Yggdrasil: Reducing Network I/O Tax with (CXL-Based) Distributed Shared Memory

In communication-intensive applications that run on hosts with high-speed network hardware, a common challenge arises from the significant burden placed on the native socket system within the OS. Researchers have devoted considerable effort to optimizing the kernel networking stack and moving the TCP/IP stack to user-space. In this paper, we describe a novel socket replacement solution, Yggdrasil, a CXL-based user-space high-performance socket system. Yggdrasil is fully compatible with Linux socket, making it a drop-in replacement for existing applications without the need for code modifications. In order to optimize performance, Yggdrasil employs CXL-based distributed shared memory (DSM) for inter-host communication whenever it is available. In cases where DSM is not accessible, Yggdrasil transparently switches back to Linux socket for communication. A key element in achieving isolation in Yggdrasil involves a trusted user-space monitoring daemon responsible for managing control plane operations like connection setup and access control. Within the data plane of Yggdrasil, a peer-to-peer model is adopted for communication between processes. To bridge the semantic gap between socket and DSM, we exploit several techniques to ensure compatibility and performance, including (1) transparent dynamic fast/slow data path navigation, (2) decentralized CXL memory management, (3) lock-free queue based QoS-aware dynamic data polling, and (4) semantics-aware memory page migration. By evaluating Yggdrasil on both emulated and real CXL hardware, we show that Yggdrasil outperforms Linux socket in Memcached throughput by 8.2 × and reduces latency by 24 ∼ 320 × in a micro benchmark across different message sizes.