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1.. SPDX-License-Identifier: GPL-2.02.. include:: <isonum.txt>3 4===========================5Power Management Strategies6===========================7 8:Copyright: |copy| 2017 Intel Corporation9 10:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>11 12 13The Linux kernel supports two major high-level power management strategies.14 15One of them is based on using global low-power states of the whole system in16which user space code cannot be executed and the overall system activity is17significantly reduced, referred to as :doc:`sleep states <sleep-states>`. The18kernel puts the system into one of these states when requested by user space19and the system stays in it until a special signal is received from one of20designated devices, triggering a transition to the ``working state`` in which21user space code can run. Because sleep states are global and the whole system22is affected by the state changes, this strategy is referred to as the23:doc:`system-wide power management <system-wide>`.24 25The other strategy, referred to as the :doc:`working-state power management26<working-state>`, is based on adjusting the power states of individual hardware27components of the system, as needed, in the working state. In consequence, if28this strategy is in use, the working state of the system usually does not29correspond to any particular physical configuration of it, but can be treated as30a metastate covering a range of different power states of the system in which31the individual components of it can be either ``active`` (in use) or32``inactive`` (idle). If they are active, they have to be in power states33allowing them to process data and to be accessed by software. In turn, if they34are inactive, ideally, they should be in low-power states in which they may not35be accessible.36 37If all of the system components are active, the system as a whole is regarded as38"runtime active" and that situation typically corresponds to the maximum power39draw (or maximum energy usage) of it. If all of them are inactive, the system40as a whole is regarded as "runtime idle" which may be very close to a sleep41state from the physical system configuration and power draw perspective, but42then it takes much less time and effort to start executing user space code than43for the same system in a sleep state. However, transitions from sleep states44back to the working state can only be started by a limited set of devices, so45typically the system can spend much more time in a sleep state than it can be46runtime idle in one go. For this reason, systems usually use less energy in47sleep states than when they are runtime idle most of the time.48 49Moreover, the two power management strategies address different usage scenarios.50Namely, if the user indicates that the system will not be in use going forward,51for example by closing its lid (if the system is a laptop), it probably should52go into a sleep state at that point. On the other hand, if the user simply goes53away from the laptop keyboard, it probably should stay in the working state and54use the working-state power management in case it becomes idle, because the user55may come back to it at any time and then may want the system to be immediately56accessible.57