Introduction to Operating Systems

Transcription

1 Module- 1 Introduction to Operating Systems by S Pramod Kumar Assistant Professor, Dept.of ECE,KIT, Tiptur Images 2006 D. M.Dhamdhare 1

2 What is an OS? Abstract views To a college student: S/W that permits access to the Internet To a programmer: S/W that makes it possible to develop programs on a computer system To a user of an application package: S/W that makes it possible to use the package To a technician in a computerized chemical plant: invisible component of a computer system that controls the plant An abstract view focuses only on essential characteristics Operating system designer also has an abstract view OS is a collection of routines that facilitates execution of user programs and use of resources 2

3 We use abstract views to present design of OS components. It has two benefits: 1. Managing complexity Abstract view contains only selected features of a system 2. Presenting a generic view For example, user interface has many variants in practice Command line interface Graphical user interface (GUI) Logical and physical organization 3

4 Fundamental goals of an operating system 1) Efficient use of computer resources 2) User convenience 3) Noninterference in the activities of its users When these goals conflict, designer makes a trade-off for Efficient use User convenience Notion of effective utilization Each OS provides a different flavor of effectiveness OS ensures efficient use of memory, CPU, and I/O devices Poor efficiency can result if a program does not use a resource allocated to it OS itself consumes CPU and memory resources, which constitutes overhead It reduces resources for user programs OS can monitor use of resources to ensure efficiency OS uses policies that ensure efficiency 4

5 User can face interference in computational activities Illegal file access OS knows which user files can be accessed by whom Achieved through authorization 5

6 Principal functions of OS: Program management Resource management Security and protection Concern OS responsibility Programs Initiation and termination of programs. Providing convenient methods so that several programs can work towards a common goal. Resources Protection Ensuring availability of resources in the system and allocating them to programs. Protect data and programs against interference from other users and their programs. 6

7 1. A Single program A single program computation consists of execution of a program on a single set of data. 2. A sequence of programs Each single program in the sequence is initiated by a user through a separate command. 3. Co-executing programs These programs have to execute at a same and interact during their execution. Resource allocations and deallocations can be done with a resource table Entry: name, address and status of a resource unit 7

8 Popular resource allocation strategies: 1. Resource partitioning OS decides a priori what resources to allocate to each user program; Resource table contains entries for partitions Simple to implement, but lacks flexibility 2. Pool-based OS allocates resources from a pool of resources Consults table and allocates the resource if it is free Less overhead of allocating and deallocating resources Achieves more efficient use of resources Resource preemption- sequential & concurrent sharing OS can interleave execution of programs on a fast CPU Scheduling decides which program should be given the CPU at any time Policy influences efficient CPU use and user service Preemption: Taking away the CPU from a program 8

9 A virtual resource is a fictitious resource Supported by OS through use of a real resource Same real resource may support several virtual ones Provides effect of having more resources Most OSs provide virtual memory May execute a program bigger than size of RAM Some OSs create virtual machines Each virtual machine can be allocated to a user Security counters threats of interference or illegal use posed by persons/programs outside OS control Authentication: only registered user can use a computer system Protection counters threats posed by users of an OS Ex: Memory protection is a HW feature used by OS to thwart disruption of programs and OS services 9

10 Intruders are outsiders who can cause interference May use or create malicious programs Trojan horses Viruses Worms Methods of addressing security threats Authentication techniques Internet firewalls 1) Nature of computation in an OS 2) Measuring Efficiency, System Performance, and User Convenience 10

11 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 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 11

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13 Batch: sequence of user jobs formed for processing by the OS Batching kernel initiates processing of jobs without requiring computer operator s intervention Control statements used to protect against interference between jobs 13

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15 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 15

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17 In multiprogramming environments, an I/O-bound program should have a higher priority than a CPU-bound program. 17

18 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 18

19 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 9/14/17 Swapping allows the kernel to service more processes than can fit into the memory Kernel performs swap-out and swap-in operations 19

20 In real-time applications, users need computer to perform some actions in a timely manner Timeliness depends on time constraints If application takes too long to respond to an activity, a failure can occur. Response requirement Deadline: time by which action should be performed 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 20

21 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 Handling network or individual computers failure requires special techniques Users must use special techniques to access resources over the network Many resources of a kind exist in system This feature is used to provide the benefits summarized in Table

22 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 22

23 Evolution (continued) Real-time operating systems Priority-based scheduling and deadline-aware scheduling Distributed operating system Lets programs share resources across network 23

24 1. Define operating system. Explain the functions of an operating system. 2. What are the two goals of an operating system? Explain briefly. 3. Briefly explain the different classes of operating systems, specifying the prime concern and key concepts used. 4. What are the operations performed by Kernel when an interrupt occurs. 5. Why I/O bound programs should be given higher priorities in a multiprogramming environment? Illustrate with timing diagram. 6. The payroll program reads the monthly attendance details of 1000 employees and prints their payroll. Reading of a record from card and printing of a line consumes 100 msec, while a read or write operation on a disk consumes 10 msec. The salary processing consumes 5 msec of CPU time per employee. Compute the program elapsed time and CPU idle time with and without spooling. 7. Explain the concept of VMOS, with example. 24