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# Copyright (c) 2016 ARM Limited
# All rights reserved
#
# The license below extends only to copyright in the software and shall
# not be construed as granting a license to any other intellectual
# property including but not limited to intellectual property relating
# to a hardware implementation of the functionality of the software
# licensed hereunder. You may use the software subject to the license
# terms below provided that you ensure that this notice is replicated
# unmodified and in its entirety in all distributions of the software,
# modified or unmodified, in source code or in binary form.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met: redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer;
# redistributions in binary form must reproduce the above copyright
# notice, this list of conditions and the following disclaimer in the
# documentation and/or other materials provided with the distribution;
# neither the name of the copyright holders nor the names of its
# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#
# 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.
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