This document discusses different CPU scheduling algorithms and their optimization criteria. It describes the First Come First Serve (FCFS) algorithm and provides examples to show how process wait times can vary based on arrival order. The Shortest Job First (SJF) algorithm is also covered, explaining how it selects the process with the shortest remaining CPU burst time to optimize average wait time. The document concludes by discussing how exponential averaging can be used to predict future CPU burst lengths for SJF scheduling.

1. Amrita
School
of
Engineering,
Bangalore
Ms. Harika Pudugosula
Teaching Assistant
Department of Electronics & Communication Engineering

2. • Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
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Scheduling
• Example:

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Scheduling Algorithm Optimization Criteria
• Max CPU utilization - keep CPU as busy as possible
• Max throughput - Number of processes that complete their execution
per time unit
• Min turnaround time - amount of time to execute a particular process
• Min waiting time - amount of time that a process spends waiting in the
ready queue
• Min response time - amount of time it takes from when a request was
submitted until the first response is produced, not output (for time-
sharing environment)

5. Scheduling Algorithms

6. First- Come, First-Served (FCFS) Scheduling
• By far the simplest CPU-scheduling algorithm is the first-come, first-served (FCFS)
scheduling algorithm
• The process that requests the CPU first is allocated the CPU first
• The implementation of the FCFS policy is easily managed with a FIFO queue
• When a process enters the ready queue, its PCB is linked onto the tail of the queue
• When the CPU is free, it is allocated to the process at the head of the queue
• The running process is then removed from the queue
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7. 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 = 0ms ; P2 = 24ms ; P3 = 27ms
• Average waiting time: (0 + 24 + 27)/3 = 17ms
P P P
1 2 3
0 24 30
27
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8. 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 = 6ms ;P2 = 0ms; P3 = 3ms
Average waiting time: (6 + 0 + 3)/3 = 3ms
This reduction is substantial. Thus, the average waiting time under an FCFS policy is
generally not minimal and may vary substantially if the processes’ CPU burst times
vary greatly
P1
0 3 6 30
P2
P3
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9. FCFS Scheduling (Cont.)
• Much better than previous case
• The FCFS scheduling algorithm is nonpremptive
• Once the CPU has been allocated to a process, that process keeps the CPU until it
releases the CPU, either by terminating or by requesting I/O
• The FCFS algorithm is thus particularly troublesome for time-sharing systems, where it
is important that each user get a share of the CPU at regular intervals
• It would be disastrous to allow one process to keep the CPU for an extended period
Convoy effect - short process behind long process
Consider one CPU-bound and many I/O-bound processes
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10. Shortest-Job-First (SJF) Scheduling
• This algorithm associates with each process the length of the process’s next CPU
burst i.e., the process which has least next CPU burst time will be assigned to CPU
• If the next CPU bursts of two processes are the same, FCFS scheduling is used to
break the tie
• The SJF algorithm can be either preemptive or nonpreemptive
• Scheduling depends on the length of the next CPU burst of a process, rather than its
total length - Shortest-next CPU-burst algorithm
• 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
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11. Example of SJF (nonpreemptive)
• Consider the following set of processes, with the length of the CPU burst given in
milliseconds:
ProcessArrival Time Burst 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 = 7milliseconds
• By comparison, if we were using the FCFS scheduling scheme, the average waiting time
would be 10.25 milliseconds
P3
0 3 24
P4
P1
16
9
P2
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12. Example of Shortest-remaining-time-first / Preemptive SJF Scheduling
Now we add the concepts of varying arrival times and preemption to the analysis
Process Arrival Time(ms)Burst Time(ms)
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
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13. 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
:
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14. Prediction of the Length of the Next CPU Burst
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15. 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
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References
1. Silberscatnz and Galvin, “Operating System Concepts,” Ninth Edition, John
Wiley and Sons, 2012.

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Thank you