System Power Management Sleep States (C) 2014 Intel Corp., Rafael J. Wysocki <[email protected]> The kernel supports up to four system sleep states generically（一般）, although three of them depend on the platform support code to implement（实行） the low-level details for each state. The states are represented by strings that can be read or written to the /sys/power/state file. Those strings may be "mem", "standby", "freeze" and "disk", where the last three always represent(代表) Power-On Suspend (if supported待机), Suspend-To-Idle（idle） and hibernation（冬眠） (Suspend-To-Disk), respectively（分别）. The meaning of the "mem" string is controlled by the /sys/power/mem_sleep file. It contains strings representing the available modes of system suspend that may be triggered by writing "mem" to /sys/power/state. These modes are "s2idle" (Suspend-To-Idle), "shallow(浅睡)" (Power-On Suspend) and "deep" (Suspend-To-RAM). The "s2idle" mode is always available, while the other ones are only available if supported by the platform (if not supported, the strings representing them are not present in /sys/power/mem_sleep). The string representing the suspend mode to be used subsequently(后来) is enclosed in square brackets. Writing one of the other strings present in /sys/power/mem_sleep to it causes the suspend mode to be used subsequently to change to the one represented by that string. Consequently, there are two ways to cause the system to go into the Suspend-To-Idle sleep state. The first one is to write "freeze" directly to /sys/power/state. The second one is to write "s2idle" to /sys/power/mem_sleep and then to write "mem" to /sys/power/state. Similarly, there are two ways to cause the system to go into the Power-On Suspend sleep state (the strings to write to the control files in that case are "standby(待机)" or "shallow" and "mem", respectively) if that state is supported by the platform. In turn, there is only one way to cause the system to go into the Suspend-To-RAM state (write "deep" into /sys/power/mem_sleep and "mem" into /sys/power/state). The default suspend mode (ie. the one to be used without writing anything into /sys/power/mem_sleep) is either "deep" (if Suspend-To-RAM is supported) or "s2idle", but it can be overridden by the value of the "mem_sleep_default" parameter in the kernel command line. The properties of all of the sleep states are described below. State: Suspend-To-Idle ACPI state: S0 Label: "s2idle" ("freeze") This state is a generic, pure software, light-weight, system sleep state. It allows more energy to be saved relative to runtime idle by freezing user space and putting all I/O devices into low-power states (possibly lower-power than available at run time), such that the processors can spend more time in their idle states. This state can be used for platforms without Power-On Suspend/Suspend-to-RAM support, or it can be used in addition to Suspend-to-RAM to provide reduced resume latency(延迟). It is always supported. State: Standby / Power-On Suspend ACPI State: S1 Label: "shallow" ("standby") This state, if supported, offers moderate（中等）, though real, power savings, while providing a relatively low-latency transition back to a working system. No operating state is lost (the CPU retains power), so the system easily starts up again where it left off. In addition to freezing user space and putting all I/O devices into low-power states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline and all low-level system functions are suspended during transitions into this state. For this reason, it should allow more energy to be saved relative to Suspend-To-Idle, but the resume latency will generally be greater than for that state. State: Suspend-to-RAM ACPI State: S3 Label: "deep" This state, if supported, offers significant（重大的） power savings as everything in the system is put into a low-power state, except for memory, which should be placed into the self-refresh mode to retain(保留) its contents. All of the steps carried out when entering Power-On Suspend are also carried out during transitions to STR. Additional operations may take place depending on the platform capabilities. In particular, on ACPI systems the kernel passes control to the BIOS (platform firmware) as the last step during STR transitions and that usually results in powering down some more low-level components that aren't directly controlled by the kernel. System and device state is saved and kept in memory. All devices are suspended and put into low-power states. In many cases, all peripheral(外围设备) buses lose power when entering STR, so devices must be able to handle the transition back to the "on" state. For at least(最小的) ACPI, STR requires some minimal boot-strapping code to resume the system from it. This may be the case on other platforms too. State: Suspend-to-disk ACPI State: S4 Label: "disk" This state offers the greatest power savings, and can be used even in the absence of low-level platform support for power management. This state operates similarly to Suspend-to-RAM, but includes a final step of writing memory contents to disk. On resume, this is read and memory is restored to its pre-suspend state. STD can be handled by the firmware or the kernel. If it is handled by the firmware, it usually requires a dedicated partition that must be setup via another operating system for it to use. Despite the inconvenience, this method requires minimal work by the kernel, since the firmware will also handle restoring memory contents on resume. For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used to write memory contents to free swap space. swsusp has some restrictive requirements, but should work in most cases. Some, albeit outdated, documentation can be found in Documentation/power/swsusp.txt. Alternatively, userspace can do most of the actual suspend to disk work, see userland-swsusp.txt. Once memory state is written to disk, the system may either enter a low-power state (like ACPI S4), or it may simply power down. Powering down offers greater savings, and allows this mechanism to work on any system. However, entering a real low-power state allows the user to trigger wake up events (e.g. pressing a key or opening a laptop lid).
- On（on） S0 - Working（s0的状态应该是freeze）
- Standby （standby） S1 - CPU and RAM are powered but not executed
- Suspend to RAM（mem) S3 - RAM is powered and the running content is saved to RAM
- Suspend to Disk，Hibernation（disk） S4 - All content is saved to Disk and power down
S1状态简称standby状态，此状态下CPU处于低功耗状态，并且没有数据保存到RAM或者disk中，此状态待机和恢复通常很快。
S3 aka STR(suspend to ram),挂起到内存，简称待机。计算机将目前的运行状态等数据存放在内存，关闭硬 盘、外设等设备，进入等待状态。此时内存仍然需要电力维持其数据，但整机耗电很少。恢复时计算机从内存读出数据，回到挂起前的状态，恢复速度较快。对 DDR的耗电情况进行优化是S3性能的关键，大多数手持设备都是用S3待机。此状态下计算机会冻结所有的活动并将当前工作状态保存到RAM中，然后关闭屏幕进入低功耗模式，通常睡眠和唤醒需要几秒
S4 aka STD(suspend to disk),挂起到硬盘，简称休眠。把运行状态等数据存放在硬盘上某个文件或者某个特定的区域，关闭硬盘、外设等设备，进入关机状态。此时计算机完全关闭，不耗电。恢复时计算机从休眠文件/分区中读出数据，回到休眠前的状态，恢复速度较慢。电子书项目中，见过一款索尼的电子书，没有定义关机状态，只定义了S4,从而提高开机速度。一般在window系统中常见到。此状态下计算机将所有活动的状态保存到磁盘中，然后处于关机状态，此模式下是不耗电的，而相比之前的模式，休眠和唤醒的速度都比较慢。
