System Programming

Practical Session 5

Liveness, Guarded Methods, and Thread Timing

Liveness

A concurrent application's ability to execute in a timely manner.

Liveness problems:

• Deadlock

• Starvation

• Livelock

Deadlock

Thread 1 Thread 2

Thread 1

acquire Printer

acquire Scanner

use printer

use scanner

release Printer

release Scanner

Thread 2

acquire Scanner

acquire Printer

use scanner

use printer

release Scanner

release Printer

1. public class Deadlock { 2.   public static void main(String[] args) { 3.     final Object resource1 = “Printer"; 4.   final Object resource2 = “Scanner"; 5.     Thread t1 = new Thread(new Runnable() { 6.       public void run() { 7.          synchronized(resource1) { 8.            System.out.println("Thread 1: locked resource 1"); 9.        

   try {Thread.sleep(50);} catch (InterruptedException e){}    

10.                synchronized(resource2) { 11.               

System.out.println("Thread 1: locked resource 2"); 12.     }/* release resource2 */ }/* release resource1 */  } } ); 13.      Thread t2 = new Thread(new Runnable() { 14.       public void run() { 15.          synchronized(resource2) { 16.           

System.out.println("Thread 2: locked resource 2"); 17.             

try{Thread.sleep(50);} catch(InterruptedException e){} 18.            synchronized(resource1) { 19.             

System.out.println("Thread 2: locked resource 1"); 20.      }/* release resource1 */ }/* release resource2 */  } } );21.      t1.start();      t2.start();    } }

Resource Ordering

All threads acquire the locks in the same order.

Thread 1

acquire Printer

acquire Scanner

use printer

use scanner

release Printer

release Scanner

Thread 2

acquire Printer

acquire Scanner

use printer

use scanner

release Printer

release Scanner

Starvation

Some threads are waiting forever for resources that are used by other threads.

Starvation

Some threads are waiting forever for resources that are used by other threads.

Example1: A solution to the dinning philosophers problem in which philosophers 1,3,5 never eat, and philosophers 2,4 eat whenever they want.

1

2

3

4

5

Starvation

Example 2: Threads with priority.

Each thread has a priority: Low or High.

Low priority threads execute only if there are no high priority threads.

High priority threads keep coming.

High

Low

LivelockThreads are unable to make progress although they are not blocked.

LivelockThreads are unable to make progress although they are not blocked.

Example: two threads trying to pass a shared corridor.

Thread protocol: if there is an obstacle, then move aside.

LivelockThreads are unable to make progress although they are not blocked.

Example: two threads trying to pass a shared corridor.

Thread protocol: if there is an obstacle, then move aside.

Guarded Methods

The guarded method model delays the execution of a thread until a condition is satisfied.

A thread that is unable to proceed, waits for condition change made by another thread.

Basic Thread Timing Example

Each thread is assigned a number from {1,2,…,k}.

In any point in time, only threads assigned a certain number run. Other threads wait.

1

3

1

1

Basic Thread Timing Example

Each thread is assigned a number from {1,2,…,k}.

In any point in time, only threads assigned a certain number run. Other threads wait.

1

3

3

1

1. class Checker { 2.     private  int m_num; 3.     public Checker(int num) { 4.         this.m_num = num; 5.     } 6.     public synchronized void  change(int num) { 7.         this.m_num = num; 8.     } 9.     public synchronized boolean check(int num){ 10.         return (num == this.m_num); 11.     } 12. }

1. public class Threads01 {

2. public static void main(String[] args) {

3.     Checker checkerObject = new Checker(0);        

4. Thread t[] = new Thread[9];

5. for (int i = 0; i < t.length ; i++)

6. t[i] = new Thread(new SleepThread(i/3+1, "NO. " +(i+1)+

7. " done, was waiting for "+(i/3+1), checkerObject));

8. for (int i = 0; i < t.length; i++)

9. t[i].start();

10.     try {  Thread.sleep(1000);

11. checkerObject.change(1); 

12. Thread.sleep(1000);

13. checkerObject.change(3); 

14. Thread.sleep(1000);

15. checkerObject.change(2); 

16.    } catch (InterruptedException e) {}

17.   }

18. }

1. class SleepThread implements Runnable { 2.     private int m_num; 3.     private String m_strToPrintWhenDone; 4.     private Checker m_checkerObject; 5.     SleepThread(int num, String strToPrintWhenDone, 

Checker checkerObject) { 6.         this.m_num = num; 7.         this.m_strToPrintWhenDone = strToPrintWhenDone; 8.         this.m_checkerObject = checkerObject; 9.     } 10.     public void run()  { 11. /* busy waiting */12.         while (!this.m_checkerObject.check(this.m_num))13. ; 14.         System.out.println(this.m_strToPrintWhenDone); 15.     } 16. }

Solution #1: Busy Waiting

1. class SleepThread implements Runnable { 2.     private int m_num; 3.     private String m_strToPrintWhenDone; 4.     private Checker m_checkerObject; 5.     SleepThread(int num, String strToPrintWhenDone, 

Checker checkerObject) { 6.         this.m_num = num; 7.         this.m_strToPrintWhenDone = strToPrintWhenDone; 8.         this.m_checkerObject = checkerObject; 9.     } 10.     public void run()  { 11.         while (!this.m_checkerObject.check(this.m_num)) 12.         { 13.             try { 14.                 Thread.sleep(100); 15.             } catch (InterruptedException e) {} 16.         } 17.         System.out.println(this.m_strToPrintWhenDone); 18.     }19. }

Solution #2: Sleep and Check

1. class SleepThread implements Runnable { 2. ………3.   public void run() 4.     { 5.         this.m_checkerObject.returnWhenCheckIsTrue(m_num);

  6.         System.out.println(m_strToPrintWhenDone); 7.     } } 8. class Checker { 9.     private int m_num; 10.     public Checker(int num) {       this.m_num = num;

    }   11.     public synchronized void change(int num){ 12.         this.m_num = num; 13.         this.notifyAll(); 14.     } 15.     public synchronized void returnWhenCheckIsTrue(int num

) { 16.         while (num != m_num) { 17.             try { 18.                  this.wait(); 19.             } catch (InterruptedException e) {} 20.         } 21.   } }

Solution #3: Wait and Notify

Tips

• Use a while loop (and not an if condition) to check the precondition

• notify() Vs. notifyAll() - which of them should you use?

• When calling wait(), notifyAll() or notify() on an object, make sure the calling thread holds the object's lock. Notice that if you do not hold the object's lock you will receive an illegal monitor runtime exception.

• After performing a wait call on the object, the thread releases the object's lock. Furthermore, before exiting the wait set of the object, the thread must re-lock the object

• A thread that releases an object's, lock will NOT release other locks it has

Advanced Synchronization Tools

CountDownLatch

Constructor:

• CountDownLatch(int value)

Methods:

• void countDown()

Decrements the latch value by 1.

• void await()

Causes the current thread to wait until the latch has counted down to zero.

1. import java.util.concurrent.*;  

2. public class Threads {

3.     public static void main(String[] args)

4.     {

5.         CountDownLatch latchObject = new CountDownLatch (3);

6.  

7.         Server s =  new Server (latchObject);

8.         Client c1 = new Client (1, latchObject);

9.         Client c2 = new Client (2, latchObject);

10.         Client c3 = new Client (3, latchObject);

11.      Thread t1=new Thread(s);

12.      Thread t2=new Thread(c1);

13.      Thread t3=new Thread(c2);

14.      Thread t4=new Thread(c3);

15.         t1.start();

16.         t2.start();

17.         t3.start();  

18.         t4.start();

19.     }

20. }

1. class Server implements Runnable {

2.   private CountDownLatch m_latchObject;   

3.   public Server(CountDownLatch latchObject)

4.   {

5.       m_latchObject = latchObject;

6.   }   

7.   public void run ()

8.   {

9.       synchronized (System.out)

10.       {

11.           System.out.println("Server initialized");

12.       }  

13.       try {     

14.  m_latchObject.await();

15.       } catch (InterruptedException e)

16.       {

17.           return;

18.       }

19.       System.out.println("Server finished");

20.   }

21. }

1. class Client implements Runnable {

2.     private int m_id;    private CountDownLatch m_latchObject;

3.     public Client(int id, CountDownLatch latchObject){

4.         m_id = id;

5.         m_latchObject = latchObject;

6.     }     

7.     public void run () {

8.         synchronized (System.out) {

9.             System.out.println("Client #" + m_id + " started");

10.         }

11.         try { Thread.sleep(200); } catch (InterruptedException e) {}

12.         synchronized (System.out){

13.             System.out.println("Client #" + m_id + " doing something");

14.         }      

15.         try {   Thread.sleep(200); } catch (InterruptedException e) {}

16.         synchronized (System.out){

17.             System.out.println("Client #" + m_id + " finished");

18.         }

19.         m_latchObject.countDown();                   

20.     }

21. }