FairHym: Improving Inter-Process Fairness on Hybrid Memory Systems

Persistent memory (PMEM) is an emerging byte-addressable memory device that sits on a memory bus like conventional DRAM. A first PMEM product, Intel® Optane™ DC Persistent Memory (DCPM), has a larger capacity and lower cost per gigabyte than DRAM, although its performance is lower than that of DRAM. Therefore, hybrid memory systems that combine DRAM and DCPM for main memory are recommended to take advantage of both of them. However, our previous work revealed significant unfairness between two processes co-running on a real hybrid memory system; the performance of a process accessing DRAM is significantly degraded by another performing frequent writes to DCPM, but not vice versa. In this work, we propose FairHym that is a dynamic frequency scaling technique to improve the inter-process fairness on hybrid memory systems. It decreases the operating frequencies of CPU cores that run a process performing frequent DCPM writes in order to throttle its access frequency and prevent DRAM accesses from being blocked. We implement FairHym as a user-level runtime system and evaluate it with 36 two-process workloads on a real server. The evaluation results show that FairHym improves a fairness metric from 0.66 to 0.86 on average, compared to the default setting that maximizes the frequencies of all cores.