Chapter 3

Overview of Operating Systems

Copyright © 2008

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Introduction

• Computing Environments and Nature of Computations• Classes of Operating Systems• Efficiency, System Performance, and User Service• Batch Processing Systems

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Introduction (continued)

• Multiprogramming Systems• Time-Sharing Systems• Real-Time Operating Systems• Distributed Operating Systems• Modern Operating Systems

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Computing Environments and Nature of Computations

• A computing environment consists of a computer system, its interfaces with other systems, and the services provided by its operating system to its users and their programs

• Evolution:– Noninteractive Computing Environments

– Interactive Computing Environments

– Real-Time, Distributed, and Embedded Environments

– Modern Computing Environments

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Computing Environments and Nature of Computations (continued)

• Noninteractive Computing Environments– OS focuses on efficient use of resources

– Computations in form of program or job

• Interactive Computing Environments– OS focuses on reducing average amount of time required

to implement an interaction between a user and his computation

– Execution of a program is called a process

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Computing Environments and Nature of Computations (continued)

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Computing Environments and Nature of Computations (continued)

• Real-Time, Distributed, and Embedded Environments– A real-time computation has specific time constraints

• OS ensures computations complete within constraints

– Distributed computing environment: enables a computation to use resources located in several computer systems through a network

– Embedded computing environment: computer system is a part of a specific hardware system

• OS has to meet the time constraints arising from the nature of the system being controlled

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Computing Environments and Nature of Computations (continued)

• Modern Computing Environments– Has features of several of the computing environments

described earlier• OS uses complex strategies to manage user computations

and resources

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Classes of Operating Systems

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Efficiency, System Performance, and User Service

• Two of the fundamental goals of an OS:– Efficiency of use

• Of a resource

– User convenience• Measurable aspect: User service

– Turnaround time

– Response time

• To a system administrator, performance of a system in its environment is more important– Typically measured as throughput

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Efficiency, System Performance, and User Service (continued)

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Batch Processing Systems

• Batch: sequence of user jobs formed for processing by the OS

• Batching kernel initiates processing of jobs without requiring computer operator’s intervention

• Card readers and printers were a performance bottleneck in the 1960s– Virtual card readers and printers implemented through

magnetic tapes were used to solve this problem• Control statements used to protect against interference

between jobs• Command interpreter read a card when currently

executing program in job wanted the next card

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Multiprogramming Systems

• Provide efficient resource utilization in a noninteractive environment

• Uses DMA mode of I/O– Can perform I/O operations of some program(s) while

using the CPU to execute some other program• Makes efficient use of both the CPU and I/O devices

• Turnaround time of a program is the appropriate measure of user service in these systems

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Multiprogramming Systems (continued)

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Multiprogramming Systems (continued)

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Multiprogramming Systems(continued)

• An appropriate measure of performance of a multiprogramming OS is throughput– Ratio of the number of programs processed and the total

time taken to process them

• OS keeps enough programs in memory at all times, so that CPU and I/O devices are not idle– Degree of multiprogramming: number of programs

– Uses an appropriate program mix of CPU-bound programs and I/O-bound programs

– Assigns appropriate priorities to CPU-bound and I/O-bound programs

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Priority of Programs

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Priority of Programs (continued)In multiprogramming environments, an I/O-bound program should have a higher priority than a CPU-bound program.

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Performance of Multiprogramming systems

• How to improve performance?

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Performance of Multiprogramming systems (continued)

When an appropriate program mix is maintained, an increase in the degree of multiprogramming would result in an increase in throughput.

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Time-Sharing Systems

• Provide a quick response to user subrequests– Round-robin scheduling with time-slicing

• Kernel maintains a scheduling queue• If time slice (δ) elapses before process completes servicing

of a subrequest, kernel preempts it, moves it to end of queue, and schedules another process

– Implemented through a timer interrupt

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Time-Sharing Systems (continued)

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Time-Sharing Systems (continued)

• Response time (rt): measure of user service– If processing of a subrequest requires δ CPU seconds

rt = n × (δ + σ)η = δ / (δ + σ)

where η: CPU efficiency, σ: scheduling overhead, n: number of users using system, δ: time required to complete a subrequest

• Actual response time would be different because– Some users may be inactive– Some programs may require > δ CPU seconds

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Time-Sharing Systems (continued)

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Swapping of Programs

• Kernel performs swap-out and swap-in operations

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Real-Time Operating Systems

• In real-time applications, users need computer to perform some actions in a timely manner– To control activities in an external system, or to

participate in them

– Timeliness depends on time constraints

• If application takes too long to respond to an activity, a failure can occur in the external system– Response requirement

– Deadline: time by which action should be performed

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Hard and Soft Real-Time Systems

• A hard real-time system meets response requirements under all conditions– It is typically dedicated to processing real-time

applications

• A soft real-time system makes best effort to meet response requirement of a real-time application– Cannot guarantee that it will be able to meet it

• Meets requirements in a probabilistic manner

– E.g., multimedia applications

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Features of a Real-Time Operating System

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Distributed Operating Systems

• A distributed computer system consists of several individual computer systems connected through a network– Each computer system could be a PC, a multiprocessor

system, or a cluster

– Many resources of a kind exist in system• This feature is used to provide the benefits summarized in

Table 3.8

– Handling network or individual computers’ failure requires special techniques

– Users must use special techniques to access resources over the network

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Distributed Operating Systems (continued)

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Special Techniques of Distributed Operating Systems

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Modern Operating Systems

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Summary

• A computing environment consists of a computer system, its interfaces with other systems, and the services provided by its OS to users and programs– Evolved with advances in computer technology:

• Batch processing systems• Multiprogramming operating system

– Priority-based scheduling

• Time-sharing operating systems– Round-robin scheduling with time-slicing

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Summary (continued)

• Evolution (continued)• Real-time operating systems

– Priority-based scheduling and deadline-aware scheduling

• Distributed operating system– Lets programs share resources across network

• Modern operating system – Modern computing environment has elements of all the

classic computing environments

– Uses different techniques for different applications