PHOTOOG Photography writings by Olivier Giroux

26Jul/070

Summary: A Model of Large Program Development (part 2 of 3)

See part 1 here. See part 3 here.

Cost Model

Towards the end of their paper, Belady and Lehman propose a simple cost model.
They present 3 activities:

  1. Activity P is programming new features.
  2. Activity A is antiregressive, it is improving the system & programmers.
  3. Activity C is complexity, it is keeping the system running & fixing bugs

When taken as time rates, all three as measured in # of programmers to assign from budget B such that:

B = P + A + C, i.e.: you must divide the budget wholly between the 3 activities.

Their observations lead them to this qualitative model between the variables:

  1. A should be linearly related to P.
  2. Under-spending in A leads to growth in C.
  3. C grows linearly with P and time (so quadratically related to total manpower).

Equilibrium between the three variables is possible but the short-term benefits of spending less in A and more in P lead C to grow monotonically in practice.


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