Virus tests to maximize availability of software systems

AbstractSoftware systems in which many user's or programmers intervene may easily contain software items — such as viruses — which will endanger the integrity of the system. This paper proposes that in addition to the conventional recovery techniques, such as dumps and roll-back recovery, system availability be enhanced by the introduction of virus tests or other types of “failure tests”. We present a model to analyze the effect of the failure rate, the frequency of virus and failure testing, and the frequency of periodic dumps, on global system availability. We assume that the “failure” rate of the system increases as time elapses beyond any individual instant at which a virus test or failure test has been carried out. Thus, we are dealing with a system in which failures will be naturally time-dependent. We compute the optimum value of the interval between dumps, and also the best time interval between virus or failure tests for this system. In order to illustrate the methodology of this work, numerical examples are presented for various time-dependent failure statistics

On modelling the performance and reliability of multimode computer systems

We present an effective technique for the combined performance and reliability analysis of multi-mode computer systems. A reward rate (or a performance level) is associated with each mode of operation. The switching between different modes is characterized by a continuous time Markov chain. Different types of service-interruption interactions (as a result of mode switching) are considered. We consider the execution time of a given job on such a system and derive the distribution of its completion time. A useful dual relationship, between the completion time of a given job and the accumulated reward up to a given time, is noted. We demonstrate the use of our technique by means of a simple example