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System Models

System Models. Advanced Operating Systems Nael Abu-halaweh. Contents Introduction. Workstation Model. Processor Pool Model. Hybrid Model. Clustering. Introduction Traditional Systems have only one processor. Processors in Distributed Systems can be organized in several ways.

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System Models

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  1. System Models Advanced Operating Systems Nael Abu-halaweh

  2. Contents • Introduction. • Workstation Model. • Processor Pool Model. • Hybrid Model. • Clustering.

  3. Introduction • Traditional Systems have only one processor. • Processors in Distributed Systems can be organized in several ways.

  4. The Workstation Model • This System consists of workstation connected by a high-speed LAN. • In some system workstations have local disks (diskful or disky workstation. • In others, workstation do not have disks (diskless). • When using diskless workstations, the file system is implemented on one or more file servers.

  5. Advantages of Diskless Workstations • Cheaper. • Ease of Maintenance. • Easier to backup. • Symmetry & flexibility.

  6. Disky Workstations • Workstation have private disks. • Local Disks can be used in the following ways: 1- Paging and Temporary Files. 2- Paging, Temporary Files & System Binaries. 3- Paging, Temporary Files, System Binaries & File Caching. 4- Complete Local File System.

  7. Advantages of the Workstation Model • The model is easy to Understand. • Users have guaranteed Response Time. • Sophisticated Graphics Programs can run very Fast. • Users have large degree of autonomy.

  8. Disadvantages of the Workstation Model • Occupy more space. • Inefficient Resource Allocation (Some Station may be idle while others demand more resources).

  9. Using Idle Workstations • At peak period 30% of the workstations are idle. • A variety of schemes have been proposed for using idle or underutilized workstation. • First attempt: the use of rsh program that comes with Berkeley Unix. • Major Flaws in rsh: 1- The user has to identify the Remote Workstation. 2- Programs execute in the Remote Machine environment. 3- User logging to the machine should accept lower performance.

  10. Using Idle Machine Key Issues • How is an Idle Ws. found? • How can a remote process be run transparently? • What happens if the machine owners comes back?

  11. Finding an Idle Machine • If no user-initiated processes are running on the machine it is considered idle. • There are substantial differences in load between idle machines. • Two categories of algorithms are used to locate idle workstations: Server Driven & Client Driven.

  12. Server Driven • Idle Station announces its availability. • To do this it registers its name, network address & properties in a registry or a database. • When a remote command is executed it looks into the registry to find a suitable Ws. • For reliability it is better to have more than one copy of the database.

  13. Server Driven (Alternative Approach) • Each Ws. has its own copy of the registry file. • Newly available Ws. broadcasts its availability on the network. • Upon receiving the idle Ws. broadcast; each client updates its registry. • Advantages: Less overhead in finding an idle Ws. & fault tolerance. • Disadvantages: All machines have to maintain their copies.

  14. Client Driven • The client looking for an idle Ws. broadcasts the request which includes the program & the resources it needs. • All available workstations reply back to the requesting client. • The requesting client picks one.

  15. Running a Remote Process Transparently • Finding an idle Ws is only the first step. • The remote environment should be set up such that it looks like the environment on the home computer. • A process needs the same file system view, working directory & environment variables. • Some system calls must be forwarded back to the home machine. (example: Keyboard) • System calls that query the state of the machine must be done on the machine on which the process is running.

  16. Processor Pool Model • A rack full of CPUs. • CPUs are allocated to users on demand. • Users are given high performance graphic terminals. • Advantages: Reducing Cost/performance, Allows easy incremental growth. • Based on the queuing theory: a Processor pool with n processors will reduce the average response time by n-fold compared to that of n personal workstations.

  17. Why have distributed Systems? • The queuing theory analysis showed that the mean response time centralized system is n times better than an n distributed single processor systems. • Advantages of Distributed systems: 1- Price/ performance. 2- Reliability. 3- Fault tolerance. 4- Uniform Response.

  18. Workstation Vs. Processor Pool Model • Processor pool is a better way of getting extra computing power. • Workstation model is suitable for users with light applications, while Processor Pool can handle large applications.

  19. Hybrid Model • A hybrid system consists of workstations & processor pool/s. • Combines the advantages of both models. • Advantages: Fast Interactive Response, Efficient Use of Resources & Simple Design.

  20. Clustering • A cluster is a group of computers working together to provide a common set of applications. • A cluster appears to the clients as a unified system. • Advantages: Load Balancing, Fail-Over & Scalability.

  21. References • A. Tanenbaum, “Distributed Operating Systems”, Prentice Hall, 1995. • “What’s New in Clustering Technologies” obtained from: http://www.microsoft.com/windowsserver2003/evaluation/overview/technologies/clustering.mspx • Filip Hanik, “Clustering Technologies in Memory Session Replication in Tomcat 4” obtained from: http://www.theserverside.com/articles/article.tss?l=Tomcat .

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