Documentation/admin-guide/thermal/intel_powerclamp.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/thermal/intel_powerclamp.rst

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Documentation/admin-guide/thermal/intel_powerclamp.rst
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=======================
Intel Powerclamp Driver
=======================

By:
  - Arjan van de Ven <arjan@linux.intel.com>
  - Jacob Pan <jacob.jun.pan@linux.intel.com>

.. Contents:

	(*) Introduction
	    - Goals and Objectives

	(*) Theory of Operation
	    - Idle Injection
	    - Calibration

	(*) Performance Analysis
	    - Effectiveness and Limitations
	    - Power vs Performance
	    - Scalability
	    - Calibration
	    - Comparison with Alternative Techniques

	(*) Usage and Interfaces
	    - Generic Thermal Layer (sysfs)
	    - Kernel APIs (TBD)

	(*) Module Parameters

INTRODUCTION
============

Consider the situation where a system’s power consumption must be
reduced at runtime, due to power budget, thermal constraint, or noise
level, and where active cooling is not preferred. Software managed
passive power reduction must be performed to prevent the hardware
actions that are designed for catastrophic scenarios.

Currently, P-states, T-states (clock modulation), and CPU offlining
are used for CPU throttling.

On Intel CPUs, C-states provide effective power reduction, but so far
they’re only used opportunistically, based on workload. With the
development of intel_powerclamp driver, the method of synchronizing
idle injection across all online CPU threads was introduced. The goal
is to achieve forced and controllable C-state residency.

Test/Analysis has been made in the areas of power, performance,
scalability, and user experience. In many cases, clear advantage is
shown over taking the CPU offline or modulating the CPU clock.


THEORY OF OPERATION
===================

Idle Injection
--------------

On modern Intel processors (Nehalem or later), package level C-state
residency is available in MSRs, thus also available to the kernel.

These MSRs are::

      #define MSR_PKG_C2_RESIDENCY      0x60D
      #define MSR_PKG_C3_RESIDENCY      0x3F8
      #define MSR_PKG_C6_RESIDENCY      0x3F9
      #define MSR_PKG_C7_RESIDENCY      0x3FA

If the kernel can also inject idle time to the system, then a

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