Efficient parallel CP decomposition with pairwise perturbation and multi-sweep dimension tree

Abstract

The widely used alternating least squares (ALS) algorithm for the canonical polyadic (CP) tensor decomposition is dominated in cost by the matricized-tensor times Khatri-Rao product (MTTKRP) kernel. This kernel is necessary to set up the quadratic optimization subproblems. State-of-the-art parallel ALS implementations use dimension trees to avoid redundant computations across MTTKRPs within each ALS sweep. In this paper, we propose two new parallel algorithms to accelerate CP-ALS. We introduce the multi-sweep dimension tree (MSDT) algorithm, which requires the contraction between an order N input tensor and the first-contracted input matrix once every $(N-1)/N$ sweeps. This algorithm reduces the leading order computational cost by a factor of $2(N-1)/N$ relative to the best previously known approach. In addition, we introduce a more communication-efficient approach to parallelizing an approximate CP-ALS algorithm, pairwise perturbation. This technique uses perturbative corrections to the subproblems rather than recomputing the contractions, and asymptotically accelerates ALS. Our benchmark results on 1024 processors on the Stampede2 supercomputer show that CP decomposition obtains a 1.25X speed-up from MSDT and a 1.94X speedup from pairwise perturbation compared to the state-of-the-art dimension-tree based CP-ALS implementations.