THE ULTRASAN MODELING ENVIRONMENT

Model-based evaluation of computer systems and networks is an increasi ngly important activity. For modeling to be used effectively, software environments are needed that ease model specification, construction, and solution. Easy to use, graphical methods for model specification t hat support solution of families of models with differing parameter va lues are also needed. Since no model solution technique is ideal for a ll situations, multiple analysis- and simulation-based solution techni ques should be supported. This paper describes UltraSAN, one such soft ware environment. The design of UltraSAN reflects its two main purpose s: to facilitate the evaluation of realistic computer systems and netw orks, and to provide a test-bed for investigating new modeling techniq ues. In UltraSAN, models are specified using stochastic activity netwo rks, a stochastic variant of Petri nets, using a graphical X-Window ba sed interface that supports large-scale model specification, construct ion, and solution. Models may be parameterized to reduce the effort re quired to solve families of models, and a variety of analysis- and sim ulation-based solution techniques are supported. The package has a mod ular organization that makes it easy to add new construction and solut ion techniques as they become available. In addition to describing the features, capabilities, and organization of UltraSAN, the paper illus trates the use of the package in the solution for the unreliability of a fault-tolerant multiprocessor using two solution techniques.