Android is NOT just 'Java on Linux'

Android is
NOT just
'Java on Linux'
2011.5.22
2011.10.26 updated

Tetsuyuki Kobayashi

1

Let's talk about inside of Android.

http://www.kmckk.co.jp/eng/kzma9/ 2
http://www.kmckk.co.jp/eng/jet_index.html

Who am I?
 20+ years involved in embedded systems
 10 years in real time OS, such as iTRON
 10 years in embedded Java Virtual Machine
 Now GCC, Linux, QEMU, Android, …
 Blogs
 http://d.hatena.ne.jp/embedded/ (Personal)
 http://blog.kmckk.com/ (Corporate)
 http://kobablog.wordpress.com/(English)
 Twitter
 @tetsu_koba
3

Android is NOT just
'Java on Linux'
 Android uses Linux kernel. Only kernel.
 User land is totally different from usual Linux
system.
 Android applications are written in Java
language.
 Class libraries are similar to Java SE but not
equal.
 Dalvik VM eats only dex code
 need to translate from Java byte code in
advance 4

Let's explore inside of Android

 Assuming you know Linux and Java
very well :)

5

Today's topic

 Android system architecture
 Init – runtime – Zygoto

 Dalvik VM

 Android specific kernel

drivers
 How to build Android

6

Today's topic


 Dalvik VM

 Android specific kernel drivers


7

System architecture

8

Java is the first class citizen in
Android
 Dalvik VM is the center of Android
runtime.
 Almost all daemon services are written in
Java.
 Application life cycle is described by
Java API

9

Java is the first class citizen in
Android
 NDK
 native library called from Java via JNI
 This is just a library. Application life
cycle is the same as Java.
 Native activity
 Only C/C++ to make Apps. (just hidden
JNI part into system.)
 not short-cut for C/C++
10

Typical Directory Tree of Android
ro: mounted as read only
/ /(root)
(root) initrd (ro)
rw: mounted as read and write

yaffs2 (ro)
/system
/system bin

etc

lib
/data yaffs2 (rw)
/data
usr

/cache yaffs2 (rw)
/cache

/mnt/sdcard removable storage (rw)
/mnt/sdcard

cf. Usual Linux system assumes all file system are read/writable. 11

Today's topic


 Dalvik VM


drivers

12

Boot sequence

13
quoted from http://hmtsay.blogspot.com/2010/10/android-startup.html

init
 located on /init
 need kernel boot parameter to add
“init=/init”
 Static linked.
 cf. typical linux init is dynamic

linked.
 Doesn't affect even dynamic link

system collapsed.
 http://blog.kmckk.com/archives/3137191.html 14

Bionic

 The standard libraries
 libc, libm, pthread, dynamic linker
 linker has implicit crash dump function
 http://kobablog.wordpress.com/2011/05/12/debuggerd-of-android/
 Came from *BSD, not glibc
 Currently, doesn't support C++ exception
and RTTI.
 latest NDK supports these by static linking.
15

Prelinking

 Locate dynamic link libraries ahead of
time.
 'apriori' command. Different from 'prelink'
command from Red Hat.
 Optimized for small embedded system
 Allocate fixed address to libraries .
 Assume 3GB memory space is large enough
to put all libraries together.
 Assume not adding/removing libraries. 16

Prelink map
build/core/prelink-linux-arm.map
# 0xC0000000 - 0xFFFFFFFF Kernel
# 0xB0100000 - 0xBFFFFFFF Thread 0 Stack
# 0xB0000000 - 0xB00FFFFF Linker
# 0xA0000000 - 0xBFFFFFFF Prelinked System Libraries
# 0x90000000 - 0x9FFFFFFF Prelinked App Libraries
# 0x80000000 - 0x8FFFFFFF Non-prelinked Libraries
# 0x40000000 - 0x7FFFFFFF mmap'd stuff
# 0x10000000 - 0x3FFFFFFF Thread Stacks
# 0x00000000 - 0x0FFFFFFF .text / .data / heap

# core system libraries
libdl.so 0xAFF00000 # [<64K]
libc.so 0xAFD00000 # [~2M]
libstdc++.so 0xAFC00000 # [<64K]
libm.so 0xAFB00000 # [~1M]
liblog.so 0xAFA00000 # [<64K]
libcutils.so 0xAF900000 # [~1M]
libthread_db.so 0xAF800000 # [<64K]
libz.so 0xAF700000 # [~1M]
libevent.so 0xAF600000 # [???]
libssl.so 0xAF400000 # [~2M]
libcrypto.so 0xAF000000 # [~4M]
libsysutils.so 0xAEF00000 # [~1M]
... 17

Zygote

quoted from http://worms.zoology.wisc.edu/dd2/echino/cleavage/intro.html

18

Zygote
fork

Zygote process Child process

classes classes classes classes
classes classes
dynamic link classes classes
dynamic link
Dalvik VM classes libraries Dalvik VM classes libraries

classes classes
classes classes
dynamic link
Dalvik VM classes libraries

Physical memory space (Actually these are mapped by pages.)
19

Zygote
 Zygote process preloads typical (approx. 1800)
classes and dynamic link libraries so that
childlen start quickly.
 Copy-on-write
 Only when new process writes page, new page is
allocated.
 All pages not be written are shared among all
zygote children.
 Exec system call is not used in zygote.
 Exec wipes the page mapping table of the process.
 It means exec discards zygote cache. 20

UID, GID of Applications
 UID(user id) and GID(group id) is used for
managing multi-user in usual Linux
system.
 Android use this mechanism to isolate
applications.
 Each application has unique UID.
 Can not read/write other application's files.
 Zygote is running as UID=0 (root). After
forking child process, its UID is changed
by setuid system call. 21

Today's topic


 Dalvik VM


drivers

22

Dalvik VM
 executes dex code, which is translated
from Java byte code
 16bit, register based
 cf. Java bytecode is 8bit, stack based
 has JIT from Android 2.2 (Froyo)
 http://blog.kmckk.com/archives/2691473.html
 has concurrent GC from Android 2.3
(Gingerbread)
 http://source.android.com/tech/dalvik/
23

Java class libraries
 Different from Java ME, which is used in
traditional Japanese phone.
 Similar to Java SE. But not equal.
 Different window/graphics. No AWT, No
Swing.
 No RMI.
 Take care to use user defined class loader
 dynamic generated classes doesn't work
because Dalvik VM doesn't eat Java class
files but Dex files. 24

Caveats of NDK programming
 Dynamic libraries built by NDK are linked
with application process.
 forked from Zygote but UID != 0 (root).
 consider about permissions.
 Don't use fork & exec system calls.
 Back ground process should be made as
android .app.Service.
 Don't use GCC's TLS extension (__thread).
 Simple Android dynamic linker does not support it.
 java.lang.ThreadLocal is available in Java. 25

3 commands to invoke Dalvik VM

 /system/bin/app_process
 This is the 'Zygote' process.
 /system/bin/dalvikvm
 Similar to usual 'java' command.
 Try 'dalvikvm -h' to show command line help.
 /system/bin/dvz
 Send request to Zygote process.
 See my blog (Sorry in Japanese)
26

Today's topic


 Dalvik VM


drivers

27

Linux kernel
 Many common Linux device drivers are
available.
 Android specific kernel drivers
 binder
 ashmem
 wake lock
 logger
 …
 http://elinux.org/Android_Kernel_Features

 These source code is not yet merged to
kernel main line repository. 28

Binder
 /dev/binder
 Base of Inter Process Method Invocation
 Not for general purpose. Tuned for specific
transaction.
 Multi-thread aware
 Have internal data per thread
 (CF. Socket have internal data per fd.)
 Doesn't use ”write” and ”read” system calls.
Write and read at once by ”ioctl”.
 http://blog.kmckk.com/archives/3676340.html 29

Ashmem
 Android / Anonymous SHared MEMory
subsystem
 $(TOP)/system/core/cutils/ashmem.h
 int ashmem_create_region(const char *name, size_t size)
→ returns fd
 int ashmem_set_prot_region(int fd, int prot)
 int ashmem_pin_region(int fd, size_t offset, size_t len)
 int ashmem_unpin_region(int fd, size_t offset, size_t len)
 Kernel reclaims not ‘pin’ ed memory
 Similar to weak reference of Java. Useful to
implement cache.
 android.os.MemoryFile from Java program
30

Wake lock

 Lock to prevent entering sleep mode.
 My memos
 eLinux wiki
 http://elinux.org/Android_Power_Management

31

Alarm

 kernel implementation to support
Android's AlarmManager.
 Wake up even when it was in sleep mode.

32

Low memory killer

 At the shortage of memory, the kernel
select a process seems low priority and
kill it. (!!)
 It's OK. because specification in the
Android application life cycle, application
should be preserve its own status.

33

Logger

 Android has unique system-wide log
system

http://elinux.org/Android_Logging_System

34

Overview of Android Logging System
Target
Java program
Java program System.out
/System.err

com.android.internal.os
Native program android.util.Log com.android.internal.os Host
Native program android.util.Log AndroidPrintStream
AndroidPrintStream

DDMS
stdout
logcat
logcat
stdout liblog
/stderr liblog adbserver
adbd
adbd adbserver
User

Kernel adb logcat

main 64KB radio
logger
logger 64KB
/dev/log/main /dev/log/main
/dev/log/radio event /dev/log/radio
system
/dev/log/event 256KB /dev/log/event
/dev/log/system 64KB /dev/log/system 35

Today's topic


 Dalvik VM


drivers

36

How to build Android
 All source code is available for download
 except Google specific services (Google
map, Android market, … )
 Easy to download source and build them
 See AOSP web site
 http://source.android.com/
 Or, my blog
37

Conclusion

 Android system architecture is totally
different from normal Linux systems.
 Android uses Linux kernel only,
further more, adding android specific
kernel drivers.
 Designed for Java applications.
 Tuned for small system.
38

Q&A

Thank you for listening!
Any comments to blogs are welcome.

39

Android is NOT just 'Java on Linux'

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