Development and performance of a mixed openMP/MPI quantum Monte Carlo code

An OpenMP version of a Quantum Monte Carlo (QMC) code has been developed. The original parallel MPI version of the QMC code was developed by the Electronic Structure of Solids HPCI consortium in collaboration with EPCC. This code has been highly successful, and has resulted in numerous publications based on results generated on the National Cray MPP systems at EPCC. Recent interest has focused on also utilizing shared-memory parallelism in the code since future HPC systems are expected to comprise clusters of SMP nodes. The code has been rewritten to allow for an arbitrary mix of OpenMP and MPI parallelism. The various issues which arose during the parallelization are discussed. The performance of the mixed OpenMP/MPI code has been assessed on an SGI Origin 2000 system and the results compared and contrasted to the original MPI version.