Linux Performance Troubleshooting: A Top-Down Diagnostic Guide

When your Linux system feels sluggish, it’s crucial to diagnose issues methodically—starting from high-level resource usage and drilling down to specific bottlenecks. This guide follows a top-down approach to identify and fix performance problems efficiently.

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Step 1: Check System-Wide Resource Usage

First, identify which resource (CPU, RAM, Disk, Network) is under the most strain.

A. Run htop (or top) for Live Monitoring

htop  # Install with `sudo apt install htop` if missing  

• Key columns to watch:
  • CPU%: Processes consuming high CPU.
  • MEM%: Memory-hungry applications.
  • IO%: Disk I/O (if high, proceed to Step 3).

B. Check System Load Averages

uptime  

• Output example: load average: 1.25, 0.85, 0.60
  • Values > (CPU cores) indicate congestion (e.g., >4 on a 4-core CPU).

C. Summarize with vmstat

vmstat 1  # Updates every second  

• Focus on:
  • wa (I/O wait): >10% means disk bottleneck.
  • free memory: Low values may trigger swapping.

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Step 2: Identify the Culprit Process

Once you know the overloaded resource, pinpoint the problematic process.

A. For CPU Issues

ps aux --sort=-%cpu | head -n 10  # Top 10 CPU hogs  

• Common offenders:
  • gnome-shell, chrome, java.
  • Fix: Restart the process or limit CPU with cpulimit.

B. For Memory Issues

ps aux --sort=-%mem | head -n 10  # Top 10 RAM users  

• If free -h shows low memory:
  • Kill leaking apps or add swap:

    sudo fallocate -l 2G /swapfile && sudo mkswap /swapfile && sudo swapon /swapfile  
    

C. For Disk I/O Issues

sudo iotop -o  # Show active I/O processes  

• High I/O processes:
  • systemd-journald (logs), pool-nemo (file manager), updatedb (file indexing).
  • Mitigate with:

    ionice -c 3 -p <PID>  # Set low I/O priority  
    

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Step 3: Diagnose Disk-Specific Problems

If iotop or vmstat show high wa (I/O wait), investigate further.

A. Check Disk Space

df -h  # Look for >90% usage  

• If full:

  sudo ncdu /  # Find large files to delete  
  

B. Test Disk Speed

sudo hdparm -Tt /dev/sdX  # Replace X with your disk (e.g., sda)  

• HDD: Expect ~80–160 MB/s.
• SSD: Expect ~300–500 MB/s (SATA) or >2000 MB/s (NVMe).

C. Check for Fragmentation (ext4)

sudo e4defrag -c /home  # Fragmentation score  

• >30%: Defragment after freeing space:

  sudo e4defrag -v /home  
  

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Step 4: Investigate Logs & Background Services

Persistent slowness may stem from misconfigured services.

A. Check System Logs

sudo journalctl -xe --no-pager | grep -i "error\|fail"  

• Common fixes:
  • Update broken packages: sudo apt --fix-broken install.
  • Restart crashed services: sudo systemctl restart <service>.

B. Disable Unnecessary Services

systemctl list-units --type=service --state=running  

• Examples to disable (if unused):
  • unattended-upgrades (auto-updates):

    sudo systemctl disable unattended-upgrades  
    

  • bluetooth, cups (printing).

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Step 5: Hardware Checks

Rule out failing hardware.

A. Run SMART Tests on Disks

sudo smartctl -a /dev/sdX | grep -i "Reallocated\|Pending\|Uncorrectable"  

• Reallocated sectors > 0: Backup data and replace the disk soon.

B. Monitor Temperature

sensors  # Install `lm-sensors` if missing  

• CPU/GPU > 90°C: Clean fans, reapply thermal paste.

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Final Recommendations

1. Regular Maintenance:
   • Clean logs: sudo journalctl --vacuum-size=200M.
   • Update packages: sudo apt update && sudo apt upgrade.
2. Upgrade Hardware:
   • Replace HDDs with SSDs.
   • Add RAM if swapping is frequent.
3. Optimize Workloads:
   • Use nice/ionice for CPU/I/O priority.
   • Schedule heavy tasks (e.g., backups) during off-hours with cron.

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By following this structured approach, you can efficiently diagnose and resolve Linux performance issues. Hopefully you found this guide helpful.