Software rejuvenation is a preventive maintenance technique to prevent failures in continuously running systems that experience software aging. In this paper, rejuvenation is modeled in a redundant computer system via a semi-Markov process in order to counteract software aging. The system can be switched into the standby unit automatically or manually in the case of a failure at the switching mechanism. Depending on the degradation of the system resources, the running node is either rejuvenated or continues serving and systempsilas steady-state availability is studied along with the optimal time to perform rejuvenation, which increases systempsilas availability. Moreover, an alternative rejuvenation modeling is introduced, in which two different actions; partial and full rejuvenation are performed according to the degradation level. The optimal rejuvenation policies for this case are also derived, leading the system to higher levels of availability.

Purpose: The purpose of this paper is to formulate and solve a new emergency evacuation planning problem. This problem addresses the needs of both able and disabled persons who are evacuated from multiple pick-up locations and transported using a heterogeneous fleet of vehicles. Design/methodology/approach: The problem is formulated using a mixed integer linear programming model and solved using a heuristic algorithm. The authors analyze the selected heuristic with respect to key parameters and use it to address theoretical and practical case studies. Findings: Evacuating people with disabilities has a significant impact on total evacuation time, due to increased loading/unloading times. Additionally, increasing the number of large capacity vehicles adapted to transport individuals with disabilities benefits total evacuation time. Research limitations/implications: The mathematical model is of high complexity and it is not possible to obtain exact solutions in reasonable computational times. The efficiency of the heuristic has not been analyzed with respect to optimality. Practical implications: Solving the problem by a heuristic provides a fast solution, a requirement in emergency evacuation cases, especially when the state of the theater of the emergency changes dynamically. The parametric analysis of the heuristic provides valuable insights in improving an emergency evacuation system. Social implications: Efficient population evacuation studied in this work may save lives. This is especially critical for disabled evacuees, the evacuation of whom requires longer operational times. Originality/value: The authors consider a population that comprises able and disabled individuals, the latter with varying degrees of disability. The authors also consider a heterogeneous fleet of vehicles, which perform multiple trips during the evacuation process.

Nowadays voice over Internet protocol (VoIP) is becoming an evolutionary technology in telecommunications. In this paper the study is focused on the resources that are allocated for VoIP calls. Resource allocation in a VoIP system and resource degradation when new demands for resources arrive at the system are modeled. To counteract resource degradation and improve availability and service reliability, we propose to perform software rejuvenation.. Moreover, the rate of resource allocation in such a system can be importantly affected by the time that an amount of resources is allocated in order to serve VoIP calls. Hence, we model software rejuvenation in a VoIP system with a semi-Markov process in order to capture the effects of time spent at resource degraded states of the system. Through the stochastic analysis of the system an optimal rejuvenation policy that maximizes system's availability is proposed and furthermore the corresponding reliability levels in means of Mean Time To Failure are derived.