PM: docs: Describe high-level PM strategies and sleep states
Reorganize the power management part of admin-guide by adding a description of major power management strategies supported by the kernel (system-wide and working-state power management) to it and dividing the rest of the material into the system-wide PM and working-state PM chapters. On top of that, add a description of system sleep states to the system-wide PM chapter. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Reviewed-by: Lukas Wunner <lukas@wunner.de>
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Rafael J. Wysocki
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@ -5,12 +5,6 @@ Power Management
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.. toctree::
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:maxdepth: 2
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cpufreq
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intel_pstate
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.. only:: subproject and html
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Indices
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=======
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* :ref:`genindex`
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strategies
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system-wide
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working-state
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===================
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System Sleep States
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===================
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::
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Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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Sleep states are global low-power states of the entire system in which user
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space code cannot be executed and the overall system activity is significantly
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reduced.
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Sleep States That Can Be Supported
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==================================
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Depending on its configuration and the capabilities of the platform it runs on,
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the Linux kernel can support up to four system sleep states, includig
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hibernation and up to three variants of system suspend. The sleep states that
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can be supported by the kernel are listed below.
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.. _s2idle:
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Suspend-to-Idle
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---------------
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This is a generic, pure software, light-weight variant of system suspend (also
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referred to as S2I or S2Idle). It allows more energy to be saved relative to
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runtime idle by freezing user space, suspending the timekeeping and putting all
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I/O devices into low-power states (possibly lower-power than available in the
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working state), such that the processors can spend time in their deepest idle
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states while the system is suspended.
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The system is woken up from this state by in-band interrupts, so theoretically
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any devices that can cause interrupts to be generated in the working state can
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also be set up as wakeup devices for S2Idle.
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This state can be used on platforms without support for :ref:`standby <standby>`
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or :ref:`suspend-to-RAM <s2ram>`, or it can be used in addition to any of the
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deeper system suspend variants to provide reduced resume latency. It is always
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supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option is set.
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.. _standby:
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Standby
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-------
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This state, if supported, offers moderate, but real, energy savings, while
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providing a relatively straightforward transition back to the working state. No
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operating state is lost (the system core logic retains power), so the system can
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go back to where it left off easily enough.
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In addition to freezing user space, suspending the timekeeping and putting all
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I/O devices into low-power states, which is done for :ref:`suspend-to-idle
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<s2idle>` too, nonboot CPUs are taken offline and all low-level system functions
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are suspended during transitions into this state. For this reason, it should
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allow more energy to be saved relative to :ref:`suspend-to-idle <s2idle>`, but
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the resume latency will generally be greater than for that state.
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The set of devices that can wake up the system from this state usually is
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reduced relative to :ref:`suspend-to-idle <s2idle>` and it may be necessary to
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rely on the platform for setting up the wakeup functionality as appropriate.
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This state is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration
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option is set and the support for it is registered by the platform with the
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core system suspend subsystem. On ACPI-based systems this state is mapped to
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the S1 system state defined by ACPI.
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.. _s2ram:
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Suspend-to-RAM
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--------------
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This state (also referred to as STR or S2RAM), if supported, offers significant
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energy savings as everything in the system is put into a low-power state, except
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for memory, which should be placed into the self-refresh mode to retain its
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contents. All of the steps carried out when entering :ref:`standby <standby>`
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are also carried out during transitions to S2RAM. Additional operations may
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take place depending on the platform capabilities. In particular, on ACPI-based
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systems the kernel passes control to the platform firmware (BIOS) as the last
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step during S2RAM transitions and that usually results in powering down some
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more low-level components that are not directly controlled by the kernel.
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The state of devices and CPUs is saved and held in memory. All devices are
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suspended and put into low-power states. In many cases, all peripheral buses
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lose power when entering S2RAM, so devices must be able to handle the transition
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back to the "on" state.
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On ACPI-based systems S2RAM requires some minimal boot-strapping code in the
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platform firmware to resume the system from it. This may be the case on other
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platforms too.
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The set of devices that can wake up the system from S2RAM usually is reduced
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relative to :ref:`suspend-to-idle <s2idle>` and :ref:`standby <standby>` and it
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may be necessary to rely on the platform for setting up the wakeup functionality
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as appropriate.
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S2RAM is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option
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is set and the support for it is registered by the platform with the core system
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suspend subsystem. On ACPI-based systems it is mapped to the S3 system state
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defined by ACPI.
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.. _hibernation:
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Hibernation
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-----------
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This state (also referred to as Suspend-to-Disk or STD) offers the greatest
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energy savings and can be used even in the absence of low-level platform support
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for system suspend. However, it requires some low-level code for resuming the
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system to be present for the underlying CPU architecture.
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Hibernation is significantly different from any of the system suspend variants.
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It takes three system state changes to put it into hibernation and two system
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state changes to resume it.
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First, when hibernation is triggered, the kernel stops all system activity and
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creates a snapshot image of memory to be written into persistent storage. Next,
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the system goes into a state in which the snapshot image can be saved, the image
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is written out and finally the system goes into the target low-power state in
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which power is cut from almost all of its hardware components, including memory,
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except for a limited set of wakeup devices.
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Once the snapshot image has been written out, the system may either enter a
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special low-power state (like ACPI S4), or it may simply power down itself.
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Powering down means minimum power draw and it allows this mechanism to work on
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any system. However, entering a special low-power state may allow additional
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means of system wakeup to be used (e.g. pressing a key on the keyboard or
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opening a laptop lid).
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After wakeup, control goes to the platform firmware that runs a boot loader
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which boots a fresh instance of the kernel (control may also go directly to
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the boot loader, depending on the system configuration, but anyway it causes
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a fresh instance of the kernel to be booted). That new instance of the kernel
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(referred to as the ``restore kernel``) looks for a hibernation image in
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persistent storage and if one is found, it is loaded into memory. Next, all
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activity in the system is stopped and the restore kernel overwrites itself with
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the image contents and jumps into a special trampoline area in the original
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kernel stored in the image (referred to as the ``image kernel``), which is where
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the special architecture-specific low-level code is needed. Finally, the
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image kernel restores the system to the pre-hibernation state and allows user
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space to run again.
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Hibernation is supported if the :c:macro:`CONFIG_HIBERNATION` kernel
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configuration option is set. However, this option can only be set if support
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for the given CPU architecture includes the low-level code for system resume.
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Basic ``sysfs`` Interfaces for System Suspend and Hibernation
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=============================================================
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The following files located in the :file:`/sys/power/` directory can be used by
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user space for sleep states control.
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``state``
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This file contains a list of strings representing sleep states supported
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by the kernel. Writing one of these strings into it causes the kernel
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to start a transition of the system into the sleep state represented by
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that string.
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In particular, the strings "disk", "freeze" and "standby" represent the
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:ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and
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:ref:`standby <standby>` sleep states, respectively. The string "mem"
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is interpreted in accordance with the contents of the ``mem_sleep`` file
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described below.
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If the kernel does not support any system sleep states, this file is
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not present.
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``mem_sleep``
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This file contains a list of strings representing supported system
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suspend variants and allows user space to select the variant to be
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associated with the "mem" string in the ``state`` file described above.
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The strings that may be present in this file are "s2idle", "shallow"
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and "deep". The string "s2idle" always represents :ref:`suspend-to-idle
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<s2idle>` and, by convention, "shallow" and "deep" represent
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:ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`,
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respectively.
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Writing one of the listed strings into this file causes the system
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suspend variant represented by it to be associated with the "mem" string
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in the ``state`` file. The string representing the suspend variant
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currently associated with the "mem" string in the ``state`` file
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is listed in square brackets.
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If the kernel does not support system suspend, this file is not present.
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``disk``
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This file contains a list of strings representing different operations
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that can be carried out after the hibernation image has been saved. The
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possible options are as follows:
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``platform``
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Put the system into a special low-power state (e.g. ACPI S4) to
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make additional wakeup options available and possibly allow the
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platform firmware to take a simplified initialization path after
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wakeup.
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``shutdown``
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Power off the system.
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``reboot``
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Reboot the system (useful for diagnostics mostly).
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``suspend``
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Hybrid system suspend. Put the system into the suspend sleep
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state selected through the ``mem_sleep`` file described above.
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If the system is successfully woken up from that state, discard
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the hibernation image and continue. Otherwise, use the image
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to restore the previous state of the system.
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``test_resume``
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Diagnostic operation. Load the image as though the system had
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just woken up from hibernation and the currently running kernel
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instance was a restore kernel and follow up with full system
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resume.
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Writing one of the listed strings into this file causes the option
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represented by it to be selected.
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The currently selected option is shown in square brackets which means
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that the operation represented by it will be carried out after creating
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and saving the image next time hibernation is triggered by writing
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``disk`` to :file:`/sys/power/state`.
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If the kernel does not support hibernation, this file is not present.
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According to the above, there are two ways to make the system go into the
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:ref:`suspend-to-idle <s2idle>` state. The first one is to write "freeze"
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directly to :file:`/sys/power/state`. The second one is to write "s2idle" to
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:file:`/sys/power/mem_sleep` and then to write "mem" to
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:file:`/sys/power/state`. Likewise, there are two ways to make the system go
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into the :ref:`standby <standby>` state (the strings to write to the control
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files in that case are "standby" or "shallow" and "mem", respectively) if that
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state is supported by the platform. However, there is only one way to make the
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system go into the :ref:`suspend-to-RAM <s2ram>` state (write "deep" into
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:file:`/sys/power/mem_sleep` and "mem" into :file:`/sys/power/state`).
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The default suspend variant (ie. the one to be used without writing anything
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into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems
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supporting :ref:`suspend-to-RAM <s2ram>`) or "s2idle", but it can be overridden
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by the value of the "mem_sleep_default" parameter in the kernel command line.
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On some ACPI-based systems, depending on the information in the ACPI tables, the
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default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported.
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@ -0,0 +1,52 @@
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===========================
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Power Management Strategies
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===========================
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::
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Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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The Linux kernel supports two major high-level power management strategies.
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One of them is based on using global low-power states of the whole system in
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which user space code cannot be executed and the overall system activity is
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significantly reduced, referred to as :doc:`sleep states <sleep-states>`. The
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kernel puts the system into one of these states when requested by user space
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and the system stays in it until a special signal is received from one of
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designated devices, triggering a transition to the ``working state`` in which
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user space code can run. Because sleep states are global and the whole system
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is affected by the state changes, this strategy is referred to as the
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:doc:`system-wide power management <system-wide>`.
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The other strategy, referred to as the :doc:`working-state power management
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<working-state>`, is based on adjusting the power states of individual hardware
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components of the system, as needed, in the working state. In consequence, if
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this strategy is in use, the working state of the system usually does not
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correspond to any particular physical configuration of it, but can be treated as
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a metastate covering a range of different power states of the system in which
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the individual components of it can be either ``active`` (in use) or
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``inactive`` (idle). If they are active, they have to be in power states
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allowing them to process data and to be accessed by software. In turn, if they
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are inactive, ideally, they should be in low-power states in which they may not
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be accessible.
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If all of the system components are active, the system as a whole is regarded as
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"runtime active" and that situation typically corresponds to the maximum power
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draw (or maximum energy usage) of it. If all of them are inactive, the system
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as a whole is regarded as "runtime idle" which may be very close to a sleep
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state from the physical system configuration and power draw perspective, but
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then it takes much less time and effort to start executing user space code than
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for the same system in a sleep state. However, transitions from sleep states
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back to the working state can only be started by a limited set of devices, so
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typically the system can spend much more time in a sleep state than it can be
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runtime idle in one go. For this reason, systems usually use less energy in
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sleep states than when they are runtime idle most of the time.
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Moreover, the two power management strategies address different usage scenarios.
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Namely, if the user indicates that the system will not be in use going forward,
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for example by closing its lid (if the system is a laptop), it probably should
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go into a sleep state at that point. On the other hand, if the user simply goes
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away from the laptop keyboard, it probably should stay in the working state and
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use the working-state power management in case it becomes idle, because the user
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may come back to it at any time and then may want the system to be immediately
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accessible.
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@ -0,0 +1,8 @@
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============================
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System-Wide Power Management
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============================
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.. toctree::
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:maxdepth: 2
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sleep-states
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@ -0,0 +1,9 @@
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==============================
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Working-State Power Management
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==============================
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.. toctree::
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:maxdepth: 2
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cpufreq
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intel_pstate
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