On The Probable Performance Of Heuristics For Bandwidth Minimization

Most research in algorithm design relies on worst-case analysis for performance comparisons. Unfortunately, worst-case analysis does not always provide an adequate measure of an algorithm's effectiveness. This is particularly true in the case of heuristic algorithms for hard combinatorial problems. In such cases, analysis of the probable performance can yield more meaningful results and can provide insight leading to better algorithms. The problem of minimizing the bandwidth of a sparse symmetric matrix by perfoming simultaneous row and column permutations, is an example of a problem for which there are well-known heuristics whose practical success has lacked a convincing analytical explanation. A class of heuristics introduced by Cuthill and McKee in 1969, and referred to here as the level algorithms, are the basis for bandwidth minimization routines that have been widely used for over a decade. At the same time, it is easy to construct examples, showing that the level algorithms can produce solutions that differ from optimal by an arbitrarily large factor. This paper provides an analytical explanation for the practical success of the level algorithms, by showing that for random matrices having optimal bandwidth no larger than k, any level algorithm will produce solutions that differ from optimal by a small constant factor. The analysis also suggests another class of algorithms with better performance. One algorithm in this class is shown to produce solutions that are nearly optimal