Case study 2
Android – Mobile OS
Sources and extra reading
Silberschatz, Galvin, Gagne. Operating System Concepts
Essentials. 8th Edition.
ISBN: 978-0-470-88920-6
A. Tanenbaum. Modern Operating Systems. 3rd Edition.
ISBN: 0136006639
H. Al-Rayes. Studying Main Differences between Android & Linux
Operating Systems. Diyala University.
Android Developer Website. http://developer.android.com
Topics covered in this session
 History of Android
 Android design principles
 Android Architecture
 Memory Management
 Threads and Scheduling
 Android File System
 Security
 Programming Interface
History of Android
- What is android? Linux 2.6 based operating system for mobile
devices.
- Open source and released under Apache Licence (Carriers
can modify it before distributing).
- Google acquired android in 2005.
- Android 1.0 released 2008.
- In 2012 Android 4.2 released
- Improvements include support for new devices:
-
Cameras
Multi core CPU
Barometer
etc
Design principles – What is required of a
mobile OS?
 Long battery life.
 Fast boot up.
 Fast response.
 Applications (Programming environment?)
 Security
 How do these differ from the needs of a desktop system?
 Consider how the above are met by the system design.
Mobile hardware differences
 Battery
 Touch screen
 Portable (Mobile CPU)
 More limited memory
 Fewer devices
Android Architecture
Source: http://developer.android.com/about/versions/index.html
Android Architecture
Source: http://developer.android.com/about/versions/index.html
Android Architecture
Source: http://developer.android.com/about/versions/index.html
Android Architecture
Source: http://developer.android.com/about/versions/index.html
Android Architecture
Source: http://developer.android.com/about/versions/index.html
Application life cycle
Memory management
 Android uses it’s own virtual machine to manage
application memory.
 Dalvik VM allows multiple instances of VM to be running
efficiently in parallel.
Process Management
 Differences between mobile app cycle and desktop
app cycle?
 Key principles:
 Android does not usually kill an app, they keep running even after you
switch, but saves state
 Task killers?
 Android kills apps when memory usage too high.
 But saves it’s state for quick restart.
 Uses Linux’s time sliced scheduling policy based on
priority
Process Priorities
 Split into background and
foreground.
 What happens if UI thread is
starved?
Disk I/O
Flash
Hard Disk Drives
Random Access
~0.1ms
5-10ms
File fragment impact
No
Greatly impacted
Total power
½ to 1/3 of HDD
Up to 15+ watts
Reliability
Reliable
Less reliable due to
mechanical parts
Write longevity
Limited number of writes.
Less of a problem
Capacity
<=512GB
Up to 4TB
Price
$1.5 - 2.0 GB
$0.10 - 0.20 GB
Why not use swapping?
 What needs storing? Consider a map application
(Lat/long).
 Swapping to flash affecting life.
Android File System
 Supports multiple different file systems (based on Linux Kernel).
 Usually yaffs2/vfat/ext4, depending on device manufacturer.
 Partitions:

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/boot (Included android kernel)
/system (Android GUI and pre-installed applications). – Read only
/recovery (Backup)
/data (User data)
/cache (Frequently accessed data)
/misc (Contains misc system settings in form of on/off switches)
/sdcard (SD card)
Security
 Android seeks to be the most secure and usable operating
system for mobile platforms by re-purposing traditional
operating system security controls to:
 Protect user data
 Protect system resources (including the network)
 Provide application isolation
 To achieve these objectives, Android provides these key
security features:




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Robust security at the OS level through the Linux kernel
Mandatory application sandbox for all applications
Secure interprocess communication
Application signing
Application-defined and user-granted permissions
Security Continued
 Assigns .unique user ID (UID) to each android application.
 Uses UNIX style file permissions due to different UIDs
 Ensures app A doesn’t read app B’s files.
 Ensures app A doesn’t exhaust B’s memory
 Ensures app A doesn’t exhaust B’s CPU usage
 Ensures app A doesn’t exhaust user Bs devices (GPS, BT)
 Linux kernel enforces security between applications
 Stops memory corruption errors causing vulnerabilities.
 Read only system partition
Security Continued
 Use of restricted APIs require
application permissions.
 Application Signing. Application
packages are signed to identify
author and prevent tampering.
Some windows 7 system files are
signed in a similar way.
Programming for Android
 Each program runs in it’s own VM.
 Need to understand an activities lifecycle.
 Make use of Android SDK
 Applications programmed in Java, with layouts coded in
XML.
 Packaged as .apk files for delivery through Android store.
 Importance of file size + data usage.