PaRSEC in Practice: Optimizing a Legacy Chemistry Application through Distributed Task-Based Execution

Task-based execution has been growing in popularity as a means to deliver a good balance between performance and portability in the post-petascale era. The Parallel Runtime Scheduling and Execution Control (PARSEC) framework is a task-based runtime system that we designed to achieve high performance computing at scale. PARSEC offers a programming paradigm that is different than what has been traditionally used to develop large scale parallel scientific applications. In this paper, we discuss the use of PARSEC to convert a part of the Coupled Cluster (CC) component of the Quantum Chemistry package NWCHEM into a task-based form. We explain how we organized the computation of the CC methods in individual tasks with explicitly defined data dependencies between them and re-integrated the modified code into NWCHEM. We present a thorough performance evaluation and demonstrate that the modified code outperforms the original by more than a factor of two. We also compare the performance of different variants of the modified code and explain the different behaviors that lead to the differences in performance.