As the core count of HPC machines continue to grow in size, issues such as fault tolerance and reliability are becoming limiting factors for application scalability. Current techniques to ensure progress across faults, for example coordinated checkpoint-restart, are unsuitable for machines of this scale due to their predicted high overheads. In this study, we present the design and implementation of a novel system for ensuring reliability which uses transparent, rank-level, redundant computation. Using this system, we show the overheads involved in redundant computation for a number of real-world HPC applications. Additionally, we relate the communication characteristics of an application to the overheads observed.