Junliang Hu | Searcharxiv

arXiv2026

Aquifer: Hierarchical Memory Pooling with CXL and RDMA for MicroVM Snapshots

Memory stranding wastes 25-35% of installed DRAM in production cloud clusters. Memory pooling over CXL and RDMA offers a remedy, but neither technology alone suffices: CXL provides low-latency, load/store-transparent access limited to a pod, while RDMA provides cluster-wide reach at higher latency with software overhead. A hierarchical architecture combining both tiers is the practical path forward, yet remains unexplored for MicroVM-based serverless computing, where snapshot restore latency is the dominant cold-start bottleneck. We present Aquifer, the first system to serve MicroVM snapshots from a hierarchical CXL+RDMA memory pool. A characterization of snapshot images reveals that the vast majority of pages are either zero or cold, enabling a hotness-based snapshot format that eliminates zero pages and places only the hot working set in the CXL pool while storing cold pages in the RDMA pool. Sharing these snapshots across hosts on CXL 2.0 multi-headed devices, which lack hardware cache coherence, requires Aquifer's ownership-based coherence protocol to ensure correctness. Finally, Aquifer uses a copy-based page serving mechanism pre-installs hot pages from CXL memory before MicroVM resume and demand-pages cold pages asynchronously from RDMA. On emulated CXL+RDMA hardware, Aquifer achieves a 2.2x geometric-mean speedup in end-to-end invocation time over Firecracker and 1.1x over the next best alternative.

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arXiv2025

Lotus: Optimizing Disaggregated Transactions with Disaggregated Locks

Disaggregated memory (DM) separates compute and memory resources, allowing flexible scaling to achieve high resource utilization. To ensure atomic and consistent data access on DM, distributed transaction systems have been adapted, where compute nodes (CNs) rely on one-sided RDMA operations to access remote data in memory nodes (MNs). However, we observe that in existing transaction systems, the RDMA network interface cards at MNs become a primary performance bottleneck. This bottleneck arises from the high volume of one-sided atomic operations used for locks, which hinders the system's ability to scale efficiently. To address this issue, this paper presents Lotus, a scalable distributed transaction system with lock disaggregation on DM. The key innovation of Lotus is to disaggregate locks from data and execute all locks on CNs, thus eliminating the bottleneck at MN RNICs. To achieve efficient lock management on CNs, Lotus employs an application-aware lock management mechanism that leverages the locality of the OLTP workloads to shard locks while maintaining load balance. To ensure consistent transaction processing with lock disaggregation, Lotus introduces a lock-first transaction protocol, which separates the locking phase as the first step in each read-write transaction execution. This protocol allows the system to determine the success of lock acquisitions early and proactively abort conflicting transactions, improving overall efficiency. To tolerate lock loss during CN failures, Lotus employs a lock-rebuild-free recovery mechanism that treats locks as ephemeral and avoids their reconstruction, ensuring lightweight recovery for CN failures. Experimental results demonstrate that Lotus improves transaction throughput by up to 2.1$\times$ and reduces latency by up to 49.4% compared to state-of-the-art transaction systems on DM.

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