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1. Nomad: Non-Exclusive Memory Tiering via Transactional Page Migration

   Xiang, Lingfeng; Lin, Zhen; Deng, Weishu; Lu, Hui; Rao, Jia Rao; Yuan, Yifan; Wang, Ren (July 2024, The Proceedings of the 18th USENIX Symposium on Operating Systems Design and Implementation (OSDI'24).)

   With the advent of byte-addressable memory devices, such as CXLmemory, persistent memory, and storage-class memory, tiered memory systems have become a reality. Page migration is the de facto method within operating systems for managing tiered memory. It aims to bring hot data whenever possible into fast memory to optimize the performance of data accesses while using slow memory to accommodate data spilled from fast memory. While the existing research has demonstrated the effectiveness of various optimizations on page migration, it falls short of addressing a fundamental question: Is exclusive memory tiering, in which a page is either present in fast memory or slow memory, but not both simultaneously, the optimal strategy for tiered memory management? We demonstrate that page migration-based exclusive memory tiering suffers significant performance degradation when fast memory is under pressure. In this paper, we propose nonexclusive memory tiering, a page management strategy that retains a copy of pages recently promoted from slow memory to fast memory to mitigate memory thrashing. To enable non-exclusive memory tiering, we develop NOMAD, a new page management mechanism for Linux that features transactional page migration and page shadowing. NOMAD helps remove page migration off the critical path of program execution and makes migration completely asynchronous. Evaluations with carefully crafted micro-benchmarks and real-world applications show that NOMAD is able to achieve up to 6x performance improvement over the state-of-the-art transparent page placement (TPP) approach in Linux when under memory pressure. We also compare NOMAD with a recently proposed hardware-assisted, access sampling-based page migration approach and demonstrate NOMAD’s strengths and potential weaknesses in various scenarios. 

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2. Tiered Memory Management Beyond Hotness

   Liu, Jinshu; Hadian, Hamid; Hanchen, Xu; Li, Huaicheng (July 2025, USENIX 19th USENIX Symposium on Operating Systems Design and Implementation (OSDI'25))

   Tiered memory systems often rely on access frequency ("hotness") to guide data placement. However, hot data is not always performance-critical, limiting the effectiveness of hotness-based policies. We introduce amortized offcore latency (AOL), a novel metric that precisely captures the true performance impact of memory accesses by accounting for memory access latency and memory-level parallelism (MLP). Leveraging AOL, we present two powerful tiering mechanisms: SOAR, a profile-guided allocation policy that places objects based on their performance contribution, and ALTO, a lightweight page migration regulation policy to eliminate unnecessary migrations. SOAR and ALTO outperform four state-of-the-art tiering designs across a diverse set of workloads by up to 12.4\texttimes{}, while underperforming in a few cases by no more than 3\%. 

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3. Flexible and Effective Object Tiering for Heterogeneous Memory Systems

   https://doi.org/10.1145/3708540  

   Kammerdiener, Brandon; Mcmichael, J Zach; Jantz, Michael; Doshi, Kshitij; Jones, Terry (March 2025, ACM Transactions on Architecture and Code Optimization)

   Computing platforms that package multiple types of memory, each with their own performance characteristics, are quickly becoming mainstream. To operate efficiently, heterogeneous memory architectures require new data management solutions that are able to match the needs of each application with an appropriate type of memory. As the primary generators of memory usage, applications create a great deal of information that can be useful for guiding memory management, but the community still lacks tools to collect, organize, and leverage this information effectively. To address this gap, this work introduces a novel software framework that collects and analyzesobject-levelinformation to guide memory tiering. The framework includes tools to monitor the capacity and usage of individual data objects, routines that aggregate and convert this information into tier recommendations for the host platform, and mechanisms to enforce these recommendations according to user-selected policies. Moreover, the developed tools and techniques are fully automatic, work on standard Linux systems, and do not require modification or recompilation of existing software. Using this framework, this study evaluates and compares the impact of a variety of design choices for memory tiering, including different policies for prioritizing objects for the fast memory tier as well as the frequency and timing of migration events. The results, collected on a modern Intel platform with conventional DDR4 SDRAM as well as Intel Optane NVRAM, show that guiding data tiering with object-level information can enable significant performance and efficiency benefits compared with standard hardware- and software-directed data-tiering strategies for a diverse set of memory-intensive workloads. 

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4. FBMM: Using the VFS for Extensibility in Kernel Memory Management

   https://doi.org/10.1145/3593856.3595908  

   Tabatabai, Bijan; Mansi, Mark; Swift, Michael M. (June 2023, HOTOS '23: Proceedings of the 19th Workshop on Hot Topics in Operating Systems)

   Modern memory hierarchies are increasingly complex, with more memory types and richer topologies. Unfortunately kernel memory managers lack the extensibility that many other parts of the kernel use to support diversity. This makes it difficult to add and deploy support for new memory configurations, such as tiered memory: engineers must navigate and modify the monolithic memory management code to add support, and custom kernels are needed to deploy such support until it is upstreamed. We take inspiration from filesystems and note that VFS, the extensible interface for filesystems, supports a huge variety of filesystems for different media and different use cases, and importantly, has interfaces for memory management operations such as controlling virtual-to-physical mapping and handling page faults. We propose writing memory management systems as filesystems using VFS, bringing extensibility to kernel memory management. We call this idea File-Based Memory Management (FBMM). Using this approach, many recent memory management extensions, e.g., tiering support, can be written without modifying existing memory management code. We prototype FBMM in Linux to show that the overhead of extensibility is low (within 1.6%) and that it enables useful extensions. 

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5. On-the-fly Page Migration and Address Reconciliation for Heterogeneous Memory Systems

   https://doi.org/10.1145/3364179  

   Islam, Mahzabeen; Adavally, Shashank; Scrbak, Marko; Kavi, Krishna (January 2020, ACM Journal on Emerging Technologies in Computing Systems)

   For efficient placement of data in flat-address heterogeneous memory systems consisting of fast (e.g., 3D-DRAM) and slow memories (e.g., NVM), we present a hardware-based page migration technique. Unlike epoch-based approaches that migrate heavily accessed (“hot”) pages from slow to fast memories at each epoch interval, we migrate a page immediately when it becomes hot (“on-the-fly”), using hardware in user-transparent manner and with minimal OS intervention. The management of physical addresses due to page relocation becomes cumbersome and requires costly OS intervention. We use a small hardware remap table to keep track of new physical addresses of the migrated pages. This limits address reconciliation to occur only at periodic evictions of old remap entries. Also, we propose a hardware-orchestrated light-weight address reconciliation process. For our studied heterogeneous memory system, on-the-fly page migration with hardware-assisted address reconciliation provides 74% and 24% IPC improvements, on average for a set of SPEC CPU2006 workloads when compared to a baseline without any page migration and a system with on-the-fly page migration using OS-based address reconciliation, respectively. Furthermore, we present an analytical model for classifying applications as page migration friendly (applications that show performance gains from page migration) or unfriendly based on memory access behavior. 

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