SpFFT Documentation

SpFFT - A 3D FFT library for sparse frequency domain data written in C++ with support for MPI, OpenMP, CUDA and ROCm. It was originally intended for transforms of data with spherical cutoff in frequency domain, as required by some computational material science codes.

For distributed computations, SpFFT uses a slab decomposition in space domain and pencil decomposition in frequency domain (all sparse data within a pencil must be on one rank). If desired, the libray can be compiled without any parallization (MPI, OpenMP, CUDA / ROCm).

Design Goals

• Sparse frequency domain input

• Reuse of pre-allocated memory

• Support of negative indexing for frequency domain data

• Unified interface for calculations on CPUs and GPUs

• Support of Complex-To-Real and Real-To-Complex transforms, where the full hermitian symmetry property is utilized.

• C++, C and Fortran interfaces

Interface Design

To allow for pre-allocation and reuse of memory, the design is based on two classes:

• Grid: Allocates memory for transforms up to a given size in each dimension.

• Transform: Is associated with a Grid and can have any size up to the Grid dimensions. A Transform holds a counted reference to the underlying Grid. Therefore, Transforms created with the same Grid share memory, which is only freed, once the Grid and all associated Transforms are destroyed.

The user provides memory for storing sparse frequency domain data, while a Transform provides memory for space domain data. This implies, that executing a Transform will override the space domain data of all other Transforms associated with the same Grid.

Note

The creation of Grids and Transforms, as well as the forward and backward execution may entail MPI calls and must be synchronized between all ranks.