Stapl-rts: An application driven runtime system

Modern HPC systems are growing in complexity, as they move towards deeper memory hierarchies and increasing use of computational heterogeneity via GPUs or other accelerators. When developing applications for these platforms, programmers are faced with two bad choices. On one hand, they can explicitly manage all machine resources, writing programs decorated with low level primitives from multiple APIs (e.g. Hybrid MPI/OpenMP applications). Though seemingly necessary for efficient execution, it is an inherently non-scalable way to write software. Without a separation of concerns, only small programs written by expert developers actually achieve this efficiency. Furthermore, the implementations are rigid, difficult to extend, and not portable. Alternatively, users can adopt higher level programming environments to abstract away these concerns. Extensibility and portability, however, often come at the cost of lost performance. The mapping of a user's application onto the system now occurs without the contextual information that was immediately available in the more coupled approach. In this paper, we describe a framework for the transfer of high level, application semantic knowledge into lower levels of the software stack at an appropriate level of abstraction. Using the stapl library, we demonstrate how this information guides important decisions in the runtime system (stapl-rts), such as multi-protocol communication coordination and request aggregation. Through examples, we show how generic programming idioms already known to C++ programmers are used to annotate calls and increase performance.