Communication Avoiding Algorithms: Analysis and Code Generation for Parallel Systems

Data movement is a critical bottleneck for future generations of parallel systems. The class of .5D communication-avoiding algorithms were developed to address this bottleneck. These algorithms reduce communication and provide strong scaling in both time and energy. As a first step towards automating the development of communication-avoiding libraries, we developed the Maunam compiler. Maunam generates efficient parallel code from a high-level, global view sketch of .5D algorithms that are expressed using symbolic data sizes and numbers of processors. It supports the expression of data movement and communication through high-level global operations such as TILT and CSHIFT as well as through element-wise copy operations. With the latter, wraparound communication patterns can also be achieved using subscripts based on modulo operations. Maunam employs polyhedral analysis to reason about communication and computation present in a .5D algorithm. After partitioning data and computation, it inserts point-to-point and collective communication as needed. Maunam also analyzes data dependence patterns and data layouts to identify reductions over processor subsets. Maunam-generated Fortran+MPI code for 2.5D matrix multiplication running on 4096 cores of a Cray XC30 supercomputer achieves 59 TFlops/s (76% of the machine peak). Our generated parallel code achieves 91% of the performance of a hand-coded version.