Hierarchical branch and bound algorithm for computational grids

Branch and Bound (B&B) algorithms are efficiently used for exact resolution of combinatorial optimization problems (COPs). They are easy to parallelize using the Master/Worker paradigm (MW) but limited in scalability when solving large instances of COPs on large scale environments such as computational grids. Indeed, the master process rapidly becomes a bottleneck. In this paper, we propose a new approach H-B&B for parallel B&B based on a hierarchical MW paradigm in order to deal with the scalability issue of the traditional MW-based B&B. The hierarchy is built dynamically and evolves over time according to the dynamic acquisition of computing nodes. The inner nodes of the hierarchy (masters) perform branching operations to generate sub-trees and the leaves (workers) perform a complete exploration of these sub-trees. Therefore, in addition to the parallel exploration of sub-trees, a parallel branching is adopted. H-B&B is applied to the Flow-Shop scheduling problem. Unlike most existing grid-based B&B algorithms, H-B&B has been experimented on a real computational grid (Grid’5000). The results demonstrate the scalability and efficiency of H-B&B.

► Unlike existing approaches, our approach deals with scalability through the use of several hierarchical levels.
► The masters spawn and handle dynamically work units with different granularity sizes.
► A dynamic adaptive load balancing is proposed to take into account the heterogeneity of grid resources.
► According to their role (master or worker), the processes of the approach use different exploration strategies.