Performability analysis of computer systems : from model specification to solution

Abstract

Computer system relialbility/availability modeling deals with the representation of changes in the structure of the system being modeled, which are generally due to faults, and how such changes affect the availability of the system. On the other hand, performance modeling involves representing the probabilistic nature of user demands and predicting the system capacity to perform useful work, under the assumption that the system structure remains constant. With the advent of degradable systems, the system may be restructurcd in responsc to faults and may continue to perform useful ,work, even though operating at lower capacity. Performability modeling considers the effect of structural changes and their impact on the overall performance of the system. The complexity of current computer systems and the variety of different problems to be analyzed, including the simultaneous evaluation of performance and availability, demonstrate the need for sophisticated tools that allow the specification of general classes of problems while incorporating powerful analytic and/or simulation techniques. Concerning model specification, a recently proposed object oriented modeling paradigm that accommodates a wide variety of applications is discussed and compared with other approaches. With respect to solution methods, a brief overview of past work on performability evaluation of Malkov models is presented. Then it is shown that many performability related measures can be calclllated using the uniformization or randomization technique by marking distinguished states and/or transitions of the Malkov model of the system being studied. Finally, the state space explosion ploblem is addressed and several techniques for dealing with the ploblem are discussed.