Monitors

Notes

• Monitors are the next level of abstraction
• A monitor is a design pattern, not an os construct but rather a design pattern.
• RAII - Resource Allocation Is Initialization
  • This is one of Stroustrup's designs in C++
  • And possibly elsewhere
  • This can actually apply as low as mutex locks
    • The lock is created in the constructor
    • The lock is destroyed/released in the destructor.
  • But the pattern can be applied higher
• With a mutex we can get into a problem if we don't call the signal/wait pair in the proper order, only once.
  • Two signals in a row, without a wait will cause a deadlock with yourself
  • Two waits without a signal can cause a lock to be unlocked when it should not.
  • Or only calling one of the two will cause a problem as well.
• These could occur because the calls using mutex/semiphores could be spread across a program (very large program)
• Encapsulation in a data structure will handle this.
• For example, for the readers and writers problem we have been playing with
• The monitor could
  • Declare the three synchronization variables as class members
  • Implement an init function that initializes them to the correct value
  • Implement the following methods
    • BeginRead
    • EndRead
    • BeginWrite
    • EndWrite
  • Implement a destructor that cleans things up.
• In each of the read/write methods, the logic for the mutex/semaphore is contained.
• We still have the problem if creating a monitor, but that could be
  • Global
  • Or a global using a singleton pattern.