The paper "NOMAD: Non-Exclusive Memory Tiering via Transactional Page Migration" by Lingfeng Xiang et al. addresses the challenges of managing tiered memory systems, particularly in the context of emerging memory technologies such as CXL memory, persistent memory, and storage-class memory. The authors critique the current approach of exclusive memory tiering, where pages are either present in fast or slow memory but not both simultaneously, and argue that this approach can lead to significant performance degradation when fast memory is under pressure. To mitigate these issues, the paper introduces *non-exclusive* memory tiering, which allows pages recently promoted from slow memory to have shadow copies in the fast memory. This approach helps to reduce memory thrashing and improves performance. The core innovation is *transactional page migration* (TPM), a mechanism that enables asynchronous page migration by copying pages without unmapping them from the page table, thus keeping the pages accessible to user programs during migration. The authors also develop NOMAD, a new page management framework for Linux that integrates non-exclusive memory tiering and TPM. NOMAD includes features such as page shadowing to manage shadow copies efficiently and a mechanism to prioritize the reclamation of shadow pages during memory pressure. Evaluations on various platforms, including an FPGA-based CXL prototype, a persistent memory system, and a commercial CXL system, show that NOMAD achieves up to 6x performance improvement over state-of-the-art transparent page placement (TPP) and outperforms other approaches like Memtis in terms of both stability and performance under memory pressure.The paper "NOMAD: Non-Exclusive Memory Tiering via Transactional Page Migration" by Lingfeng Xiang et al. addresses the challenges of managing tiered memory systems, particularly in the context of emerging memory technologies such as CXL memory, persistent memory, and storage-class memory. The authors critique the current approach of exclusive memory tiering, where pages are either present in fast or slow memory but not both simultaneously, and argue that this approach can lead to significant performance degradation when fast memory is under pressure. To mitigate these issues, the paper introduces *non-exclusive* memory tiering, which allows pages recently promoted from slow memory to have shadow copies in the fast memory. This approach helps to reduce memory thrashing and improves performance. The core innovation is *transactional page migration* (TPM), a mechanism that enables asynchronous page migration by copying pages without unmapping them from the page table, thus keeping the pages accessible to user programs during migration. The authors also develop NOMAD, a new page management framework for Linux that integrates non-exclusive memory tiering and TPM. NOMAD includes features such as page shadowing to manage shadow copies efficiently and a mechanism to prioritize the reclamation of shadow pages during memory pressure. Evaluations on various platforms, including an FPGA-based CXL prototype, a persistent memory system, and a commercial CXL system, show that NOMAD achieves up to 6x performance improvement over state-of-the-art transparent page placement (TPP) and outperforms other approaches like Memtis in terms of both stability and performance under memory pressure.