xref: /gem5/src/doc/power_thermal_model.doxygen

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# Author: David Guillen Fandos

/*! \page gem5PowerModel Gem5 Power & Thermal model

  \tableofcontents

  This document gives an overview of the power and thermal modelling
  infrastructure in Gem5. The purpose is to give a high level view of
  all the pieces involved and how they interact with each other and
  the simulator.

  \section gem5_PM_CD Class overview

  Classes involved in the power model are:

    - PowerModel: Represents a power model for a hardware component.

    - PowerModelState: Represents a power model for a hardware component
      in a certain power state. It is an abstract class that defines an
      interface that must be implemented for each model.

    - MathExprPowerModel: Simple implementation of PowerModelState that
      assumes that power can be modeled using a simple power

  Classes involved in the thermal model are:

    - ThermalModel: Contains the system thermal model logic and state.
      It performs the power query and temperature update. It also enables
      gem5 to query for temperature (for OS reporting).

    - ThermalDomain: Represents an entity that generates heat. It's
      essentially a group of SimObjects grouped under a SubSystem component
      that have its own thermal behaviour.

    - ThermalNode: Represents a node in the thermal circuital equivalent.
      The node has a temperature and interacts with other nodes through
      connections (thermal resistors and capacitors).

    - ThermalReference: Temperature reference for the thermal model
      (essentially a thermal node with a fixed temperature), can be used
      to model air or any other constant temperature domains.

    - ThermalEntity: A thermal component that connects two thermal nodes
      and models a thermal impedance between them. This class is just an
      abstract interface.

    - ThermalResistor: Implements ThermalEntity to model a thermal resistance
      between the two nodes it connects. Thermal resistances model the
      capacity of a material to transfer heat (units in K/W).

    - ThermalCapacitor. Implements ThermalEntity to model a thermal
      capacitance. Thermal capacitors are used to model material's thermal
      capacitance, this is, the ability to change a certain material
      temperature (units in J/K).

  \section gem5_thermal Thermal model

  The thermal model works by creating a circuital equivalent of the
  simulated platform. Each node in the circuit has a temperature (as
  voltage equivalent) and power flows between nodes (as current in a
  circuit).

  To build this equivalent temperature model the platform is required
  to group the power actors (any component that has a power model)
  under SubSystems and attach ThermalDomains to those subsystems.
  Other components might also be created (like ThermalReferences) and
  connected all together by creating thermal entities (capacitors and
  resistors).

  Last step to conclude the thermal model is to create the ThermalModel
  instance itself and attach all the instances used to it, so it can
  properly update them at runtime. Only one thermal model instance is
  supported right now and it will automatically report temperature when
  appropriate (ie. platform sensor devices).

  \section gem5_power Power model

  Every ClockedObject has a power model associated. If this power model is
  non-null power will be calculated at every stats dump (although it might
  be possible to force power evaluation at any other point, if the power
  model uses the stats, it is a good idea to keep both events in sync).
  The definition of a power model is quite vague in the sense that it is
  as flexible as users want it to be. The only enforced contraints so far
  is the fact that a power model has several power state models, one for
  each possible power state for that hardware block. When it comes to compute
  power consumption the power is just the weighted average of each power model.

  A power state model is essentially an interface that allows us to define two
  power functions for dynamic and static. As an example implementation a class
  called MathExprPowerModel has been provided. This implementation allows the
  user to define a power model as an equation involving several statistics.
  There's also some automatic (or "magic") variables such as "temp", which
  reports temperature.