Machine Covering in the Random-Order Model

Authors Susanne Albers, Waldo Gálvez , Maximilian Janke



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Susanne Albers
  • Department of Computer Science, Technische Universität München, Germany
Waldo Gálvez
  • Institute of Engineering Sciences, Universidad de O'Higgins, Rancagua, Chile
Maximilian Janke
  • Department of Computer Science, Technische Universität München, Germany

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Susanne Albers, Waldo Gálvez, and Maximilian Janke. Machine Covering in the Random-Order Model. In 32nd International Symposium on Algorithms and Computation (ISAAC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 212, pp. 52:1-52:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)
https://doi.org/10.4230/LIPIcs.ISAAC.2021.52

Abstract

In the Online Machine Covering problem jobs, defined by their sizes, arrive one by one and have to be assigned to m parallel and identical machines, with the goal of maximizing the load of the least-loaded machine. Unfortunately, the classical model allows only fairly pessimistic performance guarantees: The best possible deterministic ratio of m is achieved by the Greedy-strategy, and the best known randomized algorithm has competitive ratio Õ(√m) which cannot be improved by more than a logarithmic factor. Modern results try to mitigate this by studying semi-online models, where additional information about the job sequence is revealed in advance or extra resources are provided to the online algorithm. In this work we study the Machine Covering problem in the recently popular random-order model. Here no extra resources are present, but instead the adversary is weakened in that it can only decide upon the input set while jobs are revealed uniformly at random. It is particularly relevant to Machine Covering where lower bounds are usually associated to highly structured input sequences. We first analyze Graham’s Greedy-strategy in this context and establish that its competitive ratio decreases slightly to Θ(m/(log(m))) which is asymptotically tight. Then, as our main result, we present an improved Õ(∜m)-competitive algorithm for the problem. This result is achieved by exploiting the extra information coming from the random order of the jobs, using sampling techniques to devise an improved mechanism to distinguish jobs that are relatively large from small ones. We complement this result with a first lower bound showing that no algorithm can have a competitive ratio of O(log(m)/{log log(m)}) in the random-order model. This lower bound is achieved by studying a novel variant of the Secretary problem, which could be of independent interest.

Subject Classification

ACM Subject Classification
  • Theory of computation → Online algorithms
  • Theory of computation → Scheduling algorithms
Keywords
  • Machine Covering
  • Online Algorithm
  • Random-Order
  • Competitive Analysis
  • Scheduling

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