How I/O Multiplexing Gives Redis a 10× Performance Boost

Redis Core Technology

Many people attribute Redis' speed to three facts: it is an in‑memory database, it runs on a single thread, and its data structures are highly efficient. While these are true, a frequently overlooked core technology is the I/O multiplexing model.

I/O Multiplexing Overview

I/O multiplexing is an event‑driven network I/O model. Its core idea is that a single thread monitors many sockets and processes a request only when the corresponding socket becomes ready.

Assume 10,000 client connections. If each connection is handled by a dedicated thread, the system would need 10,000 threads:

Client1──Thread1
Client2──Thread2
Client3──Thread3
...
Client10000──Thread10000

This approach leads to several problems:

Huge number of threads

High memory consumption

Frequent context switches

Significant CPU time wasted on thread management

Thread switching itself is an expensive operation.

When Using I/O Multiplexing

With I/O multiplexing, the same 10,000 sockets are managed by a single event loop thread:

10000 Socket
│
▼
    epoll
│
▼
    Event Loop
│
    (one worker thread)

Advantages of this model are:

A single thread can handle a massive number of connections.

No frequent thread switches are required.

CPU utilization is higher.

Detailed Request Flow

The typical processing steps for a request are:

Client sends a request to a socket.

The operating system notifies the event loop via epoll_wait().

The Redis event loop receives the notification.

Redis reads the request from the socket.

Redis executes the command (e.g., SET or GET).

Redis sends the response back to the client.

The essence of Redis' multiplexing mechanism is to use a small number of threads to manage a large number of connections, leveraging the OS's efficient event notification capabilities to achieve high concurrency and low latency.