:

• simple
• easy to implement

Disadvantage:

• may cause convoy effect

SJF

SJF stands for Shortest Job First.

The process with the shortest burst time gets CPU first.

Advantage:

• gives low average waiting time

Disadvantage:

• difficult to know exact burst time in advance

Round Robin

Round Robin gives each process a fixed time quantum in turn.

Advantage:

• good for time-sharing systems
• fair to multiple processes

Disadvantage:

• performance depends on quantum size

Priority Scheduling

In priority scheduling, the process with highest priority gets CPU first.

It may be:

• preemptive
• non-preemptive

Main issue:

• starvation of low-priority processes

Preemptive vs Non-preemptive

• preemptive scheduling can interrupt a running process
• non-preemptive scheduling waits until process finishes or blocks

Interview-style answer

Process scheduling is the OS mechanism that selects the next process for CPU execution. Common algorithms are FCFS, SJF, Round Robin, and Priority Scheduling. Scheduling tries to improve CPU utilization, fairness, and response time.

Quick Revision Points

• FCFS is simple
• SJF gives lower waiting time
• Round Robin is common in time sharing
• Priority Scheduling may cause starvation
• scheduling improves CPU use and fairness

Viva Questions

1. What is process scheduling?
2. What is FCFS?
3. What is SJF?
4. What is Round Robin?
5. What is the difference between preemptive and non-preemptive scheduling?