Why Nginx Outperforms Apache: A Deep Dive into Select vs Epoll I/O Models

Nginx and Apache differ in I/O performance because they use different I/O models.

Apache – select model

Characteristics:

Maximum concurrent connections are limited by the number of file descriptors (FD) a process can open, defined by FD_SETSIZE (default 1024/2048), which caps the concurrency.

Efficiency drops linearly because select scans the entire FD set on each call; increasing FD_SETSIZE further reduces performance.

Kernel‑user space memory copying is required to notify the user space of FD events, leading to extra copying overhead.

Can cause blocking.

Nginx – epoll model

Characteristics:

Maximum concurrent connections are limited only by the system’s maximum open files (e.g., /proc/sys/fs/file-max), typically far larger than 2048 and dependent on available memory.

Efficiency is higher because epoll monitors only active connections, independent of total connection count.

Uses shared memory to avoid kernel‑user space copying, eliminating that overhead.

Operates in non‑blocking mode.