xSim: The extreme-scale simulator

Investigating parallel application performance at scale is an important part of high-performance computing (UPC) application development. The Extreme-scale Simulator (xSim) is a performance toolkit that permits running an application in a controlled environment at extreme scale without the need for a respective extreme-scale HPC system. Using a lightweight parallel discrete event simulation, xSim executes a parallel application with a virtual wall clock time, such that performance data can be extracted based on a processor and a network model. This paper presents significant enhancements to the xSim toolkit that provide a more complete Message Passing Interface (MPI) support and improve its versatility. These enhancements include full virtual MPI group, communicator and collective communication support, and global variables support. The new capabilities are demonstrated by executing the entire NAS Parallel Benchmark suite in a simulated HPC environment.