Do Inherently Sequential Branch-and-Bound Algorithms Exist?

In the construction of algorithms for optimization problems the Branch-and-Bound paradigm is an essential tool. Furthermore, Branch-and-Bound algorithms are traditionally regarded as well suited for parallel implementation due to the subdivision of the problem considered into essentially independent subproblems. In this paper we present experimental results for a Branch-and-Bound algorithm for the Graph Partitioning Problem showing that the traditional parallelization of a Branch-and-Bound algorithm does not always lead to an efficient parallel algorithm. The main reason seems to be lack of meaningful work, i.e. concurrent existence of subproblems which have to be solved to ensure optimality of the solution. We support this claim with experimental results.