Android Notes

Revision as of 12:18, 11 May 2009 by Tim Bird (Talk | contribs) (C Library (bionic) info: add more bionic info)

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Here are some miscellaneous notes on Android. See Android on OMAP, too.

Technical Info about system components

oom killer info

Google (Android) developer Arve Hjonevag added a lowmemorykiller feature to the staging area of the Linux kernel in January of 2009. This feature tries to reclaim memory before the system runs out (acting as a kind of cache manager, according to Arve). In Linus' 2.6.28-rc tree this appears in

Application lifecycle (and activity states) can be found here:

Applications in different stages of their lifecycle receive a different oom_adj value, which affects the probability of their being reaped.

Initial lowmemorykiller thresholds are set by /etc/init.rc (with the following lines):

# Define the oom_adj values for the classes of processes that can be
# killed by the kernel.  These are used in ActivityManagerService.
   setprop ro.FOREGROUND_APP_ADJ 0
   setprop ro.VISIBLE_APP_ADJ 1
   setprop ro.SECONDARY_SERVER_ADJ 2
   setprop ro.HIDDEN_APP_MIN_ADJ 7
   setprop ro.CONTENT_PROVIDER_ADJ 14
   setprop ro.EMPTY_APP_ADJ 15

# Define the memory thresholds at which the above process classes will
# be killed.  These numbers are in pages (4k).
   setprop ro.FOREGROUND_APP_MEM 1536
   setprop ro.VISIBLE_APP_MEM 2048
   setprop ro.SECONDARY_SERVER_MEM 4096
   setprop ro.HIDDEN_APP_MEM 5120
   setprop ro.CONTENT_PROVIDER_MEM 5632
   setprop ro.EMPTY_APP_MEM 6144

# Write value must be consistent with the above properties.
   write /sys/module/lowmemorykiller/parameters/adj 0,1,2,7,14,15

   write /proc/sys/vm/overcommit_memory 1
   write /sys/module/lowmemorykiller/parameters/minfree 1536,2048,4096,5120,5632,6144

   class_start default

   # Set init its forked children's oom_adj.
   write /proc/1/oom_adj -16

Routines to actually calculate the oom_adj value, based on application state are in the ActivityManager. See frameworks/base/services/java/com/android/server/am/

and frameworks/base/core/java/android/os/ => frameworks/base/core/jni/android_util_Process.cpp:android_os_Process_setOomAdj()

init sequence

See this blog entry for an overview of the sequence of operations performed by the 'init' program:

C Library (bionic) info

See the excellent article: for an overview of bionic.


Google developed a custom library for the C compiler (libc) called Bionic. This was necessary for three main reasons:

  • License: they wanted to keep GPL out of user-space. Bionic code uses the BSD license.
  • Size: the library has to be loaded in each process, so it needs to be small. Bionic is about 200K, or half the size of glibc (the GNU version of libc).
  • Speed: limited CPU power means it needs to be fast. Bionic has a small size and fast code paths, including a very fast and small custom pthread implementation.

Bionic has built-in support for important Android-specific services such as system properties and logging. It doesn’t support certain POSIX features, like C++ exceptions and wide chars, which were not needed on Android. Thus it’s not quite compatible with the gnu libc. All native code must be compiled against bionic, not glibc.


The bionic C library has it's own thread API, not the same as either original LinuxThreads or NPTL.


android thread library (from bionic libc) is minimalist :
 - use most of the linux kernel thread features (futex, CLONE_THREAD)
 unlike old linuxpthread.
- it doesn't support thread cancellation (see CAVEATS file)
- it misses some pthread functions : for example no barrier, missing 
*timedwait variant, ...
- very basic gdb support that only work through gdbserver
- some theoric race

Development Tips

debugging tips

Quick stack dump - "kill -3 <pid>" will put a stack trace in the log file. (Use logcat to view)

Getting a bootchart from Android

The 'init' program in Android has built-in capability to gather the data needed to produce a bootchart image. See README.BOOTCHART in the directory system/core/init directory of the Android source repository for details.


Building the kernel from scratch