Maximizing Communication Overlap with Dynamic Program Analysis

We present a dynamic program analysis approach to optimize communication overlap in scientific applications. Our tool instruments the code to generate a trace of the application's memory and synchronization behavior. An offline analysis determines the program optimal points for maximal overlap when considering several programming constructs: nonblocking one-sided communication operations, non-blocking collectives and bespoke synchronization patterns and operations. Feedback about possible transformations is presented to the user and the tool can perform the directed transformations, which are supported by a lightweight runtime. The value of our approach comes from: 1) the ability to optimize across boundaries of software modules or libraries, while specializing for the intrinsics of the underlying communication runtime; and 2) providing upper bounds on the expected performance improvements after communication optimizations. We have reduced the time spent in communication by as much as 64% for several applications that were already aggressively optimized for overlap; this indicates that manual optimizations leave untapped performance. Although demonstrated mainly for the UPC programming language, the methodology can be easily adapted to any other communication and synchronization API.