Monitors

• Semaphores are useful and powerful
• But, they require programmer to think of every timing issue; easy to miss something, difficult to debug
• Let the compiler handle the details
• Monitors are a high level language construct for dealing with synchronization
  • similar to classes in Java
  • a monitor has fields and methods
• A monitor is a software module implementing mutual exclusion
• Monitors are easier to program than semaphores
  • programmer only has to say what to protect
  • compiler actually does the protection (compiler will use semaphores to do protection)
• Natively supported by a number of programming languages: Java
  • Resources or critical sections can be protected using the keyword: synchronized keyword
  • synchronized can be applied to a method: entire method is a critical section
• Chief characteristics (see Fig. 2):
  • Local data variables are accessible only by the monitor (not externally)
  • Process enters monitor by invoking one of its procedures, but cannot directly access the monitor's internal datastructures
  • Only one process may be executing in the monitor at a time (mutual exclusion)
  • Only methods inside monitor can access fields
  • At most one thread can be active inside monitor at any one time
• Main problem: provides less control
• Allow process to wait within the monitor with condition variable, condition x,y;
• can only be used with operations wait and signal (notify() in Java);
  • operation wait(x); means that the process invoking this operation is suspended until another process invokes signal(x);
  • operation signal(x); resumes exactly one process suspended
• condition variables are not counters, they do not accumulate signals for later use the way the semaphores do. Thus if a condition variable is signaled with no waiting on it, the signal is lost
• This solution is deadlock free
• In Fig. 3, the solution for the producer-consumer problem with a monitor is given
• The class our_monitor contains the buffer, the administration variables and two synchronized methods
• when the producer is active in insert, it knows for sure that the consumer can not be active inside remove
• making it safe to update the variables and buffer without fear of race conditions

Figure 2: A monitor

Figure 3: The producer-consumer problem with a monitor.