This document compares three CPU scheduling algorithms: first-come, first-served (FCFS); shortest job first (SJF); and preemptive FCFS. It provides examples of each with processes of different burst times and calculates their average turnaround times and waiting times. FCFS has the highest average turnaround time of 27 time units. Preemptive FCFS improves this to 13 time units by allowing short jobs to preempt longer jobs. SJF has the lowest average turnaround and waiting times of 13 and 7 time units respectively by always selecting the shortest job to run next.

1. First Come First
Serve
Shortest Job First

2. 
BY: Adeel Rasheed

3. 
 FCFS is very simple - Just a FIFO queue
 Like customers waiting in line at the bank or the
post office.
FCFS (Non- Preemptive)
Process Burst Time Arrival Time
P1 24 0
P2 3 0
P3 3 0

4. Gantt Chart:
Turn Around Time:
T.A.T=Termination Time – Arrival Time
Process T.T – A.T T.A.T
P1 24 – 0 24
P2 27 – 0 27
P3 30 – 0 30
Avg T.A.T =
𝟐𝟒+𝟐𝟕+𝟑𝟎
𝟑
= 27

5. Waiting Time:
That’s time who process wait for start execution
Process Waiting Time
P1 0
P2 24
P3 27
Avg W.T =
𝟎+𝟐𝟒+𝟐𝟕
𝟑
= 17

6. 
Example

7. 
 Low priority running process anytime when a high
priority process enters into a ready state.
 Pick a little job first.
FCFS (Preemptive)
Process Burst Time Arrival Time
P1 24 0
P2 3 0
P3 3 0

8. Gantt Chart:
Turn Around Time:
T.A.T=Termination Time – Arrival Time
Process T.T – A.T T.A.T
P1 30 – 0 30
P2 3 – 0 3
P3 6 – 0 6
Avg T.A.T =
𝟑𝟎+𝟑+𝟔
𝟑
= 13

9. Waiting Time:
That’s time who process wait for start execution
Process Waiting Time
P1 6
P2 0
P3 3
Avg W.T =
𝟔+𝟎+𝟑
𝟑
= 3

10. 
 Idea behind the SJF algorithm is to pick the quickest
fastest little job that needs to be done, get it out of the
way first, and then pick the next smallest fastest job
to do next.
SJF
Process Burst Time Arrival Time
P1 6 0
P2 8 0
P3 7 0
P4 3 0

11. Gantt Chart:
 Turn Around Time:
T.A.T=Termination Time – Arrival Time
Process T.T – A.T T.A.T
P1 9 – 0 9
P2 24 – 0 24
P3 16 – 0 16
P4 3 – 0 3
Avg T.A.T =
𝟗+𝟐𝟒+𝟏𝟔+𝟑
𝟒
= 13

12. Waiting Time:
That’s time who process wait for start execution
Process Waiting Time
P1 3
P2 16
P3 9
P4 0
Avg W.T =
𝟑+𝟏𝟔+𝟗+𝟎
𝟒
= 7

13. 