1. Understand process management
• CPU Scheduling Algorithms, FCFS
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3. Recap
In last class, you have learnt
• CPU scheduling
• Scheduling criteria
– CPU utilization
– Through put
– Turn around time
– Waiting time
– Response time
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4. Objectives
On completion of this class, you would be able to know
• Types of scheduling algorithms
• FCFS scheduling algorithm
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5. Scheduling algorithms
• The CPU scheduling, selects a process in the ready
queue, for execution using scheduling algorithms
• Scheduling algorithm can be classified into
– First come, first served scheduling
– Shortest job first scheduling
– Priority scheduling
– Round robin scheduling
– Multilevel queue scheduling
– Multilevel feedback queue scheduling
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6. FCFS Scheduling Algorithm
•It is the simplest scheduling algorithm
• A process that requests the CPU first is allocated first
Hence the name First come, First serve
• It is implemented by First in First out (FIFO) Queue
• Whenever the process enters into the ready queue PCB is
linked onto the tail of the queue
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7. FCFS
Consider an example of processes P1, P2, P3 arriving at
time instant 0 and their CPU burst times are shown below:
Process Burst time (msecs)
P1 24
P2 3
P3 3
The Gantt chart below shows the result
P1 P2 P3
0 24 27 30
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8. FCFS
• Average waiting time and average turn around time are
calculated using the example in previous slide
– Waiting time = stating time – arrival time
– The waiting time for process P1
– P1= 0 – 0 = 0msec
– The waiting time for process P2
– P2 = 24 – 0 = 24msec
– The waiting time for process P3
– P3 = 27 – 0 = 27msec
• Average waiting time=(0+24+27)/3=17msec
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9. FCFS ( Disadvantages )
• Average turn around time=(24+27+30)=27msec
• The average waiting time measured under FCFS policy is long
(i.e. 17 m sec)
• For example in previous slide if processes arrive in the order P2,
P3, P1 it results in the following Gantt chart
P2 P3 P1
0 3 6 30
• The average waiting time is (6+0+3)/3=3msec
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11. FCFS (Disadvantages)
• FCFS scheduling algorithm is Non preemptive
– So, trouble some for time sharing system
• Each user gets a share of CPU at regular intervals
in timesharing system
• It is disastrous to allow are process to keep the
keep the CPU for an extended period
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12. Summary
In this class, you have learnt
• Types of scheduling algorithms
• FCFS algorithm
• Disadvantages of FCFS algorithm
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13. Frequently Asked Questions
1. List the types of CPU scheduling algorithm
2. Explain FCFS algorithm
3. Write the disadvantages of FCFS algorithm
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14. Quiz
1. FCFS algorithm is implemented by _________
a) Job queue
b) FIFO queue
c) Ready queue
d) None
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15. Quiz
2. When the process enters into ready queue PCB is linked in
to __________ of queue
a) Head
b) Tail
c) None
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16. Quiz
3. Average waiting time on FCFS algorithm is ________
a) Long
b) Short
c) None
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17. Quiz
4. FCFS algorithm is ______________
a) Preemptive
b) Non preemptive
c) None
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18. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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