Compute Express Link (CXL) 3.0 can turn stranded DRAM into a composable datacenter memory fabric, but hardware is scarce and cycle-accurate simulation is prohibitively slow for full workloads. CXLMemSim is an open-source software CXL memory simulator that keeps computation at native speed and models only CXL-affected memory behavior. Across SPEC CPU2017, Memcached, wrf, vector search, and llama.cpp, CXLMemSim stays within 11–15% of hardware and gem5 while completing a Redis-scale run in 45 minutes instead of gem5’s 73 hours. The public release at SlugLab/CXLMemSim provides PEBS/LBR/eBPF tracing, JSON topology descriptions, and reusable migration/cache policy hooks for CXL design-space exploration.