1. Lecture 16 Syed Mansoor Sarwar
Operating Systems
Lecture 16
Syed Mansoor Sarwar

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Agenda for Today
Review of previous lecture
SJF is optimal
Round-Robin scheduling
Multi-level queues scheduling
Multi-level feedback queues scheduling
Recap of lecture
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Review of Lecture 15
Shortest-Job-First (SJF)
Shortest-Remaining-Time-First (SRTF)
Exponential averaging
Priority scheduling
20 November 2018
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4. n+1 = tn + (1- ) n Review of Lecture 15 Exponential Averaging
for  = ½
n+1 = tn + (1- ) n
n+1 = tn/2 + tn-1/22 + tn-2/23 + tn-3/24 + …
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SJF is Optimal
Logical Argument: Decrease in the wait times for short processes is much more than increase in the wait times for long processes P1 P2 P3 P3 P2 P1
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Round Robin (RR)
Each process gets a small unit of CPU time, called time slice or quantum, which is usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.
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Round Robin (RR)
If there are n processes in the ready queue, the time quantum is q, and context switch time is tcs, then no process waits more than (n-1)(q+tcs) time units
Used in time-sharing systems where response time is an important performance criteria
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Round Robin (RR)
Performance
q large  FCFS
q small  q must be large with respect to context switch, otherwise overhead is too high.
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Round Robin Example
Process Burst Time
P1 53 — 33 — 13
P2 17
P3 68 — 48 — 28 — 8
P — 4
The Gantt chart with quantum 20 is: P1 P2 P3 P4 20 37 57 77 97
117
121
134
154
162
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Round Robin Example
Process Turnaround Time Waiting Time
P – 53 = 81
P – 17 = 20
P – 68 = 94
P – 24 = 97
Average waiting time = 73
Average waiting time for SJF = 38
Typically, higher average turnaround than SJF, but better response.
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11. Quantum vs Context Switch
Process Time = 10
Quantum Context
Switches
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12. Turnaround Time vs Quantum
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Multilevel Queues
Ready queue is partitioned into separate queues: - foreground (interactive) - background (batch)
Each queue has its own priority and scheduling algorithm: - foreground – RR - background – FCFS
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Multilevel Queues
Scheduling must be done across queues.
Fixed priority scheduling; i.e., serve all from foreground then from background.
Time slice – each queue gets a certain percentage of CPU time, e.g., 80% to foreground in RR and 20% to background in FCFS
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Multilevel Queues
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Recap of Lecture
SJF is optimal
Round-Robin scheduling
Multi-level queues scheduling
20 November 2018
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