How to Fix Linux Out of Memory (OOM) Killer Error

In this tutorial, you'll learn about How to Fix Linux Out of Memory (OOM) Killer Error. We cover key concepts, practical examples, and best practices.

Your application is killed unexpectedly and you find Killed in the output — the Linux OOM killer terminated the process because the system ran out of memory.

The Problem

bazel build
Killed

No error code. Running dmesg shows:

[12345.678] oom-kill: constriction=CONSTRAINT_NONE, oom_score_adj=0
[12345.678] [pid 1234] invoked oom-killer: gfp_mask=0x...
[12345.678] Out of memory: Killed process 1234 (bazel)

Step-by-Step Fix

Step 1: Check memory usage

free -h

Expected:

              total        used        free      shared  buff/cache   available
Mem:           7.6G        7.2G        100M        200M        300M        150M

If available is near zero, the system is under memory pressure.

Step 2: Find what is using memory

ps aux --sort=-%mem | head -10

Or:

top -o %MEM

Step 3: Check OOM killer logs

sudo dmesg | grep -i "oom\|killed"

Step 4: Add swap space

sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

Make permanent in /etc/fstab:

/swapfile none swap sw 0 0

Step 5: Protect critical processes

# Lower oom_score (less likely to be killed)
sudo echo -500 > /proc/<pid>/oom_score_adj

Make permanent in systemd service:

[Service]
OOMScoreAdjust=-500

Step 6: Adjust vm.overcommit

# Don't overcommit memory
sudo sysctl vm.overcommit_memory=2
sudo sysctl vm.overcommit_ratio=80

Make permanent in /etc/sysctl.conf:

vm.overcommit_memory=2
vm.overcommit_ratio=80

Step 7: Limit process memory with cgroups

sudo systemctl set-property myapp.service MemoryMax=2G

Prevention Tips

• Monitor memory with free -h and htop
• Add swap for memory pressure relief
• Set memory limits on applications
• Configure OOM score adjustment for critical services
• Use sysctl vm.swappiness=10 to reduce swap usage

Common Mistakes with out of memory

1. Forgetting deriving (Show, Eq) on custom data types needed for debugging
2. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
3. Using head and tail instead of pattern matching, causing runtime errors on empty 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.

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