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2Power Management Strategies
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7 Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
8
9The Linux kernel supports two major high-level power management strategies.
10
11One of them is based on using global low-power states of the whole system in
12which user space code cannot be executed and the overall system activity is
13significantly reduced, referred to as :doc:`sleep states <sleep-states>`.  The
14kernel puts the system into one of these states when requested by user space
15and the system stays in it until a special signal is received from one of
16designated devices, triggering a transition to the ``working state`` in which
17user space code can run.  Because sleep states are global and the whole system
18is affected by the state changes, this strategy is referred to as the
19:doc:`system-wide power management <system-wide>`.
20
21The other strategy, referred to as the :doc:`working-state power management
22<working-state>`, is based on adjusting the power states of individual hardware
23components of the system, as needed, in the working state.  In consequence, if
24this strategy is in use, the working state of the system usually does not
25correspond to any particular physical configuration of it, but can be treated as
26a metastate covering a range of different power states of the system in which
27the individual components of it can be either ``active`` (in use) or
28``inactive`` (idle).  If they are active, they have to be in power states
29allowing them to process data and to be accessed by software.  In turn, if they
30are inactive, ideally, they should be in low-power states in which they may not
31be accessible.
32
33If all of the system components are active, the system as a whole is regarded as
34"runtime active" and that situation typically corresponds to the maximum power
35draw (or maximum energy usage) of it.  If all of them are inactive, the system
36as a whole is regarded as "runtime idle" which may be very close to a sleep
37state from the physical system configuration and power draw perspective, but
38then it takes much less time and effort to start executing user space code than
39for the same system in a sleep state.  However, transitions from sleep states
40back to the working state can only be started by a limited set of devices, so
41typically the system can spend much more time in a sleep state than it can be
42runtime idle in one go.  For this reason, systems usually use less energy in
43sleep states than when they are runtime idle most of the time.
44
45Moreover, the two power management strategies address different usage scenarios.
46Namely, if the user indicates that the system will not be in use going forward,
47for example by closing its lid (if the system is a laptop), it probably should
48go into a sleep state at that point.  On the other hand, if the user simply goes
49away from the laptop keyboard, it probably should stay in the working state and
50use the working-state power management in case it becomes idle, because the user
51may come back to it at any time and then may want the system to be immediately
52accessible.
53