Preservation of non-uniform memory architecture characteristics when going from a nested OpenMP to a hybrid MPI/OpenMP approach

While the noticeable shift from serial to parallel programming in simulation technologies progresses, it is increasingly important to better understand the interplay of different parallel programming paradigms. We discuss some corresponding issues in the context of transforming a shared-memory parallel program that involves two nested levels of parallelism into a hybrid parallel program. Here, hybrid programming refers to a combination of shared and distributed memory. In particular, we focus on performance aspects arising from shared-memory parallel programming where the time to access a memory location varies with the threads. Rather than analyzing these issues in general, the focus of this position paper is on a particular case study from geothermal reservoir engineering.