Task-Based Performance Portability in HPC: Maximising long-term investments in a fast evolving, complex and heterogeneous HPC landscape

White paperInternational audienceAs HPC hardware continues to evolve and diversify and workloads become more dynamic and complex, applications need to be expressed in a way that facilitates high performance across a range of hardware and situations. The main application code should be platform-independent, malleable and asynchronous with an open, clean, stable and dependable interface between the higher levels of the application, library or programming model and the kernels and software layers tuned for the machine. The platform-independent part should avoid direct references to specific resources and their availability, and instead provide the information needed to optimise behaviour.This paper summarises how task abstraction, which first appeared in the 1990s and is already mainstream in HPC, should be the basis for a composable and dynamic performance-portable interface. It outlines the innovations that are required in the programming model and runtime layers, and highlights the need for a greater degree of trust among application developers in the ability of the underlying software layers to extract full performance. These steps will help realise the vision for performance portability across current and future architectures and problems