Asymptotic independence of queues under randomized load balancing

  • Authors:
  • Maury Bramson;Yi Lu;Balaji Prabhakar

  • Affiliations:
  • School of Mathematics, University of Minnesota, Minneapolis, USA 55455;Department of Electrical and Computer Engineering, University of Illinois, Urbana, USA 61801;Department of Electrical Engineering, Stanford University, Stanford, USA 94305

  • Venue:
  • Queueing Systems: Theory and Applications
  • Year:
  • 2012

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Abstract

Randomized load balancing greatly improves the sharing of resources while being simple to implement. In one such model, jobs arrive according to a rate-驴N Poisson process, with 驴D, D驴2, randomly chosen queues, the equilibrium queue sizes decay doubly exponentially in the limit as N驴驴. This is a substantial improvement over the case D=1, where queue sizes decay exponentially.The reasoning in Vvedenskaya et al. (Probl. Inf. Transm. 32:15---29, 1996) does not easily generalize to jobs with nonexponential service time distributions. A modularized program for treating randomized load balancing problems with general service time distributions was introduced in Bramson et al. (Proc. ACM SIGMETRICS, pp. 275---286, 2010). The program relies on an ansatz that asserts that, for a randomized load balancing scheme in equilibrium, any fixed number of queues become independent of one another as N驴驴. This allows computation of queue size distributions and other performance measures of interest.In this article, we demonstrate the ansatz in several settings. We consider the least loaded balancing problem, where an arriving job is assigned to the queue with the smallest workload. We also consider the more difficult problem, where an arriving job is assigned to the queue with the fewest jobs, and demonstrate the ansatz when the service discipline is FIFO and the service time distribution has a decreasing hazard rate. Last, we show the ansatz always holds for a sufficiently small arrival rate, as long as the service distribution has 2 moments.