Maximum TCP Connections per Server Process and per Server

During a Tencent interview, candidates were asked two questions: the maximum number of TCP connections a single server‑side process can handle, and the maximum a whole server can support.

A TCP connection is represented in the kernel by a struct socket (which contains a struct sock ) and an struct inet_sock that stores the IPv4 four‑tuple (source IP, source port, destination IP, destination port). The server process typically listens on one port (e.g., 443) bound to 0.0.0.0 , meaning the IP address is fixed while the client side varies.

The theoretical upper bound for a single process is the number of possible client IPs (2 32 ) multiplied by the number of client ports (2 16 ), i.e., 2 48 ≈ 2.8×10 14 connections. In practice this is limited by file‑descriptor and memory resources.

Linux imposes three levels of file‑descriptor limits:

System‑wide limit: cat /proc/sys/fs/file-max

User‑level limit: cat /etc/security/limits.conf

Process‑level limit: cat /proc/sys/fs/nr_open

Each TCP connection also consumes memory; an established connection occupies about 3.44 KB (≈0.81 KB + 2.19 KB + 0.19 KB + 0.25 KB). Therefore, an 8 GB server can hold roughly 8 GB / 3.44 KB ≈ 2.4 million concurrent connections.

When considering the whole server, multiple processes can listen on different ports (up to 65 535). The theoretical count becomes 2 32 × 2 16 × 2 16 , effectively unbounded, but real limits are still governed by file descriptors and memory.

In summary, the ideal maximum per process is 2 48 connections, but practical limits are in the millions, dictated by OS resources.