CPU scheduling is the basis of multi-programmed operating systems. By switching the CPU among processes, the operating system can make the computer more productive - To introduce CPU scheduling, which is the basis for multi-programmed operating systems - To describe various CPU-scheduling algorithms - To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system - To examine the scheduling algorithms of several operating systems

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

2. • Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• Thread Scheduling
• Multiple-Processor Scheduling
• Real-Time CPU Scheduling
• Operating Systems Examples
• Algorithm Evaluation
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3. Objectives
CPU scheduling is the basis of multiprogrammed operating systems. By
switching the CPU among processes, the operating system can make the
computer more productive
• To introduce CPU scheduling, which is the basis for multiprogrammed
operating systems
• To describe various CPU-scheduling algorithms
• To discuss evaluation criteria for selecting a CPU-scheduling algorithm for
a particular system
• To examine the scheduling algorithms of several operating systems
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4. Basic Concepts
• In a single-processor system, only one process can run at a time
• Others must wait until the CPU is free and can be rescheduled
• The objective of multiprogramming is to have some process running at
all times, to maximize CPU utilization
• A process is executed until it must wait, typically for the completion of
some I/O request
• In a simple computer system, the CPU then just sits idle. All this waiting
time is wasted; no useful work is accomplished
• With multiprogramming, we try to use this time productively, maximum
CPU utilization
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5. Basic Concepts
• With multiprogramming, we try to use this time productively
• Several processes are kept in memory at one time
• When one process has to wait, the operating system takes the CPU
away from that process and gives the CPU to another process
• This pattern continues...
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6. Basic Concepts
• Process execution consists of a
cycle of CPU execution and I/O
wait - CPU - I/O Burst Cycle
• CPU Burst - when the process is
being executed in the CPU
• I/O Burst - when the CPU is
waiting for I/O for further exection
• CPU burst followed by I/O burst
• Eventually, the final CPU burst
ends with a system request to
terminate execution
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7. Histogram of CPU-burst Times
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• An I/O-bound program typically has many short CPU bursts
• A CPU-bound program might have a few long CPU bursts.

8. CPU Scheduler
• Short-term/CPU scheduler selects from among the processes in ready
queue, and allocates the CPU to one of them
• Queue may be ordered in various ways
• CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
• For situations 1 and 4, there is no choice in terms of scheduling
A new process (if one exists in the ready queue) must be selected for
execution
• However, there is a choice for situations 2 and 3
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9. CPU Scheduler
• When scheduling takes place only under circumstances 1 and 4, we say
that the scheduling scheme is nonpreemptive or cooperative
• Otherwise, it is preemptive
• Under nonpreemptive scheduling, once the CPU has been allocated to
a process, the process keeps the CPU until it releases the CPU either by
terminating or by switching to the waiting state
• Under preemptive scheduling, CPU has been taken away from a
process, before the process has completed its execution or before it is
swicting to the waiting state
• Scheduling under 1 and 4 is nonpreemptive
• All other scheduling is preemptive
• Consider access to shared data
• Consider preemption while in kernel mode
• Consider interrupts occurring during crucial OS activities
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10. Dispatcher
• Dispatcher module gives control of the CPU to the process selected by
the short-term scheduler; this involves:
• switching context
• switching to user mode
• jumping to the proper location in the user program to restart that
program
• Dispatch latency – time it takes for the dispatcher to stop one process
and start another running
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11. Scheduling Criteria
• CPU utilization –
• keep the CPU as busy as possible
• Conceptually, CPU utilization can range from 0 to 100 percent
• In a real system, it should range from 40 percent (for a lightly loaded
system) to 90 percent (for a heavily loaded system).
• Throughput –
• Number of processes that complete their execution per time unit
• For long processes, this rate may be one process per hour
• For short transactions, it may be ten processes per second
• Turnaround time –
• amount of time to execute a particular process
• The interval from the time of submission of a process to the time of
completion is the turnaround time
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12. Scheduling Criteria
• Turnaround time –
• Turnaround time is the sum of the periods spent waiting to get into
memory, waiting in the ready queue, executing on the CPU, and doing
I/O
• Waiting time –
• The CPU-scheduling algorithm does not affect the amount of time
during which a process executes or does I/O
• It affects only the amount of time that a process spends waiting in the
ready queue
• Waiting time is the sum of the periods spent waiting in the ready
queue
• 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)
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13. Scheduling Algorithm Optimization Criteria
• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time
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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