This document discusses various non-preemptive scheduling algorithms, including first-come, first-served (FCFS) scheduling where processes are executed in the order of arrival. It also covers shortest-job-first (SJF) scheduling where the process with the shortest predicted CPU burst time is scheduled next. Finally, it describes priority scheduling where processes are executed based on assigned priorities, with the highest priority process being scheduled next.

1. Operating System 29
Non-preemptive Scheduling
Prof Neeraj Bhargava
Vaibhav Khanna
Department of Computer Science
School of Engineering and Systems Sciences
Maharshi Dayanand Saraswati University Ajmer

2. Non-preemptive Scheduling
• These scheduling mechanism do not force the
process to be deallocated from CPU to release
the resource as and when required.
• The processes release the resources only
when it has finished executing

3. First- Come, First-Served (FCFS) Scheduling
• The CPU, in this scheme, is scheduled to the processes
in the order of arrival at the ready queue.
• This scheme can be easily implemented using a queue
data structure.
• The processes are queued up by enqueuer (software
component of operating system), in the order of their
arrival at the tail of the queue.
• Whenever, the CPU is available, the dispatcher
(software component of operating system), removes a
process from the head of the queue and schedules it to
the CPU.

4. First- Come, First-Served (FCFS) Scheduling
Process Burst Time
P1 24
P2 3
P3 3
• Suppose that the processes arrive in the order: P1 , P2 ,
P3
The Gantt Chart for the schedule is:
• Waiting time for P1 = 0; P2 = 24; P3 = 27
• Average waiting time: (0 + 24 + 27)/3 = 17
P P P
1 2 3
0 24 30
27

5. FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order:
P2 , P3 , P1
• The Gantt chart for the schedule is:
• Waiting time for P1 = 6; P2 = 0; P3 = 3
• Average waiting time: (6 + 0 + 3)/3 = 3
• Much better than previous case
• Convoy effect - short process behind long process
– Consider one CPU-bound and many I/O-bound processes
P1
0 3 6 30
P2
P3

6. Shortest-Job-First (SJF) Scheduling
• Associate with each process the length
of its next CPU burst
– Use these lengths to schedule the process
with the shortest time
• SJF is optimal – gives minimum average
waiting time for a given set of processes
– The difficulty is knowing the length of the
next CPU request
– Could ask the user

7. Shortest-Job-First (SJF) Scheduling
• This algorithm associates with each process
the length of the number of CPU bursts
needed by it for its execution.
• The process having the smallest next CPU
burst is allocated the CPU whenever it is
available next.
• In case there is a tie, FCFS scheme is employed
to break the tie.

8. Example of SJF
ProcessArriva l TimeBurst Time
P1 0.0 6
P2 2.0 8
P3 4.0 7
P4 5.0 3
• SJF scheduling chart
• Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
P3
0 3 24
P4
P1
16
9
P2

9. Determining Length of Next CPU
Burst
• Can only estimate the length – should be similar to
the previous one
– Then pick process with shortest predicted next CPU burst
• Can be done by using the length of previous CPU
bursts, using exponential averaging
• Commonly, α set to ½
• Preemptive version called shortest-remaining-time-
first
:
Define
4.
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CPU
next
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for
value
predicted
2.
burst
CPU
of
length
actual
1.







 1
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th
n n
t
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1
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n t 

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10. Prediction of the Length of the Next CPU Burst

11. Examples of Exponential Averaging
•  =0
– n+1 = n
– Recent history does not count
•  =1
– n+1 =  tn
– Only the actual last CPU burst counts
• If we expand the formula, we get:
n+1 =  tn+(1 - ) tn -1 + …
+(1 -  )j  tn -j + …
+(1 -  )n +1 0
• Since both  and (1 - ) are less than or equal
to 1, each successive term has less weight than
its predecessor

12. Example of Shortest-remaining-time-first
• Now we add the concepts of varying arrival times and
preemption to the analysis
ProcessAarri Arrival TimeT Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
• Preemptive SJF Gantt Chart
• Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 =
6.5 msec
P4
0 1 26
P1
P2
10
P3
P1
5 17

13. Priority Scheduling
• A priority number (integer) is associated with each
process
• The CPU is allocated to the process with the highest
priority (smallest integer  highest priority)
– Preemptive
– Nonpreemptive
• SJF is priority scheduling where priority is the inverse
of predicted next CPU burst time
• Problem  Starvation – low priority processes may
never execute
• Solution  Aging – as time progresses increase the
priority of the process

14. Priority Scheduling
• A more general schedule algorithm of which both FCSC
and SJF is priority scheduling.
• This algorithm associates a priority with each process
on some pre-determined basis.
• The CPU is allocated to the process having highest
priority.
• If the priorities associated to processes are equal then
the algorithm reduces to FCSC.
• SJF is a special case of priority scheduling with
priorities associated according to the number of CPU
burst required by the processes, the process with
smallest CPU burst requirement having highest priority.

15. Example of Priority Scheduling
ProcessAarri Burst TimeT Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
• Priority scheduling Gantt Chart
• Average waiting time = 8.2 msec
1
0 1 19
P1
P2
16
P4
P3
6 18
P

16. Assignment
• What do you understand by Non Preemptive
Scheduling
• Explain First- Come, First-Served (FCFS)
Scheduling
• Explain Shortest-Job-First (SJF) Scheduling
• Explain Priority Scheduling