Android kernel code merged into mainline
现在是Linux 3.3的merge window，android的kernel code终于被merge到mainline中了，记得这个问题在一年前就开始讨论android与mainline kernel的分化问题，毕竟android是桌面Linux使用最广泛的distribution版本，把android code merge到mainline中对Linux长期的发展也有好处，当然Google也不希望自己的kernel与mainline kernel越来越远（那样Google的kernel tree维护者会整天merge code，很繁琐）。现在merge进的subsystem有：
ashmem：Anonymous Shared Memory Subsystem
config ASHMEM bool "Enable the Anonymous Shared Memory Subsystem" default n depends on SHMEM || TINY_SHMEM help The ashmem subsystem is a new shared memory allocator, similar to POSIX SHM but with different behavior and sporting a simpler file-based API. 从ashmem代码中可以看出，ashmem的后台仍是通用的share memory
binder：Android Binder IPC Driver
config ANDROID_BINDER_IPC bool "Android Binder IPC Driver" default n Binder在android中非常重要，基本上所有的跨进程通信（service间通信）都是用的binder机制。binder本质上是一种IPC，和fifo，msgqueue，shm一样运行在kernel空间。palm使用的也是openbinder。另一种mobile平台使用的IPC机制是DBUS，DBUS比binder更为人熟知，一般Fedora的机器都安装了dbus，而且时时运行着。
# ps aux|grep bus
dbus 1835 0.0 0.1 11212 716 ? Ssl Jan03 0:02 dbus-daemon --system
root 26552 0.0 0.1 4260 672 pts/1 S+ 05:00 0:00 grep bus
logger：Android log driver
这个subsystem很简单，就是在/dev目录下创建了四个文件：
#define LOGGER_LOG_RADIO "log_radio" /* radio-related messages */#define LOGGER_LOG_EVENTS "log_events" /* system/hardware events */#define LOGGER_LOG_SYSTEM "log_system" /* system/framework messages */#define LOGGER_LOG_MAIN "log_main" /* everything else */ 和syslog的7个等级差不多，android的logger实现也很简单，log放在ringbuffer中，ringbuffer的默认大小是256K
lowmemorykiller：
config ANDROID_LOW_MEMORY_KILLER bool "Android Low Memory Killer" default N ---help--- Register processes to be killed when memory is low lowmemorykiller是android中常用的的特性，iphone中也有相同的机制，当内存不够时系统会杀掉一些进程，我以前一直以为android用的是Linux mm的oom_killer，后来想一想应该不是，用Linux mm的oom_killer杀掉进程时系统应该反应已经非常迟钝了，注重用户响应速度的android不会等到那时才杀进程。
lowmemorykiller的实现原理很简单（200LOC），向mm注册一个shrinker，当系统memory压力比较大时，kswapd会调用shrinker清理内存，用户可以配置一些memory threshold来控制lowmemorykiller的行为。
pmem，ram_console，timed_gpio，timed_output，swich这几个subsystem牵涉到硬件层次比较多，暂未深入了解。
Linux在3.3 merge window中还merge了GNU的mpilib库：
config MPILIB
tristate "Multiprecision maths library" help Multiprecision maths library from GnuPG. It is used to implement RSA digital signature verification, which is used by IMA/EVM digital signature extension.config MPILIB_EXTRA bool "Multiprecision maths library - additional sources" depends on MPILIB help Multiprecision maths library from GnuPG. It is used to implement RSA digital signature verification, which is used by IMA/EVM digital signature extension. This code in unnecessary for RSA digital signature verification, and can be compiled if needed.config DIGSIG tristate "In-kernel signature checker" depends on KEYS select MPILIB help Digital signature verification. Currently only RSA is supported. Implementation is done using GnuPG MPI library
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