Lightweight Frequency-Based Tiering for CXL Memory Systems

Modern workloads are demanding increasingly larger memory capacity. Compute Express Link (CXL)-based memory tiering has emerged as a promising solution for addressing this trend by utilizing traditional DRAM alongside slow-tier CXL-memory devices in the same system. Unfortunately, most prior tiering systems are recency-based, which cannot accurately identify hot and cold pages, since a recently accessed page is not necessarily a hot page. On the other hand, more accurate frequency-based systems suffer from high memory and runtime overhead as a result of tracking large memories. In this paper, we propose FreqTier, a fast and accurate frequency-based tiering system for CXL memory. We observe that memory tiering systems can tolerate a small amount of tracking inaccuracy without compromising the overall application performance. Based on this observation, FreqTier probabilistically tracks the access frequency of each page, enabling accurate identification of hot and cold pages while maintaining minimal memory overhead. Finally, FreqTier intelligently adjusts the intensity of tiering operations based on the application's memory access behavior, thereby significantly reducing the amount of migration traffic and application interference. We evaluate FreqTier on two emulated CXL memory devices with different bandwidths. On the high bandwidth CXL device, FreqTier can outperform state-of-the-art tiering systems while using 4$\times$ less local DRAM memory for in-memory caching workloads. On GAP graph analytics and XGBoost workloads with 1:32 local DRAM to CXL-memory ratio, FreqTier outperforms prior works by 1.04$-$2.04$\times$ (1.39$\times$ on average). Even on the low bandwidth CXL device, FreqTier outperforms AutoNUMA by 1.14$\times$ on average.