# Understanding the “Out of Memory: Kill Process” Error

When Linux runs low on memory:

• Physical RAM is full

• Swap space may also be exhausted

• Kernel’s OOM Killer selects a process and terminates it to free memory

You’ll often see messages like:

Out of memory: Kill process 1234 (mysqld) score 987 or sacrifice child
Killed process 1234 (mysqld) total-vm:123456kB, anon-rss:98765kB, file-rss:1234kB

This usually affects:

• Database servers

• Web servers

• Memory-hungry applications (PHP, Node.js, Python scripts)

• Containers with limited memory

• 
  

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## Step 1 — Check Which Process Was Killed

The system log /var/log/syslog or dmesg shows OOM events.

dmesg | grep -i "killed process"

or

grep -i "oom" /var/log/syslog

This tells you:

• Process name

• PID

• Memory usage at time of kill

• 
  

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## Step 2 — Check Current Memory and Swap Usage

Run:

free -h

Example output:

              total        used        free      shared  buff/cache   available
Mem:           2.0G        1.8G        0.1G        0.1G        0.1G        0.2G
Swap:          1.0G        1.0G        0.0G

High usage or 0 free swap → system at risk of OOM.

Check which processes use most memory:

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

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## Step 3 — Free Up Memory Temporarily

A. Kill unnecessary processes

sudo kill -9 <PID>

Or restart services using too much memory:

sudo systemctl restart apache2
sudo systemctl restart mysql

B. Clear caches (safe)

sudo sync
sudo echo 3 > /proc/sys/vm/drop_caches

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## Step 4 — Check Swap and Increase if Needed

Check swap partitions:

swapon --show

Add swap if low or missing:

sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

Make it permanent by adding to /etc/fstab:

/swapfile none swap sw 0 0

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## Step 5 — Identify Memory-Hungry Applications

Common culprits:

• MySQL/PostgreSQL using large buffers

• PHP/Python scripts in loops

• Node.js apps without proper memory limits

• Docker containers with limited memory

Check running Docker containers

docker stats

Check PHP-FPM processes

ps aux | grep php-fpm

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## Step 6 — Optimize Applications and Services

• Databases: Reduce buffer/cache, optimize queries, use connection pooling

• Web servers: Limit worker processes, enable caching

• PHP/Python scripts: Avoid memory leaks, process batches instead of all at once

• Docker: Limit memory per container using --memory flag

Example Docker memory limit:

docker run -d --memory="512m" my-app

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## Step 7 — Adjust Linux OOM Behavior

Sometimes, you want to protect critical processes from being killed.

Check OOM score for a process

cat /proc/<PID>/oom_score

Lower the chance of being killed

echo -17 | sudo tee /proc/<PID>/oom_adj

Or for systemd service:

OOMScoreAdjust=-1000

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## Step 8 — Monitor Memory to Prevent OOM

Install monitoring tools:

• Netdata

• Glances

• Prometheus + Grafana

Set alerts when memory usage is high.

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## Step 9 — Add More Physical RAM

If your server constantly hits OOM despite optimization:

• Upgrade VPS RAM (1GB → 2GB or more)

• Add swap as temporary solution, but RAM is faster

• 
  

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## Beginner-Friendly Checklist

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## Conclusion

The “Out of Memory: Kill Process” error can seem alarming, but it’s Linux protecting itself. By following these steps:

• You can identify the memory-hungry process

• Free memory temporarily or increase swap

• Optimize applications to prevent future OOM events

• Adjust Linux OOM behavior for critical processes

• Monitor memory usage regularly

Even beginners can manage OOM events confidently, avoiding unexpected service downtime.