1. Process Control Management
Operating Systems
Process Control Management
Prepared by: Dhason

2. Topic & Structure of the lesson
CPU scheduling
Preemptive scheduling algorithms
Non-preemptive scheduling algorithms
Calculations using preemptive scheduling algorithms

3. Learning Outcomes At the end of this lecture YOU should be able to:
- explain the importance of CPU scheduling
- distinguish between preemptive and
non-preemptive algorithms
- calculate waiting time and turnaround time

4. CPU Scheduling process to run first from the ready queue
a CPU scheduler is tasked with choosing which
process to run first from the ready queue
a scheduling algorithm is used to choose the next
process
scheduling is a fundamental function of an operating
system
scheduling is done to ensure that the CPU is not
idle

5. Aims of CPU Scheduling fairness
- to make sure all processes get a fair share of
the CPU time
- to avoid starvation
efficiency
- to maximize CPU utilization and keep the CPU
busy close to 100% of the time
response time
to provide consistent response time
to minimize response time

6. Aims of Scheduling turnaround time
- to minimize time between submission and job
completion
throughput
- to maximize the number of jobs completed
within a given time period

7. Types of CPU scheduling
preemptive and non-preemptive scheduling:
preemptive scheduling:
- allows for processes running to be temporarily
suspended
processes releases CPU upon receiving a command from
the operating system
- preemptive scheduling algorithm:
- round robin

8. Non-preemptive Scheduling
- processes releases the CPU only after completion
- processes releases CPU voluntarily
- non-preemptive scheduling algorithms:
- first come first serve or first in first out
(FCFS), ( FIFO)
- shortest job first
- priority

9. Quick Review Questions
Why is CPU scheduling important?
What is the difference between preemptive and non-preemptive scheduling?
Give 1 example each for a preemptive and non-preemptive scheduling algorithm

10. Calculation Keywords CPU utilization / burst time throughput
turnaround time
- average turnaround time
waiting time
average waiting time

11. Round Robin
the oldest, simplest, fairest and most widely used algorithm
designed for time-sharing systems
a time quantum / time slice is defined for each process
the ready queue is treated as a circular queue
to implement round robin, the ready queue is kept as a
first in first out queue

12. Step1:- Draw a Gantt Chart to represent timings for all processes
Round Robin
Step1:- Draw a Gantt Chart to represent timings for all processes P1 P2 P3 P4 P5
Step2:- Calculate waiting time
and average value
TW(P1) = ( )=9
TW(P2) = 2
TW(P3) = 3
Tw(P4) = 5
Tw(P5) = (6+2+2)=10
TW(average)=( )/5
=5.8 Milliseconds 2 3 5 6 8 10 12 14 15 17 19
Time Slice = 2 Milliseconds
Process
Burst Time
(Milliseconds) P1 10 P2 1 P3 2 P4 P5 5

13. Round Robin
Step-3: Calculate turn-around time and average value P1 P2 P3 P4 P5 2 3 5 6 8 10 12 14 15 17 19
TT = Tw+TB
Time Slice = 2 Milliseconds
TT(P1)=(9+10)=19
Process
Burst Time
(Milliseconds) P1 10 P2 1 P3 2 P4 P5 5
TT(P2)=(2+1) = 3
TT(P3)=(3+2) = 5
TT(P4)=(5+1) = 6
TT(P5)=(10+5)=15
Average turn around time is ( )/5 = (48/5)=9.6 Milliseconds

14. Round Robin the average waiting time is usually long
performance depends on the size of the time quantum
(10 – 100 milliseconds)
process switching requires time; the time quantum
clock is already running
time quantum which is set to short would result in too
many process switches and this reduces CPU efficiency
time quantum which is too long would cause poor
response to short interactive request (would result to FIFO)

15. Question and Answer Session
Q & A

16. Next Session
CPU Scheduling
Non-preemptive Algorithms