The papers in this issue of Performance Evaluation all deal with performability modelling of computer and communication systems. This was the topic of the First International Workshop on Performability Modelling of Computer and Communication Systems (PMCCS-1), held 7-8 February 1991, at the University of Twente, Enschede, the Netherlands

Haverkort, Boudewijn R.

Niemegeers, Ignas G.

van Dijk, Nico M.

English

University of Twente Research Information

Performance Evaluation 14 (1992) 135-138 135 North-Holland Guest editorial The papers in this issue of Performance Evalua- tion all deal with performability modelling of computer and communication systems. This was the topic of the First International Workshop on Performability Modelling of  Computer and Com- munication Systems (PMCCS-1), held 7-8 Febru- ary 1991, at the University of Twente, Enschede, the Netherlands. Introduction Over the last decade the use of fault-tolerant distributed computer and communication systems has steadily grown. With this increase in usage also the need for applicable quantitative evalua- tion techniques and tools came up. It is due to the efforts of many researchers from all over the world that over the last decade performability modelling and analysis has become a mature topic that deserves attention in its own. In February 1990, at the IFIP WG 7.3 meeting in Lattrop, the idea of a workshop totally dedi- cated to performability was developed. As it was, and still is now, research contributions in the performability area are presented at a large vari- ety of conferences, such as ACM Sigmetrics, IEEE Fault-Tolerant Computer Systems, IEEE Petri Nets and Performance Models, Perfor- mance etc., as well as in a broad range of related journals. By organizing a workship especially ded- icated to performability modelling and its appli- cations, we hoped to bring together active re- searchers in the field, so as the maximize the information exchange among them. In the early organization phase already, many people reponded positively to the idea of a per- formability workshop. The initiative was also sup- ported by the IFIP working group on perfor- mance analysis (IFIP WG 7.3). We were also encouraged by the quick and positive responses of leading scientists in the field, J.F. Meyer, K.S. Trivedi and E. de Souza e Silva to give an invited presentation. We are grateful to them for having provided the workshop program with a very firm basis. The organization and success of a workshop heavily rely upon support and positive input from both the participants, the authors and the local organizations involved. In these respects we are most grateful to all of the participants for enthu- siasm, we like to thank the Tele-Informatics and Open Systems group of the University of Twente for providing the opportunity and accommoda- tion for arranging the workshop, and we are most indebted to Ms. Ellen ter Brugge for handling the administrative overhead of the workshop. The PMCCS1 workshop presentations At the workshop, a total of 17 papers were presented. Here we wish to state that only a small number next to the three invited papers could be selected, taking into account the general scope of Performance Evaluation. The general opinion of all the participants was most positive. Let us briefly summarize all contributions. R. Lepold presented the paper PENPET" A New Approach to Performability Modelling using Stochastic Petri Nets. In his presentation he dis- cussed the conceptual modelling framework un- derlying the software tool PENPET (Petri Net Based Performability Evaluation Tool). The tool has been designed to automate the difficult and tedious process of constructing Markov reward performability models of large computer systems. A. Tai, J.F. Meyer and H. Hecht presented a paper entitled A Performability Model for Real- Time Software, which deals with the performabil- ity analysis of software systems, operating in a particular environment. Both the system and the environment are modelled by stochastic activity networks. Given that there are still design faults left in the software, the overall system's ability to fulfill real-time constraints is evaluated. 0166-5316/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved 136 The paper Performability Analysis of the Inte- grated Software-Hardware Components of Real- Time Parallel and Distributed Systems by S.M.R. Islam and H.H. Ammar dealt with the evaluation of the performability of interacting software com- ponents running on parallel machines. Due to the interdependencies between the software compo- nents running in different hardware models, the failure of one module may force another module to rollback. Based on stochastic Petri nets, large Markov models are generated which are then analyzed using Laguerre transforms. In the paper Fault Injection Simulation, A.P.A. van Moorsel, B.R. Haverkort and I.G. Niemegeers discuss a new fast simulation technique that can be applied to derive performance metrics that are highly influenced by very rarely occurring events such as failures. Prerequisite to the method is that the rare event occurence probabilities are known in advance, which is, e.g., often the case when studying communication media. Simulation speed-up factors of order 100 are reported. B.R. Haverkort and I.G. Niemegeers, in their paper On the Mutual Performance-Dependability Influence in Dynamic Queueing Networks, discuss how various influence relations between a system its performance and its dependability characteris- tics can be modelled conveniently by means of dynamic queueing networks. In Product Forms and Simple Bounds for Relia- bility Networks, N.M. van Dijk outlined the fol- lowing two results for reliability networks: (i)  a Nico M. van Dijk received his M.Sc. and Ph.D. in applied mathematics from the University of Leiden, The Netherlands in 1979 and 1983. Since then he has been with the University of British Columbia, the University of Twente, the Free University of Amsterdam, and the University of Amsterdam where he is a full professor of Operation Research in the Faculty of Econometrics. His research interests involve exact product form expressions, simple performance estimates and analytic error bound results for queueing and communication networks and their application to telecommunica- tions, computer networking and manufacturing. Boudewijn R. Haverkort was born in Lichtenvoorde, the Netherlands in 1964. He received and M.Sc. and a Ph.D. degree computer science, both from the University of Twente, Enschede, the Netherlands, in 1986 and 1991, respectively. In 1986 he stayed half a year at the University of Dortmund, West-Germany, where he worked on the performance modelling of local-area network interconnection structures. Since January 1990, Dr. Haverkort is an assistant professor in the Tele-Informatics and Open Systems group of the department of computer science at the University of Twente, where he teaches undergraduate courses on performance analysis of communication networks and computer systems. His current research interests include performance, dependability and performability evaluation of fault-tolerant and distributed computer/communication systems, software tools for these evaluations, as well as formal specification techniques (Lotos) and their integration with performability analysis. Dr. Haverkort, who is a member of the IEEE Computer Society and the ACM, received a Koninklijke/Shell research prize early 1991. Ignas G. Niemegeers (S'70-M'78) was born in Gent, Belgium, in 1947. He received the M.Sc. degree in electrical engineering from the Rijksuniversiteit Gent in 1970. In 1972 he received the M.Sc. degree in computer engineering and in 1978 the Ph.D. degree from Purdue University, West Lafayette, IN. From 1978 to 1981 he was a system design engineer at Bell Telephone Mfg. Cy., Antwerp, Belgium, where he was involved in the design of the DPS 1500 system, a high capacity X.25 packet switching network. From 1981 to 1986 he was an associate professor in the Department of Electrical Engineering, at the University of Twente, Enschede, the Netherlands. Since 1986 he has been a full professor in the Tele-Informatics and Open Systems group in the Department of Computer Science of the University of Twente. His present research interests are architectures for broadband integrated communication infrastructures, parallel processing in communication switches, performance analysis, and performability of fault tolerant systems. 137 condition and insight from which one can con- clude steady-state product form expressions for the configuration of down components, and (ii) a methodology to use these insights to obtain sim- ple and guaranteed bounds for both transient and steady state measures for reliability networks that fail this condition. Examples included a series- parallel system and a hierarchical database sys- tem. P. Wartenhorst and F.A. van der Duyn Schouten in their presentation Availability Analy- sis of a Two-Unit Standby System showed how the embedding technique of Markov decision theory can be applied to obtain availability results for standby-repair systems with a control limit rule for repair. Their results extended existing results for the state independent and non-preemptive case to state-dependent and preemptive case, while further application of this technique seems promising. In Optimal Routing with Resequencing, Z. Ros- berg and S. Ayoun studied an optimal routing problem of a single server queue from which jobs are dispatched to one of two heterogeneous par- allel communication channels under the require- ment of resequencing in arrival order upon com- pletion. A holding time criterion is used. A threshold policy is proven to be optimal which shows that the routing decision is independent of the resequencing queue. In addition, explicit ex- pressions are provided to determine the critical threshold values. In the contribution Numerical Studies of sched- uled Maintenance by R. Vallejos Campos, E. de Souza e Silva and H.R. Gail, the importance of preventive maintenance policies during operation is argued to improve the availability or performa- bility of a system. Based on a recently proposed Markovian technique to evaluate scheduled maintenance policies, the impact of different pa- rameters for performability measures of systems subject to periodically scheduled maintenance is addressed. Particularly, the importance of a de- tailed model and state description was made ap- parent. O.J. Boxma, W.P. Groenendijk and J.A. West- strate in their presentation Optimal Scheduling of Preventive and Corrective Maintenance using Polling Systems, propose a polling model to deter- mine the optimal routing of a maintenance crew concerned with both preventive and corrective maintenance. A polling table was provided that minimizes a particular performance measure for the system. Selected papers The three invited speakers were given carte blanche for their papers. In fact, they were en- couraged to give survey like presentations and write their papers accordingly. Four other papers were chosen out of the regular presentations. We briefly mention the selected papers here. The invited paper by J.F. Meyer, entitled Per- formability: A Retrospective with Some Pointers to the Future, surveys the historical development of the concept of performability as it grew out of the need to access the combined performance and dependability of fault-tolerant computer systems. The invited paper by E. de Souze e Silva (co-authored by H.R. Gail), entitled Peformabil- ity Analysis of Computer Systems: From Model Specification to Solution, covers both specification techniques that have been developed for the con- struction of Markov reward models (thereby em- phasizing on the object oriented approach fol- lowed in Tangram) as well as the randomization technique for the solution of these models. The invited paper by K.S. Trivedi (co-authored by J.K. Muppala, S.P. Woolet, and B.R. Haverkort), entitled Composite Performance and Dependability Analysis, addresses the use of Markov reward models for performability mod- elling. Various performability measures and re- ward choices are surveyed and enlightened by many examples. The Markov reward models for performability are compared with completion time models and with models of queues with server breakdowns. In the paper A Combined Queueing Network and Stochastic Petri Net Approach for Evaluating the Performability of Fault-Tolerant Computer Sys- tems, H. Szczerbicka proposes a method to evalu- ate the performability of fault-tolerant computer systems by means of queueing networks. How- ever, those systems aspects that can not be mod- elled adequately by these queueing networks are modelled separately by means of stochastic Petri nets. The results of these separate analyses are subsequently embedded in the overall queueing model, thereby preserving the first and second 138 moment of the throughput. In this way, depend- ability influenced performance metrics are ob- tained. V. Grassi and L. Donatiello in their paper Sensitivity Analysis of Performability investigate the sensitivity of the expected cumulative reward over a finite time horizon with respect to system parameters such as the breakdown and repair rates. The results are based on differentiation of the uniformized expressions of the underlying Markov chain model which also lead to error bounds when numerically evaluating the sensitiv- ity. In Distribution of the Minimal Completion Time of Parallel Tasks in Multi-Reward Semi-Markov Models, A. Bobbio and L. Roberti model the completion time of parallel tasks in a randomly varying servicing system as the barrier hitting time in a multi-reward semi-Markov model. Dif- ferent reward measures are dealt with such as maximum or specific job completion times. It is shown that whenever the barrier levels have phase type distributions that than also the distribution of the hitting times have phase type distributions. This result leads to recursive computational schemes. G. Rubino and B. Sericola in their paper Inter- val Availability Analysis using Operational Periods develop a computational method to evaluate the availability of semi-Markovian systems during a given time-interval. This method relies upon in- dependence assumptions of successive opera- tional and non-operational periods. Also the cummulative operational times up to the n-th operational period or failure are hereby analyzed. Epilogue By the responses in the planning phase of the workshop, by the various discussions during the workshop and by the presented papers, both the practical interests from different disciplines as well as the fundamental theoretical problems un- derlying the topic performability have only made more apparent that performability has grown into a mature topic which is more than open for fundamental research. By organizing this work- shop and selecting papers for this special issue we primarily aimed to encourage further steps in this direction. NICO M. VAN DIJK BOUDEWIJN R. HAVERKORT IGNAS G. NIEMEGEERS 

Guest editorial to the first international workshop on performability modelling of computer and communication systems

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