This paper investigates interrupt response times and message passing latencies using OpenMPI on three different versions of the Linux kernel; one ‘standard’ server grade, one server grade with real-time pre-empt patches applied, and one with a proprietary, real-time kernel. In order to characterize these platforms, a series of measurements were made with and without a background CPU load. This paper will also highlight some system and Linux kernel tuning techniques that can improve determinism and affect system performance. The results are presented in a series of graphs showing histograms of interrupt response and MPI message latencies under the various workloads and tuning scenarios.