What is NUMA and how does it affect OS design?

Non-Uniform Memory Access (NUMA) architecture has memory physically distributed across processors - local memory is faster to access than remote. OS must: place processes near their memory (NUMA-aware scheduling), allocate memory from local node (first-touch or explicit policies), balance load while minimizing remote access (NUMA balancing), handle memory migration. Linux commands: numactl, /proc/meminfo shows NUMA stats. Critical for large multi-socket servers.