Distributed System Services Fall 2008 Siva Josyula

Outline  What is an Operating System?  Components of a computer system.  Why operating system services?  Services provided.  Other services provided.  References.

What is an Operating System??? A program acting as an intermediary between the user and the hardware. Purpose: To provide an environment, where user can execute programs in a convenient and efficient manner. The operating systems provides certain services to the users to make their tasks easier.

Components of a Computer System

Components Continued… Components of a computer-  Hardware  Operating System.  Application Programs.  Users.

Continued…  Hardware – provides basic computing resources like CPU, memory, I/O devices.  Operating system – OS controls and coordinates the use of the hardware among the various application programs for different users.  Applications programs – They define the ways in which the system resources are used to solve the computing problems of the users.  Users (people, machines, other computers).

Why Operating System Services?  OS provides an environment for the executions of the programs.  Services are provided for the convenience of the programmer, to make the programming task easier.  Services provided differ from one operating system to another operating system.

Services Provided The following are the services provided by the operating system.  Program Execution  I/O Operations  File-system manipulation  Communications

Continued…  Error Detection  Resource Allocation  Accounting  Protection

Program Execution v Operating system must be able to load a program into memory and to run that program. v The program must be able to end its execution, either normally or abnormally.

Services– I/O Operations  Running program may require I/O, which may involve a file or an I/O Device.  For some devices, special functions may be desired (like rewinding a tape drive or to blank a CRT screen)  For efficiency and protection purposes, the users usually are not able to control the I/O devices.  So it is the duty of the operating system to provide a means for the I/O since users cannot execute them directly.

File-System Manipulation  Programs need to read and write files.  Programs also need to create and delete files by name.  The operating system handles the process of creating, deleting and manipulating files.

Communications  Every process needs to exchange information with another process.  Communications may be implemented via shared memory or by the technique of message passing.

Continued…  Communications occurs in two ways. Within processes executing on the same computer. Between processes executing on different computers, tied together by a network.

Error Detection  Errors can happen any time in the system.  Errors can occur in the CPU,memory hardware,I/O Devices,and in the user program.  For each type of error,the operating system should take the appropriate action to ensure correct and consistent computing, by detecting the errors.

Resource Allocation v Multiple jobs run at the same time so resources must be allocated to each one of them. v Many different types of resources are managed by the operating system. v Some have special allocation code, whereas others have general request and release code. v The operating systems have various routines for different jobs in the system, for example CPU scheduling.

Accounting  We need to keep track of which user keeps track of how many and which kind of computer resources. v This tracking of user information can be used to find out the usage statistics. v These usage statistics are useful for the researchers who wish to re-configure the system to improve computing services.

Protection v Protection and Security is an important issue in multi-user computer systems. v Protection involves in ensuring that all access to system resources is controlled. v Protection of the system from outside users is also important. v Access to the resources must be authenticated with a password.

Other Services provided The other major services provided by the operating system are:  Memory Management  Process Management  CPU Scheduling  Primitive Services  Services by system servers  Value-added Services

Memory Management v The method of keeping track of how and where programs are loading into the main memory. v Memory is a large array of words or bytes, each with its own address. v It is a repository of quickly accessible data shared by the CPU and I/O devices.

Continued…  The operating system is responsible for the following activities : →Keeping track as to which parts of memory are currently being used and by which user. →Decide which processes need to be loaded when memory space becomes available. →Allocate and de-allocate memory space as and when it is required.

Process Management  A process is a program in execution.  Each process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task.  The OS is responsible for the following activities in connection with process management. →Process suspension and resumption. →Providing mechanisms for: process synchronization process communication

CPU Scheduling  The method of determining which process in memory is given access to the CPU so that it makes computational progress.  Two kinds of CPU Scheduling: →Preemptive Scheduling-Scheduling that occurs when the operating system decides to favor another process, preempting the currently executing process. →Non-Preemptive Scheduling-Scheduling that occurs when the currently executing process gives up the CPU voluntarily.

Primitive Services  Primitive services are those services that must be implemented in the kernel of each node in the system.  Communications is done through message passing, a defined set of send and receive primitives.  Process Servers manages creation, deletion and tracking of processes by allocating necessary resources.

Services by system servers.  Services that can be implemented anywhere in the system and performs the functions which is basic to the operation of a distributed system.  Name servers used to locate users, processes or machines.  Network servers used to give details about path selection, and routing information.  Migration servers, used to cooperate with process server and helps in moving of process form node to node.

Value-added services  Services that are not needed in the implementation of a Distributed System but useful in supporting Distributed Application  Group server, used for creation and termination of group activities.  Web Server, is good because of its simplicity.

References  Distributed operating systems & Algorithms, By Randy chow, Theodore Johnson  Operating System Concepts 6th ed - Silberschatz Galvin, Gagne.  Computer science illuminated, second edition, By Nell dale, John Lewis  Google.

THANK YOU