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.

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.

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

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.

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.

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

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.

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?

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

References A. Tanenbaum, “Distributed Operating Systems”, Prentice Hall, “What’s New in Clustering Technologies” obtained from: aluation/overview/technologies/clustering.mspx aluation/overview/technologies/clustering.mspx Filip Hanik, “Clustering Technologies in Memory Session Replication in Tomcat 4” obtained from: =Tomcat. =Tomcat