MDACache: Caching for Multi-Dimensional-Access Memories

For several emerging memory technologies, a natural formulation of memory arrays (cross-point) provides nearly symmetric access costs along multiple (e.g., both row and column) dimensions in contrast to the row-oriented nature of most DRAM and SRAM implementations, producing a Multi-Dimensional-Access (MDA) memory. While MDA memories can directly support applications with both row and column preferences, most modern processors do not directly access either the rows or columns of memories: memory accesses proceed through a cache hierarchy that abstracts many of the physical features that supply the aforementioned symmetry. To reap the full benefits of MDA memories, a co-design approach must occur across software memory layout, the mapping between the physical and logical organization of the memory arrays, and the cache hierarchy itself in order to efficiently express, convey, and exploit multidimensional access patterns. In this paper, we describe a taxonomy for different ways of connecting row and column preferences at the application level to an MDA memory through an MDA cache hierarchy and explore specific implementations for the most plausible design points. We extend vectorization support at the compiler level to provide the necessary information to extract preferences and provide compatible memory layouts, and evaluate the tradeoffs among multiple cache designs for the MDA memory systems. Our results indicate that both logically 2-D caching using physically 1-D SRAM structures and on-chip physically 2-D caches can both provide significant improvements in performance over a traditional cache system interfacing with an MDA memory, reducing execution time by 72% and 65%, respectively. We then explore the sensitivity of these benefits as a function of the working-set to cache capacity ratio as well as to MDA technology assumptions