Redis I/O Model and Thread Model Deep Dive

This article provides an in-depth analysis of Redis's high-performance design, focusing on its I/O model and thread model. The author traces the evolution of network I/O models from Blocking I/O (both single-threaded and multi-threaded variants) through Non-blocking I/O to I/O Multiplexing, explaining the fundamental concepts of blocking, non-blocking operations, and system call mechanisms.

The article then explores the Reactor pattern, covering three major models: Single Reactor Single Thread (where one thread handles connection and all I/O events, suitable for low-traffic scenarios but limited by CPU multi-core utilization), Single Reactor Multi-Thread (offloading business logic to thread pool while maintaining single thread for I/O multiplexing), and Multi Reactor Multi Thread (separating connection handling from I/O handling across multiple reactors).

Redis adopts the I/O Multiplexing model. The article details Redis's thread architecture: IO Multiplexing monitors events (connection, read, write), queues events, and dispatches them to appropriate handlers (connection response handler, command request handler, command reply handler). The author explains the complete client-server interaction flow including connection establishment and command execution processes.

Regarding Redis's single-thread model advantages: since Redis operations are memory-based with fast execution, single-thread processing maintains high performance while avoiding concurrency issues. The disadvantages become apparent under high traffic when I/O operations (read/write between kernel and application) become bottlenecks.

To address these limitations, Redis introduced multi-threaded I/O processing: IO threads handle socket reading while the main thread executes commands, then IO threads handle writing back to clients. This hybrid approach combines ideas from both single-Reactor and multi-Reactor patterns.