First-Come First-Serve (FCFS)

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Lecture 2 Processes
Lecture 1 Introduction

First-Come First-Serve (FCFS)


  • Calculate the turn around time, waiting time, average turnaround time, average waiting time, throughput and processor utilisation for the given set of processes that arrive at a given arrive time shown in the table, with the length of processing time given in milliseconds:
  • If the processes arrives as per the arrival time, the Gantt chart will be:

Shortest Job First (SJF)

  • Also known shortest job next
  • This is a non-pre-emptive type of scheduling
  • Best approach to minimize waiting time
  • The processor should know in advance how much time the process will take
  • Easy to implement in Batch systems where required CPU time is known in advance
  • Impossible to implement in interactive systems where required CPU time is not known


  • Consider the following set of processes with the following processing time which arrived at the same time.
  • Using SJF scheduling the shortest length of process will first get execution, the Gantt chart will be:


Process Time









Round Robin (RR)

  • Is a pre-emptive process scheduling algorithm
  • Each process is provided a fixed time to execute known as a Quantum
  • Once a process is executed for a given time period, it is pre-empted and other process executes for a given time period
  • No process can run for more than on quantum while others are waiting in the ready queue
  • Primarily used in time-sharing and multiuser system environment

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