Hardware-software coherence protocol for the coexistence of caches and local memories

Abstract

Cache coherence protocols limit the scalability of chip multiprocessors. One solution is to introduce a local memory alongside the cache hierarchy, forming a hybrid memory system. Local memories are more power-efficient than caches and they do not generate coherence traffic but they suffer from poor programmability. When non-predictable memory access patterns are found compilers do not succeed in generating code because of the incoherency between the two storages. This paper proposes a coherence protocol for hybrid memory systems that allows the compiler to generate code even in the presence of memory aliasing problems. Coherency is ensured by a simple software/hardware co-design where the compiler identifies potentially incoherent memory accesses and the hardware diverts them to the correct copy of the data. The coherence protocol introduces overheads of 0.24% in execution time and of 1.06% in energy consumption to enable the usage of the hybrid memory system.