3D DFT by block tensor-matrix multiplication via a modified Cannon's algorithm: Implementation and scaling on distributed-memory clusters with fat tree networks

A known scalability bottleneck of the parallel 3D FFT is its use of all-to-all communications. Here, we present S3DFT, a library that circumvents this by using point-to-point communication – albeit at a higher arithmetic complexity. This approach exploits three variants of Cannon's algorithm with adaptations for block tensor-matrix multiplications. We demonstrate S3DFT's efficient use of hardware resources, and its scaling using up to 16,464 cores of the JUWELS Cluster. However, in a comparison with well-established 3D FFT libraries, its parallel efficiency and performance were found to fall behind. A detailed analysis identifies the cause in two of its component algorithms, which scale poorly owing to how their communication patterns are mapped in subsets of the fat tree topology. This result exposes a potential drawback of running block-wise parallel algorithms on systems with fat tree networks caused by increased communication latencies along specific directions of the mesh of processing elements.