Exploiting Symmetry in Tensors for High Performance

Symmetric tensor operations arise in a wide variety of computations. However, the benefits of exploiting symmetry in order to reduce storage and computation is in conflict with a desire to simplify memory access patterns. In this paper, we propose Blocked Compact Symmetric Storage wherein we consider the tensor by blocks and store only the unique blocks of a symmetric tensor. We propose an algorithm-by-blocks, already shown of benefit for matrix computations, that exploits this storage format. A detailed analysis shows that, relative to storing and computing with tensors without taking advantage of symmetry, storage requirements are reduced by a factor O(m!) and computational requirements by a factor O(m), where m is the order of the tensor. An implementation demonstrates that the complexity introduced by storing and computing with tensors by blocks is manageable and preliminary results demonstrate that computational time is indeed reduced. The paper concludes with a discussion of how the insights point to opportunities for generalizing recent advances for the domain of linear algebra libraries to the field of multi-linear computation.