The Mythical Man Month by Frederick Brooks

Author:Frederick Brooks
Language: eng
Format: epub, pdf

space Techniques 101

Obviously, more function means more space, speed being held constant. So the first area of craftsmanship is in trading function for size. Here there comes an early and deep policy question. How much of that choice shall be reserved for the user? One can design a program with many optional features, each of which takes a little space. One can design a generator that will take an option Jist and tailor a program to it. But for any particular set of options, a more monolithic program would take less space. It's rather like a car; if the map light, cigarette lighter, and clock are priced together as a single option, the package will cost less than if one can choose each separately. So the designer must decide how fine-grained the user choice of options will be.

In designing a system for a range of memory sizes, another basic question arises. A limiting effect keeps the range of suitability from being made arbitrarily wide, even with fine-grained modulaiity of function. In the smallest system, most modules will be overlaid. A substantial part of the smallest system's resident space must be set aside as a transient or paging area into which other parts are fetched. The size of this determines the size of all modules. And breaking functions into small modules costs both performance and space. So a large system, which can afford a transient area twenty times as large, only saves accesses thereby. It still suffers in both speed and space because the module size is so small. This effect limits the maximum efficient system that can be generated from the modules of a small system.

The second area of craftsmanship is space-time trade-offs. For a given function, the more space, the faster. This is true over an amazingly large range. It is this fact that makes it feasible to set space budgets.

The manager can do two things to help his team make good space-time trade-offs. One is to ensure that they are trained in programming technique, not just left to rely on native wit and previous experience. For a new language or machine this is especially important. The peculiarities of its skillful use need to be

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