Seminar On:

Time Sharing And Distributed System

Under guidance of    S.B. Kulkarni

By :   Jayalaxmi.R.Hanni            IAR Mtech             Mech Dept

Contents:

Time Sharing.  Distributed 

Time-sharing systems: To use computer system resources efficiently, multiprogramming was introduced. The idea is to hold several jobs in memory at a time, and only assign a resource to a job that needs it on the condition that the resource is available.

Multiprogramming brought the idea of time sharing: resources could be shared between different jobs, with each job being allocated a portion of time to use a resource. Time sharing is hidden from the user each user has the impression that the whole system is serving them exclusively.

Features of Time Sharing:

In  a  time  sharing  system,  multiple  users simultaneously  access  the  system  through terminals,  with  the  operating  system interleaving  the  execution  of  each  user  program in a short burst of computation.

Time  sharing,  or  multitasking,  is  a  logical extension of multiprogramming.

Multiple jobs are executed by switching the CPU between them.

The CPU time is shared by different processes, so it is called as “Time sharing Systems”.

Time slice is defined by the OS, for sharing CPU time between processes.

Examples: Multics, Unix, etc.

The time sharing system provides the direct access to a large number of users where CPU time is divided among all the users on scheduled basis.

 The OS allocates a set of time to each user. When this time is expired, it passes control to the next user on the system.

 The time allowed is extremely small and the users are given the impression that they each have their own CPU and they are the sole owner of the CPU. 

This short period of time during that a user gets attention of the CPU; is known as a time slice or a quantum. The concept of time sharing system is shown in figure. 

In figure the user 5 is active but user 1, user 2, user 3, and user 4 are in waiting state whereas user 6 is in ready status.

As soon as the time slice of user 5 is completed, the control moves on to the next ready user i.e. user 6. In this state user 2, user 3, user 4, and user 5 are in waiting state and user 1 is in ready state. The process continues in the same way and so on.

The time-shared systems are more complex than the multi-programming systems. In time-shared systems multiple processes are managed simultaneously which requires an adequate management of main memory so that the processes can be swapped in or swapped out within a short time.

ADVANTAGES:

It refers to allocation of computer resources in time dependent fashion to several user simultaneously.

It provides a large number of users with direct access to computer for solving their problems.

In time sharing system the CPU time is divided along all user and a fixed time is allowed to each user. The programming speed of the system allows the CPU to switch from one user to another user at a very fast speed giving an illusion to each user that only his program is running.

DISADVANTAGES:

In this system only one program can be in control of the CPU at a given instance so all the programs will fall in one of the following stages.

Stages of Time Sharing:

ACTIVE STAGE

   User’s program has got control on the CPU. Only one  program can be active at given instruction of time. READY STAGE

   User’s program is ready to execute but it is waiting for its turn to get the CPU time. WAITING STAGE

   User has made no requirement for execution of his program and is waiting for completion of same input/output operations.

DISTRIBUTED SYSTEMS

A distributed system is a collection of autonomous computers linked by a computer network that appear to the users of the system as a single computer.

Features of Distributed System: [july’13]

Distributed system is collection of loosely coupled processors interconnected by a communications network

Processors variously called nodes, computers, machines, hosts (Site is location of the processor).

Heterogeneity.

Openness.

Security.

Scalability.

Failure Handling.

Concurrency.

Transparency.

Heterogeneity

Variety and differences inNetworksComputer hardwareOperating systemsProgramming languages Implementations by different developers

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Openness

Openness is concerned with extensions and improvements of distributed systems.

Detailed interfaces of components need to be published.

New components have to be integrated with existing components.

Differences in data representation of interface types on different processors (of different vendors) have to be resolved.

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Security

In a distributed system, clients send requests to access data managed by servers, resources in the networks: Doctors requesting records from hospitalsUsers purchase products through electronic commerce

Security is required for: Concealing the contents of messages: security and 

privacy Identifying a remote user or other agent correctly 

(authentication)

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Scalability

Adaptation of distributed systems to

accommodate more users

respond faster (this is the hard one)

Usually done by adding more and/or faster processors.

Components should not need to be changed when scale of a system increases.

Design components to be scalable!

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Failure Handling (Fault Tolerance)

Hardware, software and networks fail!

Distributed systems must maintain availability even at low levels of hardware/software/network reliability.

Fault tolerance is achieved by

recovery

redundancy 17

Concurrency

Components in distributed systems are executed in concurrent processes.

Components access and update shared resources (e.g. variables, databases, device drivers).

Integrity of the system may be violated if concurrent updates are not coordinated.

Lost updates

Inconsistent analysis

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Transparency

Distributed systems should be perceived by users and application programmers as a whole rather than as a collection of cooperating components.

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Types of Distributed Operating Systems

Network Operating Systems Distributed Operating Systems

Network­Operating Systems

Users are aware of multiplicity of machines.  Access to resources of various machines is done explicitly by: Remote logging into the appropriate remote machine 

Remote Desktop (Microsoft Windows)

Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism

Distributed­Operating Systems

Users not aware of multiplicity of machines   Access to remote resources similar to access to local 

resources

Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task

Computation Migration – transfer the computation, rather than the data, across the system

Distributed­Operating Systems (Cont.)

Process Migration – execute an entire process, or parts of it, at different sites Load balancing – distribute processes across network 

to even the workload

Computation speedup – sub processes can run concurrently on different sites

Hardware preference – process execution may require specialized processor

Software preference – required software may be available at only a particular site

Data access – run process remotely, rather than transfer all data locally

Network Structure

Local­Area Network (LAN) – designed to cover small geographical area. Multi access bus, ring, or star network

Speed ≈ 10 – 100 megabits/second

Broadcast is fast and cheap

Nodes:  usually workstations and/or personal computers 

a few (usually one or two) mainframes

 Depiction of typical LAN

Network Types (Cont.)

Wide­Area Network (WAN) – links geographically separated sites Point­to­point connections over long­haul lines (often 

leased from a phone company)

Speed ≈ 1.544 – 45  megabits/second

Broadcast usually requires multiple messages

Nodes:  usually a high percentage of mainframes

Communication Processors in a Wide­Area Network

Robustness

Failure detection

Reconfiguration

Failure Detection

Detecting hardware failure is difficult To detect a link failure, a handshaking protocol 

can be used Assume Site A and Site B have established a link

 At fixed intervals, each site will exchange an I­am­up message indicating that they are up and running

If Site A does not receive a message within the fixed interval, it assumes either (a) the other site is not up or (b) the message was lost

Site A can now send an Are­you­up? message to Site B

If Site A does not receive a reply, it can repeat the message or try an alternate route to Site B

If Site A does not ultimately receive a reply from Site B, it concludes some type of failure has occurred

Types of failures:­ Site B is down

­ The direct link between A and B is down­ The alternate link from A to B is down

­ The message has been lost

However, Site A cannot determine exactly why the failure has occurred

Reconfiguration

When Site A determines a failure has occurred, it must reconfigure the system: 

1. If the link from A to B has failed, this must be broadcast to every site in the system

2. If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available

When the link or the site becomes available again, this information must again be broadcast to all other sites

Advantages and Disadvantages:

Advantages Economic.

Speed.

Inherent distribution.

Reliability.

Incremental Growth.

Disadvantages:

Software.

Network.

Security.

More components to fail.

• Economics: a collection of microprocessors offer a better price/performance than mainframes. Low price/performance ratio: cost effective way to increase computing power.

• Speed: a distributed system may have more total computing power than a mainframe. Ex. 10,000 CPU chips, each running at 50 MIPS. Not possible to build 500,000 MIPS single processor since it would require 0.002 n sec instruction cycle. Enhanced performance through load distributing.

• Inherent distribution: Some applications are inherently distributed. Ex. a supermarket chain.

• Reliability: If one machine crashes, the system as a whole can still survive. Higher availability and improved reliability.

• Incremental growth: Computing power can be added in small increments. Modular expandability

• Another deriving force: the existence of large number of personal computers, the need for people to collaborate and share information.

Reasons for distributed systems

Resource sharing

sharing and printing files at remote sites

processing information in a distributed database

using remote specialized hardware devices

Computation speedup – load sharing.

Reliability – detect and recover from site failure, function transfer, reintegrate failed site.

Communication – message passing.

Previous year Questions: 

Explain the features of Distributed system and real time system[12m]

 Discuss time sharing and distributed system[10m]

 Write a short notes on Distributed OS[5m]