The document discusses process management and the First Come First Serve (FCFS) CPU scheduling algorithm. It covers: 1) FCFS is the simplest scheduling algorithm that allocates the CPU to the process that requests it first. It is implemented using a FIFO queue where new processes are added to the tail. 2) FCFS can result in long waiting times for processes. An example is provided where the average waiting time is 17ms. 3) FCFS is non-preemptive and not suitable for time-sharing systems as it does not allow regular intervals of CPU allocation between users.

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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