CS18000: Programming I

Transcription

1 CS18000: Programming I Synchronization 22 February, 2010 Prof. Chris Clifton Concurrency Example: Banking class ATM { public void withdrawcash(acct a) { Scanner sc = new Scanner(System.in); int amount = sc.nextint(); if (a.getbalance() >= amount) { a.setbalance(a.getbalance() - amount); int dispensed = dispensecash(amount); What if there wasn t enough cash? 2/24/2010 CS Chris Clifton 1

2 Concurrency Example: Banking class ATM { public void withdrawcash(acct a) { Scanner sc = new Scanner(System.in); int amount = sc.nextint(); if (a.getbalance() >= amount) { int dispensed = dispensecash(amount); a.setbalance(a.getbalance - dispensed); 2/24/2010 CS Solution: Synchronization Allow concurrency But limit when needed Once account has given one ATM the balance, it doesn t respond to the other until the balance is updated Why isn t this easy At least when we don t deal with the real world 2/24/2010 CS Chris Clifton 2

3 Why is this hard? Concurrent system architecture Two things really can happen at once! How do you control which gets to memory? CPU Cache Operations: Read or Write Bus CPU Cache Memory 2/24/2010 CS Is this problem real? public class RaceCondition extends Thread { private static int counter = 0; public static final int THREADS = 4; public static final int COUNT = ; public void run() { for ( int i = 0; i < COUNT/THREADS; i++ ) counter++; public static void main ( String[] args ) { RaceCondition[] threads = new RaceCondition[THREADS]; for ( int i = 0; i < THREADS; i++ ) { threads[i] = new RaceCondition(); threads[i].start(); try { for ( int i = 0; i < THREADS; i++ ) threads[i].join(); catch ( InterruptedException e ) { e.printstacktrace(); System.out.println("Counter:\t" + counter); 2/24/2010 CS Chris Clifton 3

4 Synchronization: Theory Two ways to do this Semaphores Monitors Both give everything we need Hardware / programming language will support one or both 2/24/2010 CS Semaphores Test value and set in one operation if (! semaphore) semaphore=true; Only use resource after testing and setting semaphore Full version: counter if (semaphore > 0) semaphore -= 1; Edsger W. Dijkstra 2/24/2010 CS Chris Clifton 4

5 Monitors Protected block of code Can t be executed concurrently Perform critical operations in monitor Don t leave until done Different threads can call the monitor But won t start until previous one done. C.A.R. Hoare Per Brinch Hansen 2/24/2010 CS Doing this in Java: synchronized A synchronized method is a monitor public synchronized void updatebalance (int amount) throws insufficientfunds { if (amount > balance) throw new insufficientfunds(); balance = balance amount; Synchronization is on a per-object basis Can run same method concurrently for different objects Cannot run two synchronized methods concurrently on the same object 2/24/2010 CS Chris Clifton 5

6 Synchronized part II: Semaphores private static Object lock = new Object(); synchronized(lock) { // update balance Only one synchronized block can run at a time for the given object (lock) 2/24/2010 CS CS18000: Programming I Synchronization 24 February, 2010 Prof. Chris Clifton 2010 Chris Clifton 6

7 Project 3 is out Concurrency Synchronization Abstractions Due 11 March Announcements Watch for updates We ve had one code improvement since it came out 2/24/2010 CS Concurrency: Coordination Synchronized blocks others until we are done What if we need another task to finish before we can complete? Example: Waiting for ATM to dispensecash(); Solution: wait(); Used in synchronized code Allows others to use that object They call notify() or notifyall() when done 2/24/2010 CS Chris Clifton 7

8 Wait Method in the Object class Can call wait() from any object, or on any object. Call wait() only in synchronized block for that object Synchronized method of that instance Synchronized(theInstance) { theinstance.wait(); Synchronized static method of class Gives up synchronization Other threads can use that object, even though we are still in synchronized block 2/24/2010 CS Wait Current thread will stop until notify() is called on that object Thread resumes when nobody else is in synchronized block for object Can also interrupt thread that is waiting interrupt() is method in Thread class causes wait() to throws InterruptedException Wait with timeout wait(long milliseconds); wait(long milliseconds, int nanoseconds); Returns on notify OR timeout 2/24/2010 CS Chris Clifton 8

9 Notify Method in the Object class Call notify() only in synchronized block for that object Synchronized method of that instance Synchronized(theInstance) { theinstance.notify(); Synchronized static method of class Otherwise you ll get an IllegalMonitorStateException Waiting thread resumes when notifying thread leaves Synchronized block/method 2/24/2010 CS notifyall Notify only wakes up ONE waiting thread What if multiple threads wait() on same object? Each waiting thread must notify() when done to wake up any others notifyall() wakes them all up Then they compete to use the synchronized object 2/24/2010 CS Chris Clifton 9

10 Recommendations on use Remember that concurrency / synchronization doesn t hold while you wait Make sure it is okay for someone else to be using / modifying the waiting instance. Make sure the wait is on the right object Synchronized(lock) { wait(); waits for notify on instance of current method, not on lock Make sure you SHOULD be awake Synchronized(lock) { while (! lock.readytocontinue() ) lock.wait(); 2/24/2010 CS Example See Wait.java and Dull.java 2/24/2010 CS Chris Clifton 10