Section: New Results

Simulation

Simgrid is a toolkit providing core functionalities for the simulation of distributed applications in heterogeneous distributed environments. Although it was initially designed to study large distributed computing environments such as grids, we have recently applied it to performance prediction of HPC configurations.

• Finite difference methods are, in general, well suited to execution on parallel machines and are thus commonplace in High Performance Computing. Yet, despite their apparent regularity, they often exhibit load imbalance that damages their efficiency. In [38], we characterize the spatial and temporal load imbalance of Ondes3D, a seismic wave propagation simulator used to conduct regional scale risk assessment. Our analysis reveals that this imbalance originates from the structure of the input data and from low-level CPU optimizations. We then show that the CHARM++ runtime can effectively dynamically rebalance the load by migrating data and computation at the granularity of an MPI rank. We propose a methodology that leverages the capabilities of the SimGrid simulation framework and allows to conduct an experimental study at low computational cost.

• The article [35] summarizes our recent work and developments on SMPI, a flexible simulator of MPI applications. In this tool, we took a particular care to ensure our simulator could be used to produce fast and accurate predictions in a wide variety of situations. Although we did build SMPI on SimGrid whose speed and accuracy had already been assessed in other contexts, moving such techniques to a HPC workload required significant additional effort. Obviously, an accurate modeling of communications and network topology was one of the key to such achievements. Another less obvious key was the choice to combine in a single tool the possibility to do both offline and online simulation.