File Descriptor Limit

On Unix-like systems, every open file, socket and pipe uses a file descriptor. The file descriptor limit is the maximum number a process, user or whole system may have open at once. Hitting it gives the error Too many open files (EMFILE).

Why it matters

Each client connection to a web server or proxy consumes at least one descriptor, and a proxy uses two (client and upstream). A busy server can hit a default of 1024 and start refusing connections while CPU is idle, which looks like a mysterious outage. Limits exist at several levels: per process, per user and system-wide.

Example

Inspect and raise the limit for a service:

ulimit -n                          # current shell limit
cat /proc/$(pidof nginx)/limits    # limit of a running process

For a systemd service, set it in the unit file, then reload and restart:

[Service]
LimitNOFILE=65536

Nginx also has worker_rlimit_nofile. Look for Too many open files in the error log to confirm.

Common mistakes

• Editing /etc/security/limits.conf for a service that systemd starts, which ignores it.
• Raising the limit while a descriptor leak is the real cause, which only delays the crash.
• Forgetting the system-wide fs.file-max ceiling.

Related

• Port exhaustion
• Saturation
• USE method
• Connection timeout

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.