CS 3733 Operating Systems

Read Section SG 6.7

• Basic properties:
  Monitors were developed in the 1970's to make it easier to avoid deadlocks. They fit well in an object-oriented paradigm.
  • A collection of precedures, variables, and data structures grouped together.
  • Processes can call the monitor procedures but cannot access the internal data structures.
  • Only one process at a time may be be active in a monitor.
  • A monitor is a language construct.
  • Compiler usually enforces mutual exclusion.
  • Monitor condition variables allow for blocking and unblocking. The operations are: x._wait and x.signal where x is a monitor condition variable. Since the monitor assures mutual exclusion, they are used without a mutex lock.
• Monitor operations:
  • x condition;
  • x.wait blocks a process.
  • x.signal unblocks exactly one process waiting for the condition variable x. If none are blocked, signal has no effect.
  • Suppose x.signal is called by P and Q is waiting. Only one process can be in the monitor. What happens:
    • Q goes: P either waits until Q leaves or waits for another condition.
    • P goes: Q either waits until P leaves or waits for another condition.
• Producer-consumer example:

      monitor ProducerConsumer {
          condition full, empty;
          int count;
  
          void enter(void) {
             if (count == N) full.wait();
             enter_item();
             count++;
             if (count == 1) empty.signal();
          }
  
          void remove(void) {
             if (count == 0) empty.wait();
             remove_item();
             count--;
             if (count == N-1) full.signal();
          }
       }
  

  How the producer and consumer programs use the monitor?

• Monitor version of producer-driven producer consumer problem (See PUP Program 10.6.)

• Practical implementations One modern language that supports monitors is Java.
  • When declaring a method in Java, the keyword synchronized is used to indicate that the method is part of a monitor.
  • There is one monitor per class.
  • Java also has condition variables.

Skill: Understand how to use monitors.