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How to Diagnose and Fix High RAM Usage on Linux Servers

#linux #devops #performance #sysadmin

How to Diagnose and Fix High RAM Usage on Linux Servers

Your server is slow and you think it's RAM. Or you got an alert saying memory is at 92%. Or your app is randomly crashing and you suspect a memory leak.

Here's the systematic approach.

Step 1: Check Overall Memory Usage 

free -h
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              total        used        free      shared  buff/cache   available
Mem:           3.8Gi       3.1Gi       142Mi       45Mi       612Mi       542Mi
Swap:          2.0Gi       1.4Gi       614Mi
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Key numbers:

  • available: what's actually usable for new processes (not "free")
  • Swap used: if swap > 0, you're RAM-constrained. Swapping is ~100x slower than RAM.

If available < 10% of total and swap is active, you have a RAM problem.

Step 2: Find Which Process Is Using RAM 

# Sort by memory usage
ps aux --sort=-%mem | head -20

# Or with more readable output
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head -20
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If you want live monitoring:

# top — press M to sort by memory
top

# htop (more readable, install if needed)
htop
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Step 3: Investigate the Top Process 

# Detailed memory breakdown for a specific PID
cat /proc/<PID>/status | grep -i mem

# Virtual vs resident memory
pmap -x <PID> | tail -1
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For Node.js processes:

# Get V8 heap statistics
kill -USR2 <PID>  # Generates heap dump if --inspect is set

# Or add this to your app
setInterval(() => {
  const mem = process.memoryUsage();
  console.log(`RSS: ${Math.round(mem.rss/1024/1024)}MB, Heap: ${Math.round(mem.heapUsed/1024/1024)}MB`);
}, 60000);
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Step 4: Check for Memory Leaks

Is RSS growing over time?

# Watch a process's memory over 10 minutes
for i in $(seq 1 10); do
  ps -o pid,rss,vsz -p <PID>
  sleep 60
done
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If RSS grows consistently without dropping, you have a leak.

Node.js specific:

# Install clinic for leak detection
npm install -g clinic
clinic heapprofile -- node server.js

# Run load against your app for a few minutes, then Ctrl+C
# Clinic generates a flamegraph showing memory allocation
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Step 5: Immediate Relief Options 

# Drop page cache (safe — kernel re-reads from disk as needed)
sync && echo 1 | sudo tee /proc/sys/vm/drop_caches

# If a specific process is leaking, restart it
pm2 restart your-app
sudo systemctl restart your-service

# Add swap if you're out of options temporarily
sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab
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Prevention: Set Memory Limits 

# For systemd services
sudo systemctl edit your-service
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[Service]
MemoryMax=512M
MemorySwapMax=0  # Prevent swap
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# For Docker
docker run --memory=512m --memory-swap=512m your-image

# For PM2
pm2 start server.js --max-memory-restart 400M
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