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  1. Shahrood University of TechnologyIT & Computer Engineering Department Modeling and Performance Evaluation of Computer Systems

  2. Chapter 1Computer System Lifecycle Performance by Design: Computer Capacity Planning by Example Daniel A. Menascé, Virgilio A.F. Almeida, Lawrence W. Dowdy  Prentice Hall, 2004

  3. Outline-1 1.1 Introduction 1.2 QoS in IT Systems 1.2.1 Response Time 1.2.2 Throughput 1.2.3 Availability 1.2.4 Reliability 1.2.5 Security 1.2.6 Scalability 1.2.7 Extensibility

  4. Outline-2 1.3 System Life Cycle 1.3.1 Requirements Analysis and Specification 1.3.2 System Design 1.3.3 System Development 1.3.4 System Testing 1.3.5 System Deployment 1.3.6 System Operation 1.3.7 System Evolution 1.4 A Reference Model for IT Systems 1.5 Concluding Remarks 1.6 Exercises Bibliography

  5. Introduction (1) • IT systems are becoming increasingly ubiquitous and help support most aspects of everyday life. • The Internet has helped accelerate the rate at which IT is integrated into most social systems. • People rely on IT systems to address most of their major human and social concerns such as • health, • education, • entertainment, • access to communication services, • access to customer support, finances,

  6. Introduction (2) • safety, • privacy, • access to government services, and • travel. • The various concerns of individuals and of the society as a whole may face major breakdowns and incur high costs if IT systems do not meet the Quality of Service (QoS) requirements of • performance, • availability, • security, and • maintainability that are expected from them.

  7. Introduction (3) • For example, a call to 911—the emergency number in the U.S.—has to be answered by a dispatcher in a few seconds or human life may be endangered. • When the stock market goes through periods of extreme ups and downs, a large number of online traders tend to flock to online trading sites, causing potential problems due to overloaded and non-responsive systems. • The inability to trade in a timely manner may cause substantial financial losses.

  8. Introduction (4) • During health crises, such as the outbreak of new diseases, people need to get easy and fast access to health insurance companies to obtain authorization to be admitted to a hospital or to undergo a medical procedure. • In times of terrorism threats, major infrastructures, such as the telephone and cellular networks, may be targeted by terrorists or, in case of attacks to other structures, may become overloaded as their capacity to process calls is stretched thin, impairing the responsiveness of such systems.

  9. Introduction (5) • This infrastructure has to be properly designed and sized to handle the extraordinary demands of battlefield information exchanges. • The operation of the military is becoming more and more dependent on an agile information and communications infrastructure to help • locate, • find, • target, and • destroy enemy forces.

  10. Introduction (6) • Most people need to interact with automated or semi-automated customer support systems and expect near immediate response. • Unfortunately, it is not uncommon for someone to be placed on hold for dozens of minutes before being connected to a human being who will take care of a problem or provide the needed information. • These situations cause significant frustration and are a major cause for companies to lose customers.

  11. Introduction (7) • The number of people signing up for access to a wide variety of communication services such as wireless and Internet access services is increasing at exponential rates. • The growth in traffic has not been met by an adequate growth in system capacity. • As a result, callers may hear the unpleasant recording "all circuits are busy, please try your call later," when trying to place a call. • People have come to expect 24 / 7, instantaneous, and extremely reliable services. 

  12. QoS in IT Systems • IT systems touch people everywhere and every effort must be made to ensure that IT systems operate reliably and dependably so that they meet the needs of society and complement the capabilities of users [1]. • This section discusses the following QoS attributes of an IT system: • response time, • throughput, • availability, • reliability, • security, • scalability, and • extensibility.

  13. Response Time (1) • Figure 1.1 shows the three major components of the response time of a search request to an e-commerce site: • browser time, • network time, and • server time. • The browser time includes • the processing and • I/O time required to send the search request and display the result page.

  14. Figure 1.1. Breakdown of response time.

  15. Response Time (2) • The network time component includes • the time spent in the transmission from the browser to the user's Internet Service Provider (ISP), • the time spent in the Internet, and • the time spent in communication between the ISP at the e-commerce site and its server. • The third component includes • all the times involved in processing the request at the e-commerce site, • all the I/O time, • the networking time internal to the e-commerce site.

  16. Response Time (3) • Any of the three components include the time spent waiting to use various resources (processors, disks, and networks). • This is called congestion (waiting) time. • The congestion time depends on • the number of requests being processed by a system. • The higher the number of requests in the system, the higher the congestion time. • In this book we will learn how to compute the congestion time through the use of performance models.

  17. Throughput (1) • The rate at which requests are completed from a computer system is called throughput and is measured in operations per unit time. • The nature of the operation depends on the computer system in question. • Examples of systems and corresponding typical throughput metrics are given in Table 1.1. • When considering a throughput metric, one has to make sure that the operation in question is well-defined. • For example, in an Online Transaction Processing (OLTP) system, throughput is generally measured in transactions per second (tps).

  18. Table 1.1. Examples of Throughput Metrics

  19. Throughput (2) • However, transactions may vary significantly in nature and in the amount of resources they require from the OLTP system. • So, in order for the throughput value to be meaningful, one has to characterize the type of transaction considered when reporting the throughput. • In some cases, this characterization is done by referring to a well established industry benchmark. • For example, the Transaction Processing Performance Council (TPC) defines a benchmark for OLTP systems, called TPC-C, that specifies a mix of transactions typical of an order-entry system.

  20. Throughput (3) • The throughput metric defined by the benchmark measures the number of orders that can be fully processed per minute and is expressed in tpm-C [17]. • The throughput is a function of the load offered to a system and of the maximum capacity of a system to process work as illustrated in Example 1.1.

  21. Example 1.1 (1) • Assume that an I/O operation at a disk in an OLTP system takes 10 msec on average. • If the disk is constantly busy (i.e., its utilization is 100%), then it will be executing I/O operations continuously at a rate of one I/O operation every 10 msec or 0.01sec. • So, the maximum throughput of the disk is 100 (= 1 / .01) I/Os per second. • But if the rate at which I/O requests are submitted to the disk is less than 100 requests/sec, then • its throughput will be equal to the rate at which requests are submitted.

  22. Example 1.1 (2) • This leads to the expression • This is expression has to be qualified by the assumption that arriving requests do not "change their mind" if the system is busy, as happens routinely in Web sites. (1.2.1)

  23. Throughput (4) • As seen in the top curve of Fig. 1.2, throughput shows an almost linear increase at light loads and then saturates at its maximum value when one of the system resources achieves 100% utilization. • However, in some cases, at high overall loads, throughput can actually decrease as the load increases further. • This phenomenon is called thrashing, and its impact on throughput is depicted in the bottom curve of Fig. 1.2.

  24. Figure 1.2. Throughput vs. load.

  25. Throughput (5) • An example of thrashing occurs when a computer system with insufficient main memory spends a significant amount of CPU cycles and I/O bandwidth to handle page faults as opposed to process the workload. • This may occur because at high loads there are too many processes competing for a fixed amount of main memory. • As each process gets less memory for its working set, the page fault rate increases significantly and the throughput decreases.

  26. Throughput (6) • The operating system continuously spends its time handling extra overhead operations (due to increased load), which diminishes the time the CPU can be allocated to processes. • This increases the backlog even further, leading to a downward performance spiral that can cripple the system, in a way similar to a traffic jam. • An important consideration when evaluating computer systems is to determine the maximum effective throughput of that system and how to achieve it. • More on this will be discussed in Chapter 3.

  27. Availability (1) • Imagine that you access an online bookstore and get as a result the page shown in Fig. 1.3. • You are likely to become frustrated and may turn to another online bookstore to buy the book you are looking for. • The consequences of system unavailability can be far more reaching than a loss of customers. • The credibility and reputation of a company are vital. • As mentioned by Schneider [15], service interruptions can even threaten lives and property.

  28. Figure 1.3. Availability problems.

  29. Availability (2) • Availability is defined as the fraction of time that a system is up and available to its customers. • For example, a system with 99.99% availability over a period of thirty days would be unavailable • For many systems (e.g., an online bookstore), this level of unavailability would be considered excellent. • However, for other systems (e.g., defense systems, 911 services), even 99.99% would be unacceptable. (1.2.2 )

  30. Availability (3) • The two main reasons for systems to be unavailable are • failures and • overloads. • Failures may prevent users from accessing a computer system. • For example, the network connection of a Web site may be down and no users may be able to send their requests for information.

  31. Availability (4) • Alternatively, overloads occur when all components are operational but • the system does not have enough resources to handle the magnitude of new incoming requests. • This situation usually causes requests to be rejected. • For instance, a Web server may refuse to open a new TCP connection if the maximum number of connections is reached. • Failures must be handled rapidly to avoid extended down times. • The first step for failure handling is failure detection.

  32. Availability (5) • Then, the causes of the failures must be found so that the proper resources (e.g., people and materiel) may be put in place to bring the system back to its normal operational state. • Thus, failure handling comprises failure detection, • failure diagnosis, and • failure recovery. • One of the reasons for controlling and limiting the number of requests that are handled concurrently by an IT system is to guarantee good quality of service for the requests that are admitted.

  33. Availability (6) • This is called admission control and is illustrated in Fig. 1.4, which shows two response time curves versus system load. • If no admission control is used, response time tends to grow exponentially with the load. • In the case of admission control, the number of requests within the system is limited so that response time does not exceed a certain threshold. • This is accomplished at the expense of rejecting requests. • Thus, while accepted requests experience an acceptable level of service, the reject ones may suffer very large delays to be admitted.

  34. Figure 1.4. Impact of admission control on response time.

  35. Reliability • The reliability of a system is • the probability that it functions properly and continuously over a fixed period of time [8]. • Reliability and availability are closely related concepts but are different. • When the time period during which the reliability is computed becomes very large, the reliability tends to the availability.

  36. Security (1) • Security is a combination of three basic attributes: • Confidentiality: • only authorized individuals are allowed access to the relevant information. • Data Integrity: • information cannot be modified by unauthorized users. • Non-repudiation: • senders of a message are prevented from denying having sent the message.

  37. Security (1) • To enforce these properties, systems need to implement authentication mechanisms [5] to guarantee that each side in a message exchange is assured that the other is indeed the person they say they are. • Most authentication mechanisms used to provide system security are based on one or more forms of encryption. • Some encryption operations may be very expensive from the computational standpoint. • The tradeoffs between security and performance have been studied in [6, 7, 9, 14].

  38. Scalability • A system is said to be scalable if its performance does not degrade significantly as the number of users, or equivalently, the load on the system increases. • For example, the response time of system A in Fig. 1.5 increases in a non-linear fashion with the load, while that of system B exhibits a much more controlled growth. • System A is not scalable while system B is.

  39. Figure 1.5. Scalability. System A is not scalable while system B is.

  40. Extensibility • Extensibility is the property of a system to easily evolve to cope with new functional and performance requirements. • It is not uncommon for new functionalities to be required once a new system goes into production. • Even a careful requirements analysis cannot necessarily uncover or anticipate all the needs of system users. • Changes in the environment in which the system has to operate (e.g., new laws and regulations, different business models) may require that the system evolve to adapt to new circumstances.

  41. System Life Cycle (1) • Addressing performance problems at the end of system development is a common industrial practice that can lead to • using more expensive hardware than originally specified, • time consuming performance-tuning procedures, and, • in some extreme cases, to a complete system redesign [3]. • It is therefore important to consider performance as an integral part of a computer system life cycle and not as an afterthought.

  42. System Life Cycle (2) • The methods used to assure that QoS requirements are met, once a system is developed, are part of the discipline called Performance Engineering (PE) [16]. • This section discusses the seven phases of the life cycle of any IT system: • requirements analysis and specification, • design, • development, • testing, • deployment, • operation, and • evolution as illustrated in Fig. 1.6.

  43. Requirements Analysis and specification System development System design testing deployment evolution operation System Life Cycle (3) • The inputs and outputs of each phase are discussed, • the tasks involved in each phase are described, and • QoS issues associated with each phase are addressed. Figure 1.6. System life cycle.

  44. Requirements Analysis and Specification • During this phase of the life cycle of a computer system, the analysts, in conjunction with users, gather information about what they want the system to do. • The result of this analysis is a requirements specifications document that is divided into two main parts: • Functional requirements • Non-functional requirements

  45. Functional requirements (1) • The functional requirements specify the set of functions the system must provide with the corresponding inputs and outputs as well as the interaction patterns between the system and the outside world (users). • For example, the functional requirements of an online bookstore could indicate that the site must provide a search function that allows users to search for books based on keywords, ISBN, title, and authors. • The specification indicates how the results of a search are displayed back to the user.

  46. Functional requirements (2) • The functional requirements usually include information about the physical environment and technology to be used to design and implement the system. • In the same example, the specification could say that the online bookstore site should use Web servers based on UNIX and Apache and • that it should also provide access to wireless users using the Wireless Application Protocol (WAP) [19].

  47. Non-functional requirements • The non-functional requirements deal mainly with the QoS requirements expected from the system. • Issues such as performance, availability, reliability, and security are specified as part of the non-functional requirements. • A qualitative and quantitative characterization of the workload must be given so that the QoS requirements can be specified for specific workload types and levels. • For example, a non-functional requirement could specify that • "at peak periods, the online bookstore is expected to receive 50 search requests/sec and • respond within 2 seconds to 95% of the requests."

  48. System Design (1) • System design is the stage in which the question "How will the requirements be met?" is answered. • In this phase, • the system architecture is designed, • the system is broken down into components, • major data structures, including files and databases, are designed, • algorithms are selected and/or designed, and • pseudo code for the major system components is written. • It is also during this phase that the interfaces between the various components are specified.

  49. System Design (2) • These interfaces may be of different types, including • local procedure calls, • Remote Procedure Calls (RPC) and • message exchanges of various types. • The current trend in software engineering is to reuse as many proven software solutions as possible. • While this approach is very attractive from the point of view of • shortening the duration of the design and development phases, • it may pose risks in terms of performance.

  50. System Design (3) • Designs that perform well in one type of environment and under a certain type of workload may perform very poorly in other settings. • For example, a search engine used in a low volume online retailer may perform very poorly when used in an e-commerce site that receives millions of requests per day. • As the workload intensity scales up, • different techniques, • different algorithms, and • different designs may have to be adopted to satisfy the non-functional requirements.