High-performance design patterns for modern Fortran

Abstract

This paper presents ideas for using coordinate-free numerics in modern Fortran to achieve code flexibility in the partial differential equation (PDE) domain. We also show how Fortran, over the last few decades, has changed to become a language well-suited for state-of-the-art software development. Fortran's new coarray distributed data structure, the language's class mechanism, and its side-effect-free, pure procedure capability provide the scaffolding on which we implement HPC software. These features empower compilers to organize parallel computations with efficient communication. We present some programming patterns that support asynchronous evaluation of expressions comprised of parallel operations on distributed data. We implemented these patterns using coarrays and the message passing interface (MPI). We compared the codes' complexity and performance. The MPI code is much more complex and depends on external libraries. The MPI code on Cray hardware using the Cray compiler is 1.5-2 times faster than the coarray code on the same hardware. The Intel compiler implements coarrays atop Intel's MPI library with the result apparently being 2-2.5 times slower than manually coded MPI despite exhibiting nearly linear scaling efficiency. As compilers mature and further improvements to coarrays comes in Fortran 2015, we expect this performance gap to narrow.

A Monte Carlo-based Poisson's equation solver parallelized with Coarray Fortran

“…CAF's speedup performance was found to be 3.6% higher on average than the ideal value. This superlinear speedup is caused by cache effects [20]. More data can be stored in caches as the number of processors increases.…”

OpenCoarrays

“…Furthermore, in studies with the Cray compiler, the Burgers solver exhibits 87% parallel efficiency in weak scaling on 16,384 cores. The solver exhibits linear scaling (sometimes super-linear) [7] and a separate, upcoming paper provides comparisons to a MPI version of the solver. It uses coarrays mainly for scalar transfers.…”