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cpu_frequency.md

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CPU Frequency Scaling & Power Management

1️⃣ What is CPU Frequency Scaling?

CPU frequency scaling is a power management technique that adjusts the CPU speed dynamically to balance performance and power consumption. It is controlled by CPU governors.

2️⃣ What is a CPU Governor?

A governor is a kernel module that determines how CPU frequency is adjusted. Different governors are available:

Governor Description
performance Runs CPU at max speed always. Best for high-performance tasks.
powersave Runs CPU at minimum speed to save power.
ondemand Increases speed when needed, but lowers it when idle.
conservative Like ondemand, but increases speed gradually.
schedutil Uses kernel scheduler to adjust CPU dynamically (modern method).
userspace Allows manual frequency setting by users or scripts.

3️⃣ Essential Tools for CPU Management

🔹 cpupower

A modern tool for managing CPU frequency and power states.

Installation

  • Ubuntu/Debian 
    sudo apt install linux-tools-common linux-tools-$(uname -r) -y
  • RHEL/CentOS/Rocky Linux 
    sudo dnf install kernel-tools -y

Usage

  • Check Current CPU Frequency Info 
    cpupower frequency-info
  • Set CPU Governor to Performance Mode 
    sudo cpupower frequency-set -g performance
  • Set CPU Governor to Power-Saving Mode 
    sudo cpupower frequency-set -g powersave
  • Manually Set CPU Frequency 
    sudo cpupower frequency-set -f 2.5GHz

🔹 cpufreq-set

An older tool from cpufrequtils package for CPU frequency control.

Installation

  • Ubuntu/Debian 
    sudo apt install cpufrequtils -y
  • RHEL/CentOS 
    sudo yum install cpufrequtils -y

Usage

  • Check Available Governors 
    cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_governors
  • Set Performance Governor 
    sudo cpufreq-set -g performance
  • Set Maximum CPU Frequency 
    sudo cpufreq-set -u 3.0GHz
  • Set Minimum CPU Frequency 
    sudo cpufreq-set -d 1.5GHz

4️⃣ Other Useful Tools

🛠 turbostat (Intel Only)

  • Provides detailed CPU power consumption and frequency information.
  • Install: 
    sudo apt install linux-tools-$(uname -r)
  • Run: 
    sudo turbostat

🛠 powertop (Power Consumption Analysis)

  • Helps identify power-hungry processes.
  • Install: 
    sudo apt install powertop
  • Run: 
    sudo powertop

🛠 auto-cpufreq (Automatic CPU Scaling)

  • Dynamically adjusts CPU speed based on load.
  • Install: 
    sudo apt install auto-cpufreq
  • Run: 
    sudo auto-cpufreq --monitor

5️⃣ System Administrator Procedures

🔹 Check CPU Governors and Current Scaling

cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

or

cpupower frequency-info

🔹 Set Performance Mode for High-Performance Tasks

sudo cpupower frequency-set -g performance

Useful for: ✅ Compiling code (e.g., building Glibc)
✅ Gaming & high-performance applications
✅ Virtual machines requiring stable performance

🔹 Revert to Power-Saving Mode

sudo cpupower frequency-set -g ondemand

✅ Extends battery life on laptops
✅ Reduces CPU heat & fan noise

🔹 Make CPU Frequency Changes Persistent

Add the command to /etc/rc.local or create a systemd service.

1️⃣ Create a script:

sudo nano /etc/init.d/cpufreq_performance

Add:

#!/bin/bash
cpupower frequency-set -g performance

Save & exit.

2️⃣ Make it executable:

sudo chmod +x /etc/init.d/cpufreq_performance

3️⃣ Enable at startup:

sudo update-rc.d cpufreq_performance defaults

6️⃣ Troubleshooting

🔹 cpupower or cpufreq-set Command Not Found?

  • Install: 
    sudo apt install linux-tools-common cpufrequtils -y

🔹 Changes Reset After Reboot?

  • Add commands to /etc/rc.local
  • Use systemd service to apply settings on boot

🔹 Settings Not Applying?

  • Check if another power management tool (like TLP) is overriding settings: 
    sudo systemctl status tlp
    If running, stop and disable it: 
    sudo systemctl stop tlp
sudo systemctl disable tlp

7️⃣ Best Practices for System Administrators

✅ Use performance mode when compiling or running heavy workloads.
✅ Use ondemand or powersave mode for general use and laptops.
✅ Monitor CPU load and frequency using htop or turbostat.
✅ Automate CPU settings using systemd or boot scripts.
✅ Regularly check CPU temperature and power consumption to optimize efficiency.

💡 Summary

  • CPU governors control frequency scaling.
  • Use cpupower or cpufreq-set to manage CPU frequency.
  • System admins should automate settings using scripts/systemd.
  • Monitor CPU power and temperature for optimal performance.

🚀 Let me know if you need further guidance!