free barcode add-in excel 2007 Performance Tuning in Software

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212 Performance Tuning
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Tuning the performance of a system involves adjusting various parameters and design choices to improve its performance for a speci c application Various aspects of a database-system design ranging from high-level aspects such as the schema and transaction design, to database parameters such as buffer sizes, down to hardware issues such as number of disks affect the performance of an application Each of these aspects can be adjusted so that performance is improved
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2121 Location of Bottlenecks
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The performance of most systems (at least before they are tuned) is usually limited primarily by the performance of one or a few components, called bottlenecks For instance, a program may spend 80 percent of its time in a small loop deep in the code, and the remaining 20 percent of the time on the rest of the code; the small loop then is a bottleneck Improving the performance of a component that is not a bottleneck does little to improve the overall speed of the system; in the example, improving the speed of the rest of the code cannot lead to more than a 20 percent improvement overall, whereas improving the speed of the bottleneck loop could result in an improvement of nearly 80 percent overall, in the best case Hence, when tuning a system, we must rst try to discover what are the bottlenecks, and then to eliminate the bottlenecks by improving the performance of the components causing them When one bottleneck is removed, it may turn out that another component becomes the bottleneck In a well-balanced system, no single component is the bottleneck If the system contains bottlenecks, components that are not part of the bottleneck are underutilized, and could perhaps have been replaced by cheaper components with lower performance For simple programs, the time spent in each region of the code determines the overall execution time However, database systems are much more complex, and can be modeled as queueing systems A transaction requests various services from the database system, starting from entry into a server process, disk reads during execution, CPU cycles, and locks for concurrency control Each of these services has a queue associated with it, and small transactions may spend most of their time wait-
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VII Other Topics
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Performance Tuning
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ing in queues especially in disk I/O queues instead of executing code Figure 216 illustrates some of the queues in a database system As a result of the numerous queues in the database, bottlenecks in a database system typically show up in the form of long queues for a particular service, or, equivalently, in high utilizations for a particular service If requests are spaced exactly uniformly, and the time to service a request is less than or equal to the time before the next request arrives, then each request will nd the resource idle and can therefore start execution immediately without waiting Unfortunately, the arrival of requests in a database system is never so uniform, and is instead random If a resource, such as a disk, has a low utilization, then, when a request is made, the resource is likely to be idle, in which case the waiting time for the request will be 0 Assuming uniformly randomly distributed arrivals, the length of the queue (and correspondingly the waiting time) go up exponentially with utilization; as utilization approaches 100 percent, the queue length increases sharply, resulting in excessively long waiting times The utilization of a resource should be kept low enough that queue length is short As a rule of the thumb, utilizations of around 70 percent are considered to be good, and utilizations above 90 percent are considered excessive, since they will result in signi cant delays To learn more about the theory of queueing systems, generally referred to as queueing theory, you can consult the references cited in the bibliographical notes
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