First-Come First-Serve (FCFS)

First-Come First-Serve (FCFS)

• Jobs are scheduled on the order they are received
• Is non-pre-emptive
• Easy to understand and implement
• Its implementation is based on FIFO queue
• Poor in performance as average wait time is high

Exercise

• 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

Exercise

• 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:

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