The document discusses the Shortest Job First (SJF) CPU scheduling algorithm. SJF is a preemptive scheduling policy that selects the waiting process with the shortest execution time/burst time to execute next. The algorithm involves sorting processes by arrival time and selecting the process with the shortest burst time. An example shows the Gantt chart and calculations for average turnaround time and waiting time using SJF. Advantages include shorter jobs being favored and minimum average waiting time, while disadvantages are potential starvation of longer jobs and inability for short-term CPU scheduling.

1. SHORTEST JOB
FIRST(SJF)
PRESENTED BY
NAME:SHUBHAM GUPTA
UNIVERSITY ROLL:12000221054
BRANCH:INFORMATION
TECHNOLOGY
SUBJECT CODE:PCC-CS502

2. TOPIC OVERVIEW
CPU SCHEDULING
PREEMTIVE & NON PREEMPTIVE
CLASSIFICATION OF PREEMPTIVE & NON PREEMPTIVE
INTRODUCTION OF SHORTEST JOB FIRST(SJF)
ALGORITHM OF SJF
EXAMPLE OF SJF
ADVANTAGE AND DISADVANTAGE OF SJF

3. CPU SCHEDULING
CPU Scheduling is a process of determining which process will own
CPU for execution while another process is on hold. The main task of
CPU scheduling is to make sure that whenever the CPU remains idle,
the OS at least select one of the processes available in the ready
queue for execution. The selection process will be carried out by the
CPU scheduler. It selects one of the processes in memory that are
ready for execution.

4. TYPE OF CPU SCHEDULING
Non preemptive –Non-Preemptive scheduling is used when a
process terminates , or when a process switches from running state
to waiting state.
Preemptive –Preemptive scheduling is used when a process switches
from running state to ready state or from the waiting state to the
ready state. This scheme is known as the Shortest-Remaining-Time-
First (SRTF).

5. CLASSIFICATION OF PREEMTIVE
AND NON PREEMTIVE
CPU
SCHEDULING
PRE-EMTIVE
NON PRE-
EMTIVE
SRTM
LRTM
LJF
SJF
FCFS
ROUND ROBIN

6. INTRODUCTION OF SHORTEST JOB
FIRST(SJF)
The shortest job first (SJF) or shortest job next, is a scheduling
policy that selects the waiting process with the smallest
execution time to execute next.
SJN, also known as Shortest Job Next (SJN)
It can be preemptive or non-preemptive both.
Preemptive SJF is also called Shortest Remaining Time First.

7. ALGORITHM OF SJF
Sort all the processes according to the arrival time.
Then select that process that has minimum arrival time and minimum
Burst time.
After completion of the process make a pool of processes that arrives
afterward till the completion of the previous process and select that
process among the pool which is having minimum Burst time.

8. BEFORE GOING TO EXAMPLE YOU
MUST KNOW THIS TERM
Arrival Time: Time at which the process arrives in the ready queue.
Completion Time: Time at which process completes its execution.
Burst Time: Time required by a process for CPU execution.
Turn Around Time: Time Difference between completion time and
arrival time.
Turn Around Time = Completion Time – Arrival Time
Waiting Time(W.T): Time Difference between turn around time and
burst time.
Waiting Time = Turn Around Time – Burst Time

9. EXAMPLE OF NON PREEMPTIVE SJF
CONSIDER THE SET OF 5 PROCESSES WHOSE ARRIVAL TIME AND BURST TIME ARE GIVEN BELOW-
Process Id Arrival time Burst time
P1 3 1
P2 1 4
P3 4 2
P4 0 6
P5 2 3

10. GANNT
CHART
Process Id Arrival time Burst time
P1 3 1
P2 1 4
P3 4 2
P4 0 6
P5 2 3
P4 P1 P3 P5 P2
0 6 12
9
7 16
COMPLETION
TIME (CT)

11. Process Id Arrival Time
Burst
Time
Completio
n time
Turn around
Time(CT-AT)
Waiting
Time(TAT-
BT)
P1 3 1 7 7-3 = 4 4-1 = 3
P2 1 4 16 16-1 = 15 15-4 = 11
P3 4 2 9 9-4 = 5 5-2 = 3
P4 0 6 6 6-0 = 6 6-6 = 0
P5 2 3 12 12-2 = 10 10-3 = 7
GANNT
CHART
Average Turn Around time = (4 + 15 + 5 + 6 + 10) / 5 = 40
/ 5 = 8
Average waiting time = (3 + 11 + 3 + 0 + 7) / 5 = 24 / 5 =
4.8
P4 P1 P3 P5 P2
0 6 12
9
7 16
COMPLETION
TIME (CT)

12. ADVANTAGE & DISADVANTAGE OF
SJF
Advantage:
1. Shortest jobs are favored.
2. It is probably optimal, in that it gives the minimum average waiting
time for a given set of processes.
Disadvantage:
1. SJF may cause starvation if shorter processes keep coming. This
problem is solved by aging.
2. It cannot be implemented at the level of short-term CPU
scheduling.

13. THANK
YOU