13.3 Top-Down (Requirements-Driven) Performance Modeling

13.3.1 Specifying Targets – the Project-Management Analogy

When taking a top-down modeling approach, the problem facing us can be stated quite simply, as follows:

• Design input: Specifications (in particular, performance objectives)
• Design output: Functional architecture
• Design performance: Who knows

Now, this is very similar to the issues faced when devising a project plan (Figure 13.7). The overall end-to-end project timescales are defined by the needs of the project. What must be done now is to produce a plan to ensure that the work gets finished on time. Such plans are developed first in their simplest form, concentrating on the major work phases. Each phase may then be further refined to show more detail. What we end up with is a model showing the work activities (and their relationships) of the project. Moreover, these activities have defined time allocations or targets. We can now functionally...