224 CHAPTER 9 PERFORMANCE AND (Yahoo web space) SCALABILITY TESTING
224 CHAPTER 9 PERFORMANCE AND SCALABILITY TESTING Here s a coarse comparison: back in 1997, I bought a 1995 Mazda RX7 (the Batmobilelooking car). It is a high-performance sports car boasting acceleration from 0 to 60 mph in less than five seconds and a top speed of over 160 mph. It corners fast and handles incredibly, but it has a limitation referred to as unpredictable oversteer it can take corners very fast, but once you pass a certain threshold, the car s behavior is unpredictable. Shortly after buying the car, I took my friend Chris for a ride, and we discovered that threshold. I approached a tight corner, and instead of letting off the gas, I accelerated. The car performed two 360-degree spins before coming to a (thankfully) safe stop. Needless to say, Chris and I needed to take another short trip around the neighborhood to restart our hearts, but I never pushed the car past the limit that I discovered. Let s bring this back to performance and scalability testing: you may be comfortable with the performance of your application in general and even with its performance at your current or projected usage, but until you discover its breaking point, you will always experience uncertainty every time marketing runs another promotion or a new, high-profile customer endorses your application. The key to attaining this confidence is to perform the following tasks: Assess the performance of your application at expected load. Determine the load that causes your application to exceed its SLAs. Determine the load that causes your application to reach its saturation point and enter the buckle zone. Construct a performance degradation model from expected usage to exceeded SLAs to saturation point. With this information in hand, you will be well equipped to project the effects of changes in usage patterns on your environment and intelligently recommend environment changes to your CIO. Performance vs. Scalability The terms performance and scalability are commonly used interchangeably, but the two are distinct: performance measures the speed with which a single request can be executed, while scalability measures the ability of a request to maintain its performance under increasing load. For example, the performance of a request may be reported as generating a valid response within three seconds, but the scalability of the request measures the request s ability to maintain that three-second response time as the user load increases. Scalability asks the following questions about the request: At the expected usage, does the request still respond within three seconds? For what percentage of requests does it respond in less than three seconds? What is the response time distribution for requests that do not respond within three seconds?
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