Scaling Shared Memory Multiprocessing Applications in Non-cache-coherent Domains

Abstract

Due to the slowdown of Moore's Law, systems designers have begun integrating non-cache-coherent heterogeneous computing elements in order to continue scaling performance. Programming such systems has traditionally been difficult - developers were forced to use programming models that exposed multiple memory regions, requiring developers to manually maintain memory consistency. Previous works proposed distributed shared memory (DSM) as a way to achieve high programmability in such systems. However, past DSM systems were plagued by low-bandwidth networking and utilized complex memory consistency protocols, which limited their adoption. Recently, new networking technologies have begun to change the assumptions about which components are bottlenecks in the system. Additionally, many popular shared-memory programming models utilize memory consistency semantics similar to those proposed for DSM, leading to widespread adoption in mainstream programming. In this work, we argue that it is time to revive DSM as a means for achieving good programmability and performance on non-cache-coherent systems. We explore optimizing an existing DSM protocol by relaxing memory consistency semantics and exposing new cross-node barrier primitives. We integrate the new mechanisms into an existing OpenMP runtime, allowing developers to leverage cross-node execution without changing a single line of code. When evaluated on an x86 server connected to an ARMv8 server via InfiniBand, the DSM optimizations achieve an average of 11% (up to 33%) improvement versus the baseline DSM implementation.