The Speed of Convergence in Congestion Games under Best-Response Dynamics

  • Authors:
  • Angelo Fanelli;Michele Flammini;Luca Moscardelli

  • Affiliations:
  • Department of Computer Science, University of L'Aquila, L'Aquila, 67100;Department of Computer Science, University of L'Aquila, L'Aquila, 67100;Department of Computer Science, University of L'Aquila, L'Aquila, 67100

  • Venue:
  • ICALP '08 Proceedings of the 35th international colloquium on Automata, Languages and Programming, Part I
  • Year:
  • 2008

Quantified Score

Hi-index 0.00

Visualization

Abstract

We investigate the speed of convergence of congestion games with linear latency functions under best response dynamics. Namely, we estimate the social performance achieved after a limited number of rounds, during each of which every player performs one best response move. In particular, we show that the price of anarchy achieved after k rounds, defined as the highest possible ratio among the total latency cost, that is the sum of all players latencies, and the minimum possible cost, is $O(\sqrt[2^{k-1}] {n})$, where n is the number of players. For constant values of k such a bound asymptotically matches the $\Omega(\sqrt[2^{k-1}] {n}/k)$ lower bound that we determine as a refinement of the one in [7]. As a consequence, we prove that order of loglogn rounds are not only necessary, but also sufficient to achieve a constant price of anarchy, i.e. comparable to the one at Nash equilibrium. This result is particularly relevant, as reaching an equilibrium may require a number of rounds exponential in n. We then provide a new lower bound of $\Omega(\sqrt[2^k-1] {n})$ for load balancing games, that is congestion games in which every strategy consists of a single resource, thus showing that a number of rounds proportional to loglogn is necessary and sufficient also under such a restriction.Our results thus solve the important left open question of the polynomial speed of convergence of linear congestion games to constant factor solutions.