DAXFS: A Lock-Free Shared Filesystem for CXL Disaggregated Memory
This addresses the need for efficient, scalable filesystems in disaggregated memory systems for data centers and high-performance computing, representing a novel method rather than an incremental improvement.
The paper tackles the problem of enabling lock-free multi-host coordination for shared memory filesystems using CXL, presenting DaxFS which achieves over 99% CAS accuracy under cross-host contention and up to 2.68x higher random write throughput compared to tmpfs.
CXL (Compute Express Link) enables multiple hosts to share byte-addressable memory with hardware cache coherence, but no existing filesystem exploits this for lock-free multi-host coordination. We present DaxFS, a Linux filesystem for CXL shared memory that uses cmpxchg atomic operations, which CXL makes coherent across host boundaries, as its sole coordination primitive. A CAS-based hash overlay enables lock-free concurrent writes from multiple hosts without any centralized coordinator. A cooperative shared page cache with a novel multi-host clock eviction algorithm (MH-clock) provides demand-paged caching in shared DAX memory, with fully decentralized victim selection via cmpxchg. We validate multi-host correctness using QEMU-emulated CXL 3.0, where two virtual hosts share a memory region with TCP-forwarded atomics. Under cross-host contention, DaxFS maintains >99% CAS accuracy with no lost updates. On single-host DRAM-backed DAX, DaxFS exceeds tmpfs throughput across all write workloads, achieving up to 2.68x higher random write throughput with 4 threads and 1.18x higher random read throughput at 64 KB. Preliminary GPU microbenchmarks show that the cmpxchg-based design extends to GPU threads performing page cache operations at PCIe 5.0 bandwidth limits.