Many random walks are faster than one

  • Authors:
  • Noga Alon;Chen Avin;Michal Koucky;Gady Kozma;Zvi Lotker;Mark R. Tuttle

  • Affiliations:
  • Tel Aviv University, Tel Aviv, Israel;Ben Gurion University of the Negev, Beer-Sheva, Israel;Czech Academy of Sciences, Prague, Czech Rep;Weizmann Institute of Science, Rehovot, Israel;Ben Gurion University of the Negev, Beer-Sheva, Israel;Intel, Hudson, MA, USA

  • Venue:
  • Proceedings of the twentieth annual symposium on Parallelism in algorithms and architectures
  • Year:
  • 2008

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Abstract

We pose a new and intriguing question motivated by distributed computing regarding random walks on graphs: How long does it take for several independent random walks, starting from the same vertex, to cover an entire graph? We study the cover time - the expected time required to visit every node in a graph at least once - and we show that for a large collection of interesting graphs, running many random walks in parallel yields a speed-up in the cover time that is linear in the number of parallel walks. We demonstrate that an exponential speed-up is sometimes possible, but that some natural graphs allow only a logarithmic speed-up. A problem related to ours (in which the walks start from some probablistic distribution on vertices) was previously studied in the context of space efficient algorithms for undirected s-t-connectivity and our results yield, in certain cases, an improvement upon some of the earlier bounds.