This document discusses CPU scheduling and scheduling criteria. It covers the CPU scheduler or short term scheduler which selects processes from the ready queue to allocate the CPU to. It describes various scheduling criteria like CPU utilization, throughput, turnaround time, waiting time and response time that are used to compare scheduling algorithms. The goal of scheduling is to always keep the CPU busy while maximizing throughput and minimizing waiting times and turnaround times of processes.

1. Understand process management
• CPU Scheduling and Scheduling Criteria
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2. Recap
In the last class, you have learnt
• Scheduling Concepts
• Types of scheduling queues namely
– Ready queue
– Device queue
• Types of schedulers namely
– Short term scheduler
– Medium term scheduler
– Long term scheduler
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4. Objective
On completion of this class, you would be able
to know
• About CPU scheduling
• Scheduling Criteria
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5. CPU Scheduling
•Uniprocessors, only one process running at all the time
If there are more processes they must wait
•The objective of multiprogramming is
Several processes are kept in memory at a time
Run a process at all time
Maximize CPU utilization
•Idea of multiprogramming is simple,
A process is executed until it must wait, for an I/O or an event
The OS takes away processor from that process and gives it to
another process 5

6. CPU Scheduler
• Also called short term scheduler
• The CPU scheduler selects a process from a ready queue
i.e.,
Ready to execute
CPU is allocated it to
• Ready queue need not be a FIFO queue
• Ready queue may be implemented as a FIFO, Priority, tree
or simply linked list
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7. Scheduling Criteria
• Many criteria exists for comparing various
scheduling algorithms namely
• CPU utilization
• Throughput
• Turn around Time
• Waiting Time
• Response Time
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8. Scheduling Criteria
CPU Utilization
• Always keep CPU busy
• Utilization ranges from 0-100%
• In real systems utilization ranges from 40% ( lightly
loaded systems ) to 90% for heavily loaded systems
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9. Scheduling Criteria
Throughput :
• Number of processes completed per time unit
• This rate may be,
– One process per hour for long process
– 10 to 12 processes per second for short process
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10. Scheduling Criteria
Turn Around Time:
• The interval of time between submission of a
process and to the time of completion of a process
• It is the sum of the periods spent by a process
– Waiting to get memory
– Waiting in ready queue
– Executing on the CPU
– Doing I/O
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11. Scheduling Criteria
Waiting Time
• Sum of the periods spent by a job waiting in the ready
queue
Response Time
• It is a measure of time from submission of a request
until time of first response
• i.e., it is the amount of time it takes to start responding,
– But not the time that it takes to output that response
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12. Summary
In this class, you have learnt
• CPU scheduling
• Various scheduling criteria
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13. Frequently Asked Questions
1. Explain the role of CPU scheduler
2. List and explain various scheduling criteria
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14. Quiz
1. CPU scheduler is also called short term scheduler
a) True
b) False
2. Response time is always equal to Turn around
time
a) True
b) False
3. We always want to keep the CPU busy
a) True
b) False 14

15. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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