The Taub Faculty of Computer Science Events and Talks

In a paper of 1974 Dijkstra introduced the notion of self-stabilizing algorithms and presented three such algorithms for the problem of mutual exclusion on a ring of $n$ processors. The third algorithm is the most interesting of these three but is rather non intuitive. In 1986 a proof of its correctness was presented by Dijkstra, but the question of determining its worst case complexity --- that is, providing an upper bound on the number of moves of this algorithm until it stabilizes --- remained open. In this work we solve this question and prove an upper bound of $3\frac{13}{18} n^2 + O(n)$ for the complexity of this algorithm. We also show a lower bound of $1\frac{5}{6} n^2 - O(n)$ for the worst case complexity. For computing the upper bound, we use two techniques: potential functions and amortized analysis. We also present a new-three state self-stabilizing algorithm for mutual exclusion and show a tight bound of $\frac{5}{6} n^2 + O(n)$ for the worst case complexity of this algorithm. In 1995 Beauquier and Debas presented a similar three-state algorithm, with an upper bound of $5\frac{3}{4}n^2+O(n)$ and a lower bound of $\frac{1}{8}n^2-O(n)$ for its stabilization time. For this algorithm we prove an upper bound of $1\frac{1}{2}n^2 + O(n)$ and show a lower bound of $n^2 - O(n)$. As far as the worst case performance is considered, the algorithm of Beauquier and Debas is better than the one of Dijkstra and our algorithm is better than both.