之前在Android过程调度中介绍过Android应用的调度策略。Android对调度策略和优先级的治理是很审慎的，个别状况下不须要利用和服务调整本人的优先级。然而，一些有零碎开发能力的厂商有时会对优先级进行批改。如果没有通过充分考虑，可能会对系统运行产生不良影响，尤其是批改实时调度策略。
Linux kernel RT priority
目前，Linux对实时调度的优先级治理是比拟宽泛的，能够在0~99间调整优先级。然而，这样也同样带来了优先级治理问题。社区大神就有埋怨这个问题(Controlling realtime priorities in kernel threads)，提交了补丁来勾销实时调度优先级调整的大部分能力。先看一下以后内核中对SCHED_FIFO优先级的治理。
| Subsystem | Priority | Description |
|---|---|---|
| Arm bL switcher | 1 | The Arm big.LITTLE switcher thread |
| crypto | 50 | Crypto engine worker thread |
| ACPI | 1 | ACPI processor aggregator driver |
| drbd | 2 | Distributed, replicated block device request handling |
| PSCI checker | 99 | PSCI firmware hotplug/suspend functionality checker |
| msm | 16 | MSM GPU driver |
| DRM | 1 | Direct rendering request scheduler |
| ivtv | 99 | Conexant cx23416/cx23415 MPEG encoder/decoder driver |
| mmc | 1 | MultiMediaCard drivers |
| cros_ec_spi | 50 | ChromeOS embedded controller SPI driver |
| powercap | 50 | “Powercap” idle-injection driver |
| powerclamp | 50 | Intel powerclamp thermal management subsystem |
| sc16is7xx | 50 | NXP SC16IS7xx serial port driver |
| watchdog | 99 | Watchdog timer driver subsystem |
| irq | 50 | Threaded interrupt handling |
| locktorture | 99 | Locking torture-testing module |
| rcuperf | 1 | Read-copy-update performance tester |
| rcutorture | 1 | Read-copy-update torture tester |
| sched/psi | 1 | Pressure-stall information data gathering |
从上表能够看出，Kernel中实时调度的优先级次要集中在三个地位：高优先级(99)、失常优先级(50)、低优先级(1)。所以，其实并不需要太多优先级的阶层，过多只会带来凌乱。
Android RT priority
Android对于实时调度优先级的设置是通过三思而行的，要保障实时性又不能烦扰内核运行。其中，Sensor设置了很高的优先级，因为其对实时性非常敏感。其余的实时调度线程都维持在低优先级上，同样因为音频对工夫敏感，而赋予了绝对高的优先级，但还是维持在低优先级上。下表列出Android零碎(9.0)中实时调度优先级的配置，
| Subsystem | Priority | Description |
|---|---|---|
| SensorManager EventQueue | 98 | SensorManager EventQueue thread |
| SensorManager | 10 | SensorManager looper |
| FastMixer/FastCapture | 3 | AudioFlinger fast mixer and fast capture |
| AudioApp | 2 | AudioFlinger playback or record threads |
| Audio HAL | 3/2 | Flow AudioFlinger |
| AAudioService | 2 | Aaudio threads |
| SurfaceFlinger | 2 | SurfaceFlinger Main, DispSync and Event threads. |
| CameraService | 1 | When “camera.fifo.disable” isn’t 1. |
| SurfaceFlinger handleMessage | 1 | SCHED_FIFO when the display is turned on, SCHED_OTHER when turned off. |
| UI and RenderThreads | 1 | ActivityManagerService use SCHED_FIFO for UI and RenderThreads if “sys.use_fifo_ui” isn’t 0. |
下面的两个表格总结了Linux kernel和Android零碎中实时调度策略优先级的配置，零碎开发人员能够做为参考。当开发人员须要批改或配置本人的实时调度策略是，须要充沛了解以后优先级的状态，以及应用实时调度策略的目标，从而抉择一个适合的优先级。
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