Approximate Strong Equilibrium in Job Scheduling Games

  • Authors:
  • Michal Feldman;Tami Tamir

  • Affiliations:
  • School of Business Administration and Center for the Study of Rationality, Hebrew University of Jerusalem,;School of Computer Science, The Interdisciplinary Center, Herzliya, Israel

  • Venue:
  • SAGT '08 Proceedings of the 1st International Symposium on Algorithmic Game Theory
  • Year:
  • 2008

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Abstract

A Nash Equilibriun (NE) is a strategy profile that is resilientto unilateral deviations, and is predominantly used in analysis ofcompetitive games. A downside of NE is that it is not necessarilystable against deviations by coalitions. Yet, as we show in thispaper, in some cases, NE does exhibit stability against coalitionaldeviations, in that the benefits from a joint deviation arebounded. In this sense, NE approximates strong equilibrium(SE) [6].We provide a framework for quantifying the stability and theperformance of various assignment policies and solution concept inthe face of coalitional deviations. Within this framework weevaluate a given configuration according to three measurements: (i)IRmin: the maximal number α,such that there exists a coalition in which the minimum improvementratio among the coalition members is α(ii) IRmax: the maximum improvement ratio among thecoalition's members. (iii) DRmax: themaximum possible damage ratio of an agent outside thecoalition.This framework can be used to study the proximity betweendifferent solution concepts, as well as to study the existence ofapproximate SE in settings that do not possess any suchequilibrium. We analyze these measurements in job scheduling gameson identical machines. In particular, we provide upper and lowerbounds for the above three measurements for both NE and thewell-known assignment rule Longest Processing Time(LPT)(which is known to yield a NE). Most of our bounds are tight forany number of machines, while some are tight only for threemachines. We show that both NE and LPT configurations yield smallconstant bounds for IRminand DRmax. As for IRmax, itcan be arbitrarily large for NE configurations, while a small boundis guaranteed for LPT configurations. For all three measurements,LPT performs strictly better than NE.With respect to computational complexity aspects, we show thatgiven a NE on m≥ 3 identical machines and a coalition,it is NP-hard to determine whether the coalition can deviate suchthat every member decreases its cost. For the unrelated machinessettings, the above hardness result holds already for m≥2 machines.