1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 .. include:: <isonum.txt> 2 .. include:: <isonum.txt>
3 3
4 =========================== 4 ===========================
5 Power Management Strategies 5 Power Management Strategies
6 =========================== 6 ===========================
7 7
8 :Copyright: |copy| 2017 Intel Corporation 8 :Copyright: |copy| 2017 Intel Corporation
9 9
10 :Author: Rafael J. Wysocki <rafael.j.wysocki@in 10 :Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
11 11
12 12
13 The Linux kernel supports two major high-level 13 The Linux kernel supports two major high-level power management strategies.
14 14
15 One of them is based on using global low-power 15 One of them is based on using global low-power states of the whole system in
16 which user space code cannot be executed and t 16 which user space code cannot be executed and the overall system activity is
17 significantly reduced, referred to as :doc:`sl 17 significantly reduced, referred to as :doc:`sleep states <sleep-states>`. The
18 kernel puts the system into one of these state 18 kernel puts the system into one of these states when requested by user space
19 and the system stays in it until a special sig 19 and the system stays in it until a special signal is received from one of
20 designated devices, triggering a transition to 20 designated devices, triggering a transition to the ``working state`` in which
21 user space code can run. Because sleep states 21 user space code can run. Because sleep states are global and the whole system
22 is affected by the state changes, this strateg 22 is affected by the state changes, this strategy is referred to as the
23 :doc:`system-wide power management <system-wid 23 :doc:`system-wide power management <system-wide>`.
24 24
25 The other strategy, referred to as the :doc:`w 25 The other strategy, referred to as the :doc:`working-state power management
26 <working-state>`, is based on adjusting the po 26 <working-state>`, is based on adjusting the power states of individual hardware
27 components of the system, as needed, in the wo 27 components of the system, as needed, in the working state. In consequence, if
28 this strategy is in use, the working state of 28 this strategy is in use, the working state of the system usually does not
29 correspond to any particular physical configur 29 correspond to any particular physical configuration of it, but can be treated as
30 a metastate covering a range of different powe 30 a metastate covering a range of different power states of the system in which
31 the individual components of it can be either 31 the individual components of it can be either ``active`` (in use) or
32 ``inactive`` (idle). If they are active, they 32 ``inactive`` (idle). If they are active, they have to be in power states
33 allowing them to process data and to be access 33 allowing them to process data and to be accessed by software. In turn, if they
34 are inactive, ideally, they should be in low-p 34 are inactive, ideally, they should be in low-power states in which they may not
35 be accessible. 35 be accessible.
36 36
37 If all of the system components are active, th 37 If all of the system components are active, the system as a whole is regarded as
38 "runtime active" and that situation typically 38 "runtime active" and that situation typically corresponds to the maximum power
39 draw (or maximum energy usage) of it. If all 39 draw (or maximum energy usage) of it. If all of them are inactive, the system
40 as a whole is regarded as "runtime idle" which 40 as a whole is regarded as "runtime idle" which may be very close to a sleep
41 state from the physical system configuration a 41 state from the physical system configuration and power draw perspective, but
42 then it takes much less time and effort to sta 42 then it takes much less time and effort to start executing user space code than
43 for the same system in a sleep state. However 43 for the same system in a sleep state. However, transitions from sleep states
44 back to the working state can only be started 44 back to the working state can only be started by a limited set of devices, so
45 typically the system can spend much more time 45 typically the system can spend much more time in a sleep state than it can be
46 runtime idle in one go. For this reason, syst 46 runtime idle in one go. For this reason, systems usually use less energy in
47 sleep states than when they are runtime idle m 47 sleep states than when they are runtime idle most of the time.
48 48
49 Moreover, the two power management strategies 49 Moreover, the two power management strategies address different usage scenarios.
50 Namely, if the user indicates that the system 50 Namely, if the user indicates that the system will not be in use going forward,
51 for example by closing its lid (if the system 51 for example by closing its lid (if the system is a laptop), it probably should
52 go into a sleep state at that point. On the o 52 go into a sleep state at that point. On the other hand, if the user simply goes
53 away from the laptop keyboard, it probably sho 53 away from the laptop keyboard, it probably should stay in the working state and
54 use the working-state power management in case 54 use the working-state power management in case it becomes idle, because the user
55 may come back to it at any time and then may w 55 may come back to it at any time and then may want the system to be immediately
56 accessible. 56 accessible.
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