Average-Case and Smoothed Competitive Analysis of the Multilevel Feedback Algorithm

  • Authors:

  • Luca Becchetti;Stefano Leonardi;Alberto Marchetti-Spaccamela;Guido Schfer;Tjark Vredeveld

  • Affiliations:

  • Dipartimento di Informatica e Sistemistica, Universit di Roma La Sapienza, Via Salaria 113, 00198 Rome, Italy;Dipartimento di Informatica e Sistemistica, Universit di Roma La Sapienza, Via Salaria 113, 00198 Rome, Italy;Dipartimento di Informatica e Sistemistica, Universit di Roma La Sapienza, Via Salaria 113, 00198 Rome, Italy;Institut fr Mathematik, Technische Universitt Berlin, Strae des 17. Juni 136, 10623 Berlin, Germany;Department of Quantitative Economics, Maastricht University, P.O. Box 616, 6200 MD Maastricht, The Netherlands

  • Venue:
  • Mathematics of Operations Research
  • Year:
  • 2006

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Abstract

In this paper, we introduce the notion of smoothed competitive analysis of online algorithms. Smoothed analysis has been proposed by Spielman and Teng [25] to explain the behavior of algorithms that work well in practice while performing very poorly from a worst-case analysis point of view. We apply this notion to analyze the multilevel feedback algorithm (MLF) to minimize the total flow time on a sequence of jobs released over time when the processing time of a job is only known at time of completion. The initial processing times are integers in the range [1, 2K]. We use a partial bit randomization model, i.e., the initial processing times are smoothed by changing the k least significant bits under a quite general class of probability distributions. We show that MLF admits a smoothed competitive ratio of O((2k/)3 (2k/)2 2K-k), where denotes the standard deviation of the distribution. In particular, we obtain a competitive ratio of O(2K-k) if (2k). We also prove an (2K-k) lower bound for any deterministic algorithm that is run on processing times smoothed according to the partial bit randomization model. For various other smoothing models, including the additive symmetric smoothing one, which is a variant of the model used by Spielman and Teng [25], we give a higher lower bound of (2K). A direct consequence of our result is also the first average-case analysis of MLF. We show a constant expected ratio of the total flow time of MLF to the optimum under several distributions including the uniform one.