ANDROID OS CSE120 FA10 Xiao Ma xiao xiao ma com WHY ANDROID the definition of open mkdir android cd android repo init u git android git kernel org platform manifest git repo sync make What does it mean to researchers What does it mean to users OUTLINE Android platform architecture OS kernel libraries and devices Android programming model Delvik Virtual Machine Energy efficiency How to write efficient code FIRST THING FIRST What is the difference between a mobile OS and a desktop server OS ARCHITECTURE ANDROID ANDROID ANDROID BASED ON LINUX Android uses Linux 2 6 kernel as the hardware abstraction What are the essences an OS should provide Memory management process management IPC No virtual memory specially implemented IPC Drivers and architecture support How to port Android to a new device Using Linux vs Writing a new OS from scratch Do all Linux kernel implementations work well on mobile devices APPLICATION LIBRARY GNU libs glibc is too big and complicated for mobile phones so Android implements its own special version of libc Bionic libc Smaller size 200K glibc is more than 400K Strip out some complicated C features the most significant one no C exception Very special and small pthread implementation heavily based on kernel futexes Bionic libc does not fully support POSIX and is not compatible with glibc which means PROCESS MANAGEMENT What s the difference between mobile apps cycle and desktop apps cycle Two key principles Android usually do not kill an app i e apps keep running even after you switch to other apps Android kills apps when the memory usage goes too high but it saves app state for quick restart later on Do they make sense to mobile apps APPLICATION LIFE CYCLE EXAMPLE System Home Home Home At the Home screen EXAMPLE System Home Mail Home List Home List Start the Mail app and read the list EXAMPLE System Home Mail Home List Message List Message Home Click on one of the message and see its content EXAMPLE System Home Mail Browser Home List Message List Message Browser Home Browser Click a link in the message EXAMPLE System Home Browser Home List Message Browser Home Browser Now we have enough space to start the Map app EXAMPLE System Home Home List Message Browser Map Home Browser Browser Map Map Start the Map app EXAMPLE System Home Home List Message Browser Home Browser Browser Map Go back to the browser EXAMPLE System Home Home List Message List Message Home Browser Mail The Mail app is resumed and shows the previous message EXAMPLE System Home Home List Home Browser Mail List Go back to the mail list EXAMPLE System Home Home Home Browser Mail Go back to the Home screen DEBATE Swapping model VS Android s life cycle model DISK I O Flash Hard Disk Drive Random access 0 1ms 5 10ms File fragment impact No Greatly impacted Total power 1 2 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 2 3TB Price 1 5 2 GB 0 1 0 2 GB LIMITED WRITES Flash drives have the well known problem of limited number of writes in the life time 10 000 100 000 times Solution What can applications do How about operating system Controllers Hardware MEMORY MANAGEMENT Linux kernel does most of the job Page based memory management Virtual address to physical address mapping NO virtual memory Why do we still need virtual to physical address mapping Why does Android not support virtual memory POWER MANAGEMENT DALVIK VM Why does Android let developers use Java DALVIK VM A special Java virtual machine VM designed to run with limited system resource Memory efficiency Register machine vs Stack machine modern JVM fewer instructions faster execution why does the number of instructions matter Running multiple VMs more efficiently DEX FILE Java class files are converted into dex files that Dalvik executes Java byte code is converted into Dalvik byte code during this process MEMORY EFFICIENCY Shared constant string pool Share clean even some dirty memory between processes as much as possible dex files are mapped as read only by mmap Memory efficient JIT implementation JIT itself is about 100K Code cache and supporting data structure takes another 100K for each application SHARED STRING POOL public interface Zapper public String zap String s Object o public class Blort implements Zapper public String zap String s Object o public class ZapUser public void useZap Zapper z z zap SHARED STRING POOL SHARED MEMORY PROGRAMMING MODEL Each application is running in its own process An application can have one or more components activities services broadcast receivers and content providers A task an application from user s point of view consists of several activities from one or multiple applications An application keeps running until the system kills it because of memory shortage POWER SAVING Picture is from Google I O 09 talk Coding for Life Battery Life That Is GZIP TEXT DATA Use GZIP for text data whenever possible Compressing is implemented by native code CHECK NETWORK TYPE Wifi and 3G are much more energy efficient so wait for Wifi or 3G when transferring big chunk of data UPDATE BIN Use setInexactRepeating so the system can bin your update together with others Picture is from Google I O 09 talk Coding for Life Battery Life That Is WORK OFFLOADING Naive offloading Speech to text OCR More sophisticated offloading fine grained offloading MAUI Making Smartphones Last Longer with Code Offload MobiSys 10 Running two versions of the app on the mobile device and a powerful server Decide when what to offload on the fly EFFICIENT CODE for int i initializer i 0 i int limit calculate limit for int i 0 i limit i Type array get array for Type obj array for int i 0 i array length i for int i 0 i this var i Iterable Type list get list for Type obj list EFFICIENT CODE Try to rest for the most of the time be nice to other processes Avoid allocation short lived objects need to be garbaged collected long lived objects take precious memory Make a method static if it does not access member variables Avoid internal getter setters Use floating point numbers only when you have to Prefer int over enum Use static final for constants
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