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转自:【Android休眠】之Android休眠机制___2017__的博客-CSDN博客_android系统休眠机制
一、休眠概述
休眠,简而言之就是设备在不需要工作的时候把一些部件、外设关掉(掉电或让它进入低功耗模式)。
为什么要休眠呢?一言以蔽之:省电。
休眠分主动休眠和被动休眠。主动休眠:比如我电脑不用了,就通过设置让系统进入休眠模式;被动休眠:系统检测到自己闲的慌,为了节约故,自己就休眠去了。
废话不叙。
二、Android休眠
休眠是内核的核心工作,而Android是基于Linux内核的,所以Android休眠和内核有着千丝万缕的联系;由于Android的特殊应用场景:移动设备,所以Android休眠和内核又有着特别的需求。
1、联系:
Android设备停止使用,系统没有什么事情可做,进入休眠状态的功能最终是由内核去实现的;每一类硬件都有自己的驱动,具体的驱动决定怎么进入休眠以及处于何种层次的休眠。比如:对于platform_device,就按照platform_driver定义的规则,在suspend调用的时候,去做上面提到的事情:
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- struct platform_driver {
- int (*probe)(struct platform_device *);
- int (*remove)(struct platform_device *);
- void (*shutdown)(struct platform_device *);
- int (*suspend)(struct platform_device *, pm_message_t state);
- int (*resume)(struct platform_device *);
- struct device_driver driver;
- const struct platform_device_id *id_table;
- };
2、Android的特别需求:
比如对于自己的电脑,不用让它休眠好了;但是对于我们形影不离的手机,在休眠的时候还要睁一只眼:来电了要通知你,QQ啊微信啊什么的由信息了也要通知你,所以Android在Linux内核休眠机制之上,提出了“Opportunistic Suspend”。
三、休眠实践
絮絮叨叨这么多,下面让我们切切实实体验下休眠。
1、休眠模式
休眠是分好几种模式的,不同模式实现方式、耗电量不同,以下来自Documentation/power/states.txt:
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- The kernel supports four power management states generically, though
- one is generic and the other three are dependent on platform support
- code to implement the low-level details for each state.
- This file describes each state, what they are
- commonly called, what ACPI state they map to, and what string to write
- to /sys/power/state to enter that state
- state: Freeze / Low-Power Idle
- ACPI state: S0
- String: "freeze"
- This state is a generic, pure software, light-weight, low-power state.
- It allows more energy to be saved relative to 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 Standby/Suspend-to-RAM
- support, or it can be used in addition to Suspend-to-RAM (memory sleep)
- to provide reduced resume latency.
- State: Standby / Power-On Suspend
- ACPI State: S1
- String: "standby"
- This state offers minimal, though real, power savings, while providing
- a very 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.
- We try to put devices in a low-power state equivalent to D1, which
- also offers low power savings, but low resume latency. Not all devices
- support D1, and those that don't are left on.
- State: Suspend-to-RAM
- ACPI State: S3
- String: "mem"
- This state offers significant power savings as everything in the
- system is put into a low-power state, except for memory, which is
- placed in self-refresh mode to retain its contents.
- System and device state is saved and kept in memory. All devices are
- suspended and put into D3. In many cases, all peripheral buses lose
- power when entering STR, so devices must be able to handle the
- transition back to the On state.
- For at least ACPI, STR requires some minimal boot-strapping code to
- resume the system from STR. This may be true on other platforms.
- State: Suspend-to-disk
- ACPI State: S4
- String: "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.
虽说kernel支持上述四种休眠模式,但具体哪几种可用取决于你的硬件。那么怎么知道自己的Android设备支持的休眠模式呢?
答案:通过/sys/文件系统。查询支持的休眠模式可以cat文件/sys/power/state:
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- cat /sys/power/state
- freeze mem
如果我们往/sys/power/state文件echo上面的某一种模式的字符串,系统就会进入相应的休眠模式:
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- echo "mem" > /sys/power/state
如果你搜索过Android休眠相关的内容,在老版本的Android(4.4版本之前)会见有提到PowerManager的setPowerState()方法,该方法即是通过以上方式使系统进入休眠。但自从引入Autosleep后,就不在这么做了,setPowerState()方法也销声匿迹。
2、/sys/power/目录下文件
文件简介:
- /sys/power/state:用来控制系统的Power状态。读取该文件可以获取系统支持的休眠模式,写入该文件休眠模式的一种,系统进入到指定的休眠模式。如上所示例。
- /sys/power/autosleep:从Android wakelocks补丁集中演化而来,用于取代Android wakelocks中的自动休眠功能。向该文件写入/sys/power/state返回值的某一种,系统会在适当的时候进入指定的休眠的模式;读取该文件返回之前写入的数值。
- /sys/power/wake_lock、/sys/power/wake_unlock:即我们常说的休眠锁,如果应用持有休眠锁,系统将无法进入休眠模式。在Android wakelocks时代,写wake_lock获取锁,写wake_unlock释放锁;在AutoSleep时代,具体参见【Android休眠】之AutoSleep
- wakeup_count:用于解决“system suspend和system wakeup events之间的同步问题”。
- /sys/power/pm_async:状态切换开关,允许/禁止User空间对设备进行异步的suspend和resume操作。
- /sys/power/pm_freeze_timeout:系统在执行休眠动作的时候要冻结(freeze)用户控件的进程和内核空间的允许冻结的内核线程,执行这些操作要耗时间吧?该文件指定所需时间的最大值。
四、其他需要明了的问题
1、Android设备屏幕暗下来的时候,并不是立即就进入了休眠模式;当所有唤醒源都处于de-avtive状态后,系统才会进入休眠。
2、Android设备连着adb线到其他设备的情况下,设备是不会进入休眠模式的。
3、有休眠操作就有唤醒,就需要唤醒源。唤醒源有很多种,在内核注册,比如常用的Power按键。
4、曾经困惑的一个问题:系统怎么知道自己应该进入休眠模式了?它的判断依据是什么?
- 在wakelock时代,系统休眠过程中去检测休眠锁;如果系统中没有其他部件持有休眠锁,就尝试进入休眠模式,没有异常事件发生的话就进入休眠模式。
- Android从4.4开始使用autosleep机制,只要不存在任何active的唤醒源(wakeup_source)了,就进入休眠模式。
5、系统Power Manager整体流程:
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