Threads II: Synchronization

Tom Kelliher, CS 245

Oct. 13, 2008

Administrivia

Announcements

Assignment

Wednesday: Deliverables, Exam I.

Friday: Read Chapter 6.

From Last Time

Introduction to threads.

Outline

1. Thread Synchronization

2. Lab

Coming Up

Exam I, Game2D case study.

Thread Synchronization

1. The Producer/Consumer problem:
   
   1. Producer and Consumer work at different rates.

   2. Fixed amount of buffering (queuing) between them.

   3. With no synchronization, items can ``disappear'' or be multiply consumed.

2. A solution: the monitor.
   1. Lock and condition variables.

   2. A thread must secure the lock before executing within the monitor.

   3. Once in the monitor, a thread checks its condition variables:
      1. Producer: queue full.

      2. Consumer: queue empty.

      If the condition is false, the thread waits -- exits the monitor and sleeps. When it awakes, it must recheck its condition.

   4. Once a thread has modified state within the monitor, it will notify waiting threads, allowing them to recheck their conditions. (Must first re-obtain lock.)

3. Example:

   class Queue
   {
      private int val;
      private boolean full = false;
      private boolean empty = true;
   
      public Queue()
      {
         val = 0;
      }
   
      public synchronized void put(int v)
      {
         while (full)
            try
            {
               wait();
            }
            catch (Exception e)
            {
            }
   
         val = v;
         full = true;
         empty = false;
         notify();
      }
   
      public synchronized int get()
      {
         while (empty)
            try
            {
               wait();
            }
            catch (Exception e)
            {
            }
   
         full = false;
         empty = true;
         notify();
         return val;
      }
   }
   

Lab

Tom Kelliher