Analyzing the impact of overlap, offload, and independent progress for MPI.

The overlap of computation and communication has long been considered to be a significant performance benefit for applications. Similarly, the ability of the Message Passing Interface (MPI) to make independent progress (that is, to make progress on outstanding communication operations while not in the MPI library) is also believed to yield performance benefits. Using an intelligent network interface to offload the work required to support overlap and independent progress is thought to be an ideal solution, but the benefits of this approach have not been studied in depth at the application level. This lack of analysis is complicated by the fact that most MPI implementations do not sufficiently support overlap or independent progress. Recent work has demonstrated a quantifiable advantage for an MPI implementation that uses offload to provide overlap and independent progress. The study is conducted on two different platforms with each having two MPI implementations (one with and one without independent progress). Thus, identical network hardware and virtually identical software stacks are used. Furthermore, one platform, ASCI Red, allows further separation of features such as overlap and offload. Thus, this paper extends previous work by further qualifying the source of the performance advantage: offload, overlap, or independent progress.