Fixing "Too Many Open Files" Errors

Understanding the Error

Linux limits how many files (including sockets and pipes) a process can open simultaneously. When this limit is reached, you see:

Too many open files
socket: Too many open files
accept: Too many open files (24)

Check Current Limits

# System-wide limit
cat /proc/sys/fs/file-max

# Current user's soft/hard limits
ulimit -Sn  # Soft limit (usually 1024)
ulimit -Hn  # Hard limit

# For a running process
cat /proc/PID/limits | grep "Max open files"

# How many files a process has open
ls /proc/PID/fd | wc -l

Find Which Process Hit the Limit

# List processes by open file count
for pid in /proc/[0-9]*; do
    count=$(ls "$pid/fd" 2>/dev/null | wc -l)
    name=$(cat "$pid/comm" 2>/dev/null)
    [ "$count" -gt 100 ] && echo "$count $name (PID: $(basename $pid))"
done | sort -rn | head -10

Fix: Increase Limits

For the Current Session

ulimit -n 65535

Permanent — System-Wide

# /etc/security/limits.conf
*    soft    nofile    65535
*    hard    nofile    65535
root soft    nofile    65535
root hard    nofile    65535

For Systemd Services

# In the service file's [Service] section
LimitNOFILE=65535

Or globally for all services:

# /etc/systemd/system.conf
DefaultLimitNOFILE=65535

sudo systemctl daemon-reload
sudo systemctl restart your-service

System-Wide Maximum

# /etc/sysctl.conf
fs.file-max = 2097152
fs.nr_open = 1048576

sudo sysctl -p

Verify the Fix

# Check new limits for running service
cat /proc/$(pgrep -f nginx)/limits | grep "Max open files"
# Max open files            65535                65535                files

Common Services That Need Higher Limits

Warning Increasing file limits without investigating why so many files are open can mask connection leaks. Always check for leaked sockets or unclosed file handles in your application.