How to Fix Linux ulimit: Too Many Open Files Error

In this tutorial, you'll learn about How to Fix Linux ulimit: Too Many Open Files Error. We cover key concepts, practical examples, and best practices.

Your application crashes with Too many open files or EMFILE — the process has reached the system limit on open file descriptors.

The Problem

java.io.IOException: Too many open files

Or:

socket: Too many open files

Or:

Error: EMFILE: too many open files, watch

Step-by-Step Fix

Step 1: Check current limits

ulimit -n

Expected for default:

1024

Step 2: Check hard and soft limits

ulimit -Sn  # soft limit
ulimit -Hn  # hard limit

Step 3: Check current open file count

# For a specific PID
ls -la /proc/<PID>/fd | wc -l

# For current user
lsof -u $USER | wc -l

Step 4: Increase limit temporarily

ulimit -n 65535

Step 5: Increase limit permanently for a user

Edit /etc/security/limits.conf:

user    soft    nofile    65535
user    hard    nofile    65535

Or for all users:

*       soft    nofile    65535
*       hard    nofile    65535

Step 6: Increase system-wide limit

sudo sysctl fs.file-max=2097152

Make permanent in /etc/sysctl.conf:

fs.file-max=2097152

Step 7: Apply changes

Re-login or:

sudo sysctl -p

Verify:

ulimit -n  # should show 65535

Step 8: Configure for systemd services

For systemd services, add to the service unit:

[Service]
LimitNOFILE=65535

Then:

sudo systemctl daemon-reload
sudo systemctl restart myservice

Prevention Tips

• Set appropriate open file limits in provisioning scripts
• Monitor open file counts with lsof
• Fix file descriptor leaks in application code
• Close file handles, database connections, and sockets properly
• Set limits higher than expected peak usage

Common Mistakes with ulimit error

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world LINUX code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ