CS 3733 Operating Systems

• Monitors:
  
  • 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: monitor_wait and monitor_signal. Since the monitor assures mutual exclusion, they are used without a mutex lock.
  • Monitor operations:
    • monitor_wait(V) blocks a process.
    • monitor_signal(V) unblocks a process waiting for the condition variable V.
    • monitor_signal causes a new process to enter the monitor, replacing the original.
    • Alternatively, it could be required that monitor_signal be the last statement in a monitor routine.
    • If a condition variable is signaled with nobody waiting, the signal is lost.
  • Figure 2-14, page 48 of Tanenbaum shows a solution to the producer-consumer using monitors.
    • count is only modified inside the monitor so it is protected.
    • enter_item and remove_item can be ordinary procedures as they are called only inside the monitor.
    • condition variables do not have values --- better to think of them as part of an object.

  • 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.

• Message passing systems
  • Can be used across networks.
  • Messages can be lost.
  • Usually has some sort of handshaking such as an acknowledgement.
  • Must have a way of identifying sender and receiver.
  • Must have a way of authenticating a message.
  • Messages that are not yet read must be buffered by the operating system or must go to a mailbox.
  • If no buffering get the rendezvous mechanism.
  • A pipe is a buffering mechanism that doesn't preserve message boundaries.