Chapter 3
Overview of Operating Systems
Copyright © 2008
Introduction
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•
•
•
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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3.3
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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:
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–
–
–
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/Obound 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
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Copyright © 2008
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