Framework for Extensible, Asynchronous Task Scheduling (FEATS) in Fortran

Abstract

Most parallel scientific programs contain compiler directives (pragmas) such as those from OpenMP [1], explicit calls to runtime library procedures such as those implementing the Message Passing Interface (MPI) [2], or compiler-specific language extensions such as those provided by CUDA [3]. By contrast, the recent Fortran standards empower developers to express parallel algorithms without directly referencing lower-level parallel programming models [4,5]. Fortran's parallel features place the language within the Partitioned Global Address Space (PGAS) class of programming models. When writing programs that exploit data-parallelism, application developers often find it straightforward to develop custom parallel algorithms. Problems involving complex, heterogeneous, staged calculations, however, pose much greater challenges. Such applications require careful coordination of tasks in a manner that respects dependencies prescribed by a directed acyclic graph. When rolling one's own solution proves difficult, extending a customizable framework becomes attractive. The paper presents the design, implementation, and use of the Framework for Extensible Asynchronous Task Scheduling (FEATS), which we believe to be the first task-scheduling tool written in modern Fortran. We describe the benefits and compromises associated with choosing Fortran as the implementation language, and we propose ways in which future Fortran standards can best support the use case in this paper.