What does "Too many open files" mean and how do you resolve it?
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Quick Answer
The process hit its file-descriptor limit. Raise the ulimit (per-process and system-wide in limits.conf / systemd LimitNOFILE), and fix leaks where the app never closes sockets/files.
Detailed Answer
Every socket and file uses a descriptor; high-connection services exhaust the default limit. Check with ulimit -n and cat /proc/<pid>/limits; count open fds with lsof -p <pid> | wc -l. Raise the soft/hard limits in /etc/security/limits.conf or the systemd unit (LimitNOFILE), and investigate descriptor leaks (connections not closed) rather than only raising the ceiling.
Code Example
ulimit -n lsof -p <pid> | wc -l # raise: LimitNOFILE=65536 in the systemd unit
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Interview Tip
Pair raising the limit with looking for an fd leak — bumping the ceiling alone just delays the crash.
linuxfile-descriptorsulimittroubleshooting