A representation of an operating system is called a model. A model which is used to represent the behavior of a system in terms of its performance is called a performance model. Performance models are used predict the values of performance metrics of a system from a set of operating system parameters and workload parameters. Examples of performance measures are response time, throughput, utilization and queue length.

Depending on the nature of the transactions, models are classified as "Open", "Closed" and "Mixed" models.

• An open system model has an infinite customer population. Customers arrive to the system, receive service, and exit. The number of customers in the system is not bounded.
• A closed system model is used to represent a backlogged situation in which a departing customer is immediately replaced by an equivalent arriving customer. The number of customers in the system at any time is fixed.
• A mixed model is a combination of two separate submodels, a closed model and an open model, that share the common resources of a computer system.

Models are further classified as "Single Class" and "Multiple Class" models.

• In the single class, there exists only one class of transactions or jobs. Here, all transactions are indistinguishable with respect to their device service demands and routing behavior.
• In the multiple class, the workload consists of several types of transactions. Here, transactions may be partitioned into different classes. Each class has its unique service demands and routing behavior.

This OS Performance workbench demonstrates the Single Class performance model for Open and Closed algorithms. When the 'Open' button is pressed, the diagram of the computer system for the Open model is presented along with the facility of user input in the Open case. Once the user has provided the values, pressing the 'Solve' button will result in the calculation and display of the performance measures. This calculation is done using the algorithm for the Open model. Similarly, when the 'Closed' button is pressed, the diagram of the computer system for the Closed model is presented along with the facility of user input in Closed case. Once the user has provided the values, pressing the 'Solve' button will result in the calculation and display of the performance measures. This calculation is done using the algorithm for the Closed model. Pressing the 'Reset' button will set the workbench back to the initial configuration.