How I/O Multiplexing Works in Redis?

Redis uses I/O multiplexing to enable its single-threaded core to efficiently handle thousands of concurrent client connections. This technique allows the Redis server to monitor multiple sockets simultaneously and process data on those that are ready, without blocking on any single connection.

Single-threaded core

Redis processes all commands sequentially in a single main thread, which eliminates the overhead of thread creation, context switching, locks, and race conditions.

Non-blocking I/O

All client sockets are set to non-blocking mode. This means that an I/O operation (read/write) returns immediately if no data is available, rather than pausing the entire thread.

Event Loop

The main thread runs an event loop that uses a specific I/O multiplexing system call provided by the operating system (e.g., epoll on Linux, kqueue on macOS/BSD, evport on Solaris).

Kernel Notification

Instead of constantly polling all connections (which wastes CPU), the event loop blocks in a single, efficient system call (like epoll_wait). The operating system kernel is responsible for monitoring all the file descriptors (sockets) and notifying Redis when one or more of them are ready for an I/O operation.

Event Handling

When the system call returns, Redis's event loop processes only the "ready" sockets one by one. It reads the command, executes it from memory, and writes the response back to the client.