Week 05 Notes for CS 3750 -- Fall 2001

• Take Roll.
  
  
• Announcements
  
  
  • We will have quiz #1 on Wednesday, October 10. The quiz will cover chapters 1-5. You will find some review materials here:
    
    Fall 2001 Review Materials
    
    Note: these review materials are up to date -- geared toward the latest edition of the text.
  
  
  
• Check what's new in schedule
  
  
• Chapter Seven Notes -- Process Synchronization
  
  
  • 7.1 Background -- The unsuccessful attempt at a new solution to the bounded buffer problem illustrates that concurrent processes accessing shared memory may need to synchronize and provide mutually exclusive access in order to preserve data integrity.
    
    
  • 7.2 The Critical Section Problem
    
    
    • Processes P1 ... Pn each have a "critical" section of code C1 ... Cn. We want to program all the processes so at most one of the processes is executing in its critical section at any time (mutual exclusion). Typically the reason is that the critical section of code contains accesses to shared memory.
      
      
    • We want to solve the problem by giving each process a module of "entry code" to be executed just prior to entering its critical section and "exit code" to be executed just after exiting its critical section. The goal is that the processes will perform all the communication necessary to maintain mutually exclusive access by executing the entry and exit code.
      
      
    • In order that our solution to the critical section problem be truly satisfactory, we require that the following goals be achieved:
      
      
      1. Mutual Exclusion: If process Pi is executing in its critical section, then none of the other processes can be executing in their critical section.
         
         
      2. Progress: If no process is executing in its critical section and some processes wish to enter their critical sections, then only those processes that are not executing in their remainder section can participate in the decision on which will enter its critical section next, and this selection cannot be postponed indefinitely.
         
         
      3. Bounded Waiting: There exists a bound on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted.
         
         
    • To understand the criteria for a solution to a critical section problem, you must understand completely the term "indefinite postponement" -- an event is postponed indefinitely if there is no bound on how much time can go by before the event occurs.
      
      
    • Algorithm 1 (mutual exclusion, but not progress)
      
      
    • Algorithm 2 (mutual exclusion, but not progress -- transpose two lines and you lose mutual exclusion
      
      
    • Algorithm 3 (works! -- Petersen 1981)
      
      
    • Multiple Process Solutions
      
      
      • The Bakery Algorithm (Lamport 1974)
        
        
      • The Eisenberg-McGuire Algorithm (1972)
        
        
  • 7.3 Synchronization Hardware
    
    
  • 
    
    
•