Summary

TIP: BE CAREFUL WITH GENERALIZATION

The abstract technique of generalization can thus be quite useful in systems design, where one good idea can be made slightly broader and thus solve a larger class of problems. However, be careful when generalizing; as Lampson warns us “Don’t generalize; generalizations are generally wrong”“Hints for Computer Systems Design” by Butler Lampson. ACM Operating Systems Review, 15:5, October 1983. Lampson, a famous systems researcher, loved using hints in the design of computer systems. A hint is something that is often correct but can be wrong; in this use, a signal() is telling a waiting thread that it changed the condition that the waiter was waiting on, but not to trust that the condition will be in the desired state when the waiting thread wakes up. In this paper about hints for designing systems, one of Lampson’s general hints is that you should use hints. It is not as confusing as it sounds..

One could view semaphores as a generalization of locks and condition variables; however, is such a generalization needed? And, given the difficulty of realizing a condition variable on top of a semaphore, perhaps this generalization is not as general as you might think.