1.1. Performance is Important and Difficult (D.1)

Adequate performance, in the sense of delay and throughput, is essential to the useful functioning of most software products. However it is often the last aspect of a pro-gram to be analyzed, only after integration and during final testing. At such a late stage the necessary changes may be difficult and expensive. Projects are delayed, go over cost targets, or fail altogether because of performance problems. This problem has been described by Smith as the “fix-it-later ” approach, and she believes many programmers are averse to even considering performance in their designs. The situation is made worse by the rush to market, and by the greater use of dis-tributed systems such as client-server systems, communicating over the Internet or simi-lar internal networks. Distribution and concurrency make these systems much harder to understand. If one wants to estimate performance earlier, for example when designing the architecture of software, there is a frustrating lack of machinery to help. The standard approaches to design have limited places for performance requirements or information. It isn’t clear what information is needed, and it is easy to get into a maze of detail, with n