Stability using fluid limits: Illustration through an example "Push-Pull" queuing network Yoni Nazarathy* EURANDOM Contains Joint work with Gideon Weiss and Erjen Lefeber Universiteit Gent October 14, 2010 * Supported by NWO-VIDI Grant of Erjen Lefeber
K umar S eidman R ybko S toylar
Purpose of the talk Part 1: Outline research on Multi-Class Queueing Networks (with Infinite Supplies) - N., Weiss, Ongoing work with Lefeber Part 2: An overview of “the fluid limit” method for stability of queueing networks Key papers: - Rybko, Stolyar Dai Bramson/Mandelbaum/Dai/Meyn… Recommended Book: - Bramson, Stability of Queueing Networks, 2009
PART 1: MULTI-CLASS QUEUEING NETWORKS (WITH INFINITE SUPPLIES)
Continuous Time, Discrete Jobs 2 job streams, 4 steps Queues at pull operations Infinite job supply at 1 and 3 2 servers The Push-Pull Network Control choice based on No idling, FULL UTILIZATION Preemptive resume Push Pull Push Pull
“interesting” Configurations: Processing Times
Policies Policy: Pull priority (LBFS) Policy: Linear thresholds Typical Behavior: 2, ,3 Typical Behavior: Server: “don’t let opposite queue go below threshold” Push Pull Push 1,3
8 is strong Markov with state space. A Markov Process Queue Residual
Stability Results Theorem (N., Weiss): Pull-priority,, is PHR Theorem (N., Weiss): Linear thresholds,, is PHR Theorem (in progress) (Lefeber, N.):, pull-priority, is PHR if More generally, when there is a matrix such that is PHR when e.g: Theorem (Lefeber, N.):, pull-priority, if, is PHR Current work: Generalizing to servers
Heuristic Modes Graph for M=3 Pull-Priority
Heuristic Stable Fluid Trajectory of M=3 Pull-Priority Case
PART 2: THE “FLUID LIMIT METHOD” FOR STABILITY
Main Idea Establish that an “associated” deterministic system is “stable” The “framework” then implies that is “stable” Nice, since stability of is sometimes easier to establish than directly working
Stochastic Model and Fluid Model
Comments on the Fluid Model T is Lipschitz and thus has derivative almost everywhere Any Y=(Q,T) that satisfies the fluid model is called a solution In general (for arbitrary networks) a solution can be non-unique
Stability of Fluid Model Definition: A fluid model is stable, if when ever, there exists T, such that for all solutions, Definition: A fluid model is weakly stable, if when ever Main Results of “Fluid Limit Method” Stable Fluid Model Positive Harris Recurrence Weakly Stable Fluid Model Technical Conditions on Markov Process (Pettiness) Rate Stability: Association of Fluid Model To Stochastic System
Association of Fluid Model and Stochastic System
Lyapounov Proofs for Fluid Stability When, it stays at 0. When, at regular points of t,. Need: for every solution of fluid model:
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QUESTIONS?