How Zero‑Copy (sendfile) Boosts Web Server Performance on Linux

Since Linux kernel 2.2, a mechanism called “zero‑copy” has been introduced, and many application servers such as Apache and Nginx now support it to boost performance.

Zero‑copy is realized through the sendfile system call.

Before zero‑copy, data transfer relied on the read and write system calls. In a typical web service, a request that reads a file from disk and sends it over a network involves multiple copies: hardware → kernel space → user space → kernel space again, each requiring a context switch.

Reading data into user space with read, then writing it back to the kernel with write, incurs costly context switches. When a web server handles many concurrent requests, these calls become a performance bottleneck.

Zero‑copy eliminates the copy to the user‑space buffer by establishing a direct mapping between the disk area and memory, so data never passes through user space.

When a web server uses the sendfile system call, it avoids kernel‑user context switches, dramatically reducing CPU and memory overhead and improving overall server performance.