linux-server-rapl-powermonitor

Power efficiency is an important factor for small-scale servers, such as those running in a home or lab environment. While a hardware watt-meter is the most direct way to measure power usage, not everyone has access to one (I certainly don't!). This script provides a software-based solution to measure the power consumption of your Intel CPU(s) using Intel's Running Average Power Limit (RAPL) interface.

This utility is specifically designed for Linux servers and offers an easy way to estimate CPU package power consumption.

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How It Works

The script leverages Intel's RAPL feature, which provides hardware-level energy usage data for various domains of a CPU, including the entire CPU package. It achieves this by accessing the Model-Specific Registers (MSRs) exposed by the processor.

Key Steps:

1. Detect CPU Packages: It identifies all CPU packages using the topology data available in /sys/devices/system/cpu/cpu%d/topology/physical_package_id.

2. Read MSR Values: It reads energy consumption data from the MSR registers (0x611) of each package and calculates power usage using the energy unit provided by the 0x606 register.

3. Calculate Power: By sampling the energy values over a defined interval, it computes the average power usage in Watts.

Note: The methodology is derived from a RAPL MSR reading utility created by Vince Weaver. You can find the original code and further details here.

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Requirements

To use this utility, ensure your system meets the following prerequisites:

• Intel RAPL Support: Only Intel processors with RAPL (typically Sandy Bridge and newer) are supported. Unfortunately, AMD CPUs are not supported.
• MSR Module: The Linux MSR driver must be enabled. Load it using:

  sudo modprobe msr
  

Explanation

• Multiple CPU Packages
  The script identifies each CPU’s package by reading
  /sys/devices/system/cpu/cpuN/topology/physical_package_id.
  This ensures only one CPU per socket for RAPL reads.

• Reading RAPL MSRs
  Intel exposes power/energy data via /dev/cpu/<n>/msr.

  • MSR_RAPL_POWER_UNIT (0x606) holds scaling factors.
  • MSR_PKG_ENERGY_STATUS (0x611) reports cumulative package energy usage.

• Energy Unit
  Bits [12:8] of 0x606 store raw_unit. The energy tick is:

  $$ \mathrm{energy_{unit}} = \frac{1}{2^{\mathrm{raw_{unit}}}} $$

  Since different CPUs can have different raw_unit values, reading it at runtime is required.

• Power Measurement
  The script reads an initial energy value, waits (e.g., 5s), then reads again.
  The difference (in Joules) over that interval (in seconds) yields power in Watts, printed per package and in total.

Disclaimer

I have not tested the complete accuracy of this script using a watt-meter. This script is only meant to give a rough estimate of the power used by your CPUs.

Furthermore, this has only been tested on Ubuntu 22.04 LTS, on a dual Xeon E5-2690 v4 processor setup.