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February 26, 2014: Prof. John T. Tsitsiklis (MIT)

Abstract: It is well known that resource pooling (or, equivalently, the use of flexible resources that can serve multiple types of requests) significantly improves the performance of service systems. On the other hand, complete resource pooling often results in higher infrastructure (communication and coordination) costs. This leads us to explore the benefits that can be derived by a limited amount of resource pooling, and the question whether a limited amount of pooled resources can deliver most of the benefits of complete resource pooling.

More concretely, we propose and analyze a multi-server model that captures such a performance tradeoff. In our model, a fraction p of an available resource is deployed as a flexible/pooled resource (e.g., it can serve a most-loaded station) while the remaining fraction 1−p is allocated to local servers that can only serve requests addressed specifically to their respective stations. Using a fluid model approach, we demonstrate a surprising phase transition in the steady-state delay, as p changes: in the limit of a large number of stations, and even if p is small, the average queue length in steady state scales (as a function of traffic intensity) at a much slower (exponentially smaller) rate.

We also discuss an alternative model of limited flexibility in which there are n arrival streams and each one of n servers is capable of serving only a small fraction of these streams.

(Joint work with Kuang Xu.)

Biography: John N. Tsitsiklis received the B.S. degree in Mathematics (1980), and the B.S. (1980), M.S. (1981) and Ph.D. (1984) degrees in Electrical Engineering, all from M.I.T., where he is currently a Clarence J Lebel Professor of Electrical Engineering. He has served as a co-director of the MIT Operations Research Center (2002-5), and a co-associate director of the Laboratory of Information and Decision Systems. As of 2013, he is serving as the Chair of the Council of the Harokopio University, in Greece.

His research interests are in the fields of systems, optimization, communications, control, and operations research. He has coauthored four books and several journal papers in these areas. He is a coauthor of Parallel and Distributed Computation: Numerical Methods (1989, with D. Bertsekas), Neuro-Dynamic Programming (1996, with D. Bertsekas), Introduction to Linear Optimization (1997, with D. Bertsimas), and Introduction to Probability (2002, with D. Bertsekas).

He has been a recipient of an IBM Faculty Development Award (1983), an NSF Presidential Young Investigator Award (1986), an Outstanding Paper Award by the IEEE Control Systems Society (1986), the M.I.T. Edgerton Faculty Achievement Award (1989), the Bodossaki Foundation Prize (1995), and a co-recipient of two INFORMS Computing Society prizes (1997, 2012), and a Sigmetrics Best paper award (2013). He is a Fellow of the IEEE (1999) and of INFORMS (2007). In 2007, he was elected to the National Academy of Engineering. In 2008, he was conferred the title of Doctor honoris causa from the Universite catholique de Louvain (Belgium).