1.. SPDX-License-Identifier: GPL-2.0 2.. include:: <isonum.txt> 3 4=========================== 5Power Management Strategies 6=========================== 7 8:Copyright: |copy| 2017 Intel Corporation 9 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 in 16which user space code cannot be executed and the overall system activity is 17significantly reduced, referred to as :doc:`sleep states <sleep-states>`. The 18kernel puts the system into one of these states when requested by user space 19and the system stays in it until a special signal is received from one of 20designated devices, triggering a transition to the ``working state`` in which 21user space code can run. Because sleep states are global and the whole system 22is affected by the state changes, this strategy is referred to as the 23:doc:`system-wide power management <system-wide>`. 24 25The other strategy, referred to as the :doc:`working-state power management 26<working-state>`, is based on adjusting the power states of individual hardware 27components of the system, as needed, in the working state. In consequence, if 28this strategy is in use, the working state of the system usually does not 29correspond to any particular physical configuration of it, but can be treated as 30a metastate covering a range of different power states of the system in which 31the individual components of it can be either ``active`` (in use) or 32``inactive`` (idle). If they are active, they have to be in power states 33allowing them to process data and to be accessed by software. In turn, if they 34are inactive, ideally, they should be in low-power states in which they may not 35be accessible. 36 37If all of the system components are active, the system as a whole is regarded as 38"runtime active" and that situation typically corresponds to the maximum power 39draw (or maximum energy usage) of it. If all of them are inactive, the system 40as a whole is regarded as "runtime idle" which may be very close to a sleep 41state from the physical system configuration and power draw perspective, but 42then it takes much less time and effort to start executing user space code than 43for the same system in a sleep state. However, transitions from sleep states 44back to the working state can only be started by a limited set of devices, so 45typically the system can spend much more time in a sleep state than it can be 46runtime idle in one go. For this reason, systems usually use less energy in 47sleep 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 should 52go into a sleep state at that point. On the other hand, if the user simply goes 53away from the laptop keyboard, it probably should stay in the working state and 54use the working-state power management in case it becomes idle, because the user 55may come back to it at any time and then may want the system to be immediately 56accessible. 57