Transcript Process Control Management

Operating Systems
Process Control Management
Prepared by: Dhason
Topic & Structure of the lesson
Process Control Management
• CPU scheduling
• Preemptive scheduling algorithms
• Non-preemptive scheduling algorithms
• Calculations using preemptive scheduling algorithms
Operating Systems
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Learning Outcomes
Process Control Management
• 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
Operating Systems
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CPU Scheduling
Process Control Management
•
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
Operating Systems
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Aims of CPU Scheduling
Process Control Management
• 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
Operating Systems
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Aims of Scheduling
Process Control Management
• turnaround time
- to minimize time between submission and job
completion
• throughput
- to maximize the number of jobs completed
within a given time period
Operating Systems
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Types of CPU scheduling
Process Control Management
• 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
-
Operating Systems
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Non-preemptive Scheduling
Process Control Management
• 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
Operating Systems
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Quick Review Questions
Process Control Management
• Why is CPU scheduling important?
• What is the difference between preemptive and nonpreemptive scheduling?
• Give 1 example each for a preemptive and nonpreemptive scheduling algorithm
Operating Systems
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Calculation Keywords
Process Control Management
• CPU utilization / burst time
• throughput
• turnaround time
- average turnaround time
• waiting time
- average waiting time
Operating Systems
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Round Robin
Process Control Management
• 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
Operating Systems
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Round Robin
Process Control Management
Step1:- Draw a Gantt Chart to represent timings for all
processes
P1
P2 P3 P4 P5
0
2 3
5
6
8
Step2:- Calculate waiting time
and average value
TW(P1) = (0+6+2+1)=9
TW(P2) = 2
TW(P3) = 3
Tw(P4) = 5
Tw(P5) = (6+2+2)=10
TW(average)=(9+2+3+5+10)/5
=5.8 Milliseconds
Operating Systems
P1
P5
10
P1 P5 P1 P1
12
14
15
17 19
Time Slice = 2
Milliseconds
Burst Time
Process
(Milliseconds)
P1
10
P2
1
P3
2
P4
1
P5
5
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Round Robin
Process Control Management
Step-3: Calculate turn-around time and average value
P1
0
P2 P3 P4 P5
2
3
TT(P1)=(9+10)=19
TT(P2)=(2+1) = 3
TT(P3)=(3+2) = 5
TT(P4)=(5+1) = 6
5
P1
6
8
10
TT = Tw+TB
P5
P1 P5 P1 P1
12
TT(P5)=(10+5)=15
Average turn around time is
(19+3+5+6+15)/5 = (48/5)=9.6
Milliseconds
Operating Systems
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15
17 19
Time Slice = 2
Milliseconds
Burst Time
Process
(Milliseconds)
P1
10
P2
1
P3
2
P4
1
P5
5
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Round Robin
Process Control Management
• 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)
Operating Systems
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Question and Answer Session
Process Control Management
Q&A
Operating Systems
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Next Session
Process Control Management
CPU Scheduling
Non-preemptive Algorithms
Operating Systems
Slide 16 of 24