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We present surprising examples of MCQN with traffic intensity rho < 1 that are unstable under some policies and prepare the background for rigorous treatment of stability.
We study the supermarket model and show that choosing the shortest of just a few randomly chosen queues is almost as good as JSQ. Another issue with many-server systems is specialization, with several customer and server types, and limited compatibility between them
We continue the discussion of control of transient MCQN. We formulate a fluid optimization problem that we can solve using a separated continuous linear programming (SCLP) algorithm.We then describe a method of tracking the optimal fluid solution, using virtual infinite queues and maximum pressure policy. We show that this procedure is asymptotically optimal for high-volume systems, as exemplified by semiconductor wafer fabs.
We discuss fluid and diffusion approximations to the GI/GI/1 queue by scaling time and space. We also introduce the GI/GI/1 queueing system and study it under many-server scaling. The three types of scaling, fluid, diffusion, and many-server, form the backbone for Parts IV, V, andVI of the book, where we use them to study networks of queues. These approximations allow us to obtain a much better idea of how queues evolve over time than can be obtained from an exact discrete state Markov description.
Applications of queueing network models have multiplied in the last generation, including scheduling of large manufacturing systems, control of patient flow in health systems, load balancing in cloud computing, and matching in ride sharing. These problems are too large and complex for exact solution, but their scale allows approximation. This book is the first comprehensive treatment of fluid scaling, diffusion scaling, and many-server scaling in a single text presented at a level suitable for graduate students. Fluid scaling is used to verify stability, in particular treating max weight policies, and to study optimal control of transient queueing networks. Diffusion scaling is used to control systems in balanced heavy traffic, by solving for optimal scheduling, admission control, and routing in Brownian networks. Many-server scaling is studied in the quality and efficiency driven Halfin–Whitt regime and applied to load balancing in the supermarket model and to bipartite matching in ride-sharing applications.