This document presents an overview of priority scheduling algorithms. It discusses basic concepts of scheduling, criteria like CPU utilization and turnaround time. It provides an example of priority scheduling with and without preemption. Advantages include ease of use and suitability for varying time requirements. A disadvantage is potential indefinite blocking of low priority processes.

1. Welcome
to Our
Presentation

2. Priority
Scheduling
Algorithms
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3. Submitted by-
Name ID
Asikul Islam 143-15-
Kanchon Kumar 143-15-
Saiful Islam 143-15-
Sonot Kumar 143-15-
Sohel Al Mamun 143-15-
Tuhinur Rahman 143-15-
Muine al Yamine
Submitted to-
Dept. Of CSE

4. Overview 4
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Example of Priority Scheduling
 Advantages & Disadvantages

5. Basic Concepts
 Main objective of multiprogramming is to keep on
running processes all the time for maximum CPU
utilization.
 Scheduling is fundamental function of OS.
 The task of selecting the processes in memory that are
ready to execute, and allocating them to the CPU is
performed by the CPU Scheduler.
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6. CPU Scheduler
 CPU scheduling decisions may take place when
a process:
o 1. Switches from running to waiting state
o 2. Switches from running to ready state
o 3. Switches from waiting to ready
o 4. Terminates
 Scheduling under 1 and 4 is non preemptive.
 All other scheduling is preemptive.
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7. CPU Scheduler
 Nonpreemptive
Once a process is allocated the CPU, it does
not leave unless:
o it has to wait, e.g., for I/O request
o it terminates
 Preemptive
o OS can force (preempt) a process from CPU at anytime
o E.g., to allocate CPU to another higher-priority process
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CONT…

8. Scheduling Criteria
 CPU utilization: keep the CPU as busy as possible
◦ Maximize
 Throughput: No of processes that complete their
execution per time unit
◦ Maximize
 Turnaround time: amount of time to execute a
particular process (time from submission to
termination)
◦ Minimize
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9. Scheduling Criteria 9
CONT…
 Waiting time: amount of time a process has been
waiting in the ready queue (sum of time waiting in
ready queue)
o Minimize
 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)
o Minimize

10. Scheduling Algorithms
 First Come, First Served
 Shortest Job First
 Priority
 Round Robin
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11.  A priority number (integer) is associated with each
process.
 Lager the CPU burst lower the priority.
 The CPU is allocated to the process with the highest
priority (smallest integer ≡ highest priority)
 Starvation (Infinity blocking): low priority processes
may never execute.
 Aging: as time progresses increase the priority of the
process.
Priority 11
CONT…

12. Example of Priority Scheduling (Non-
Preemptive) 12
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
 Gantt Chart
 Average waiting time = (6 + 0 + 16 + 18 + 1)/5 = 8.2
 Average Turn Around Time= (1+6+16+18+19)/5 = 12
P2 P1P5
61 160
P3
18
P4
19

13. Example of Priority Scheduling
(Preemptive)
Process Arrival Time Burst Time Priority
P1 0 5 2
P2 4 8 1
P3 6 2 4 p4
8 6 3
Average WT: ([(0-0)+(12-4)]+(4-4)+(19-6)+(13-8))/4
= (8+0+13+5)/4 = 6.5
Average TAT: ((5+8)+(8+0)+(2+13)+(6+5))/4
= (13+8+15+11)/4 = 47/4 = 11.75
P1
P4P2
4 190 13
P3
2112
P1

14. Advantages of Priority
 Easy to use
 User friendly
 Aging :- As time increases , increase in the priority
of a process .
 Simplicity .
 Suiteble for aplications with varying time and
resource requirement .

15. Disadvantages of Priority
 If system eventually crashes , all low priority
processes get lost .
 Indefinite blocking or Starvation .

16. Preemptive vs nonpreemptive
schedulingCPU scheduling decisions may take place when a process:
1. switches from running to waiting state
e.g., I/O request
2. switches from running to ready state
e.g., when interrupt or timeout occurs
3. switches from waiting to ready
e.g., completion of I/O
4. Terminates
scheduling under 1 and 4 is nonpreemptive
 once a process starts, it runs until it terminates or willingly gives up control
simple and efficient to implement – few context switches
examples: Windows 3.1, early Mac OS
all other scheduling is preemptive
 process can be "forced" to give up the CPU (e.g., timeout, higher priority process)
more sophisticated and powerful
examples: Windows 95/98/NT/2K, Mac OS-X, UNIX

17.  Preemptive vs Non-Preemptive Scheduling
 Scheduling is non-preemptive if once the CPU has been allocated
to a process, the process can keep the CPU until it releases it,
either by terminating or switching to the waiting state.
 Scheduling is preemptive if the CPU can be taken away from a
process during execution.

18. Priority scheduling
each process is assigned a numeric priority
 CPU is allocated to the process with the highest priority
 priorities can be external (set by user/admin) or internal
 (based on resources/history)
 SJF is priority scheduling where priority is the predicted
 next CPU burst time
priority scheduling may be preemptive or nonpreemptive
 priority scheduling is not fair
 starvation is possible – low priority processes may never execute
 can be made fair using aging – as time progresses, increase the
priority
 Aging is a technique of gradually increasing the priority of
processes that wait in the system for a long time