Explain Redis's single-threaded model. Why is it fast despite being single-threaded, and how has this changed in Redis 6+?

Redis processes commands in a single thread, avoiding lock contention. Speed comes from in-memory ops and efficient I/O multiplexing (epoll/kqueue). Redis 6 introduced threaded I/O for reading/writing network data, while command execution stays single-threaded. Redis 7.4 adds async I/O improvements.