Towards Using Free Memory to Improve Microarchitecture Performance

A computer system's memory is designed to accommodate the worst-case workloads with the highest memory requirement; as such, memory is underutilized when a system runs workloads with common-case memory requirements. Through a large-scale study of four production HPC systems, we find that memory underutilization problem in HPC systems is very severe. As unused memory is wasted memory, we propose exposing a compute node's unused memory to its CPU(s) through a user-transparent CPU-OS codesign. This can enable many new microarchitecture techniques that transparently leverage unused memory locations to help improve microarchitecture performance. We refer to these techniques as Free-memory-aware Microarchitecture Techniques (FMTs). In the context of HPC systems, we present a detailed example of an FMT called Free-memory-aware Replication (FMR). FMR replicates in-use data to unused memory locations to effectively reduce average memory read latency. On average across five HPC benchmark suites, FMR provides 13% performance and 8% system-level energy improvement.