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Chapter 3 System Performance and Models

Chapter 3 System Performance and Models. 3.1 Introduction. A system is the part of the real world under study. Composed of a set of entities interacting among themselves and with the environment. A model is an abstract representation of a system.

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Chapter 3 System Performance and Models

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  1. Chapter 3System Performance and Models

  2. 3.1 Introduction • A system is the part of the real world under study. Composed of a set of entities interacting among themselves and with the environment. • A model is an abstract representation of a system. • The system behavior is dependent on the input data and actions from the environment.

  3. Abstraction • The most important concept in analysis and design • A high-level description of a collection of objects • Only the relevant properties and features of the objects are included.

  4. System A system has: • Structure • Behavior The model of a system is simpler than the real system in its structure and behavior. But it should be equivalent to the system.

  5. Using Models A user can: • Manipulate the model by supplying it with a set of inputs • Observe its behavior or output • Predict the behavior of the real system by analyzing the behavior of the model.

  6. Behavior of a Model Depends on: • The passage of time • Input data • Events generated by the environment

  7. 3.2 General Concepts of Model The most general categories of models are: • Physical models (scale models) • Graphical models • Mathematical models. Mathematical models are the most flexible ones.

  8. 3.3 Simple Models of Computer SystemsModel of a Simple Batch System

  9. Simulation Results For every simulation run there are two types of output: • Trace - sequence of events that occur during the simulation period • Performance measures - summary statistics about the simulation.

  10. 3.4 Performance of Computer Systems • Measures that indicate how well (or bad) the system is being studied carrying out its functions, with respect to some aspects • In studying a system, usually several performance measures are necessary.

  11. Approaches for Studying Performance • Measurements on the real system • Simulation models • Analytical (mathematical) models

  12. Performance Study A complete performance study includes the definition of the following components: • A set of relevant objectives and goals • Performance metrics • System parameters • System factors • System workload parameter

  13. 3.4.1 Performance Measures • Average wait time: the average period that jobs wait in system since their arrival time • Average throughput: average number of jobs completed in some specified time interval. • CPU Utilization: the portion of time the CPU is used relative to total observation interval. • Resource Utilization: the proportion of the interval the resource is used. • Response time: the average time interval that the system takes to respond to a particular command or request of a user process.

  14. Performance Measures (cont’d) • Average turnaround time: the average time interval that elapses from the time a job is submitted until the time the system writes the output results (also called sojourn time). • Availability: the fraction of time that an external observer finds the system capable of carrying out some work. • Reliability: the mean time between failures. • Capacity: maximum achievable throughput under ideal workload conditions. • Fairness: a measure of the variability of throughput across the various types of jobs or processes. • Speedup: a factor of gains in speed usually achieved by adding more processors to a system.

  15. Goal of Performance Study • Reduce waiting periods for the processes • Improve the processor utilization • Maximize throughput

  16. 3.4.2 Workload and System Parameters(These are for the simulation model, Skip ) • The performance measures depend on the current workload of the system • The workload for a system can be characterized by another series of measures, which are made on the input to the system • Errors in characterizing the workload may have serious consequences.

  17. Workload Parameters • Inter-arrival time • Task size • I/O request rate • I/O service rate • Memory request

  18. System Parameters • System memory • Processor speed • Number and type of processors • Degree of multi-programming • Length of time slice • Number and type of I/O ports

  19. Skip: 3.5 and 3.6 (Run simulator)

  20. 3.7 System Capacity and Bottleneck • The capacity of a system is determined by its maximum performance • The nominal capacity of a system is given by the maximum achievable throughput under ideal conditions • The usable capacity is the maximum throughput achievable under specified constraints.

  21. Bottleneck • The computer system reaches capacity when one or more of its servers or resources reach a utilization close to 100%. • The bottleneck of the system will be localized in the server or resource with a utilization close to 100%, while the other servers and resources each have utilization significantly below 100%.

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