Scheduling groups of tasks with precedence constraints on three dedicated processors

Renata Mansini, M. Grazia Speranza, Zsolt Tuza

Research output: Contribution to journalArticle

7 Citations (Scopus)


We study the problem of scheduling groups of tasks with precedence constraints on three dedicated processors. Each task requires a specified set of processors. Up to three precedence constraints are considered among groups of tasks requiring the same set of processors. The objective of the problem is to find a nonpreemptive schedule which minimizes the maximum completion time (makespan). This scheduling problem is equivalent to the problem of finding an extension of the constraint graph (i.e. the graph which represents the conflicts between tasks and the precedence constraints) to a comparability graph with minimum (over all the extensions) maximum clique weight. The problem is NP-hard in the strong sense. A normal schedule is such that all the tasks requiring the same set of processors are scheduled consecutively. With a normal schedule the problem reduces to the quotient graph of the constraint graph. In this paper we obtain tight approximation results for the minimum makespan of a normal schedule through tight results on the minimum increase of the maximum clique weight when the (partially oriented) quotient graph is extended to a comparability graph.

Original languageEnglish
Pages (from-to)141-168
Number of pages28
JournalDiscrete Applied Mathematics
Issue number1-3
Publication statusPublished - Jan 5 2004


  • Approximation algorithm
  • Comparability graph
  • Maximum clique weight
  • Normal schedule
  • Scheduling

ASJC Scopus subject areas

  • Discrete Mathematics and Combinatorics
  • Applied Mathematics

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