COMP2310 - Week 6 - Centralised Synchronisation

Monitors

• Form of encapsulation -> Centralise all operations on a shared data structure in one place

  • Encapsulate data along with operations which operate on it.

• Prohibits access other than by the monitor procedures and functions

• Assures mutual exclusion of all monitor procedures and functions

monitor buffer;
    export append, take;

    var (* declare protected vars *)

    procedure append (I : integer);
        ...
    
    procedure take (var I : integer);
        ...
    
begin
    (* initialisation)
end;

Condition Synchronisation

• Hoare Monitors
  • Condition variables implemented by semaphores (Wait and Signal)
  • Queues for tasks suspended on condition variables are realised
  • Suspended task releases its lock on the monitor, enabling another task to enter

    • If they don't, no other task could access, possible deadlock

    • More efficient evaluation of guards -> Task leaving monitor can evaluate all guards, allowing right tasks to be activated

    • Blocked tasks may be ordered and livelocks prevented

Monitors with Condition Synchronisation

• Signal is allowed only as the last action of a process before it leaves the monitor

• Signal operations have the side-effect of executing a return statement

  • I.e. nothing can go after the signal operation

• Signal operation of unblocking another process has the side-effect of blocking the current process

  • Executes again once monitor is unlocked again

• Signal operation which unblocks a process does not block the caller, however unblocked process must re-gain access to the monitor

Monitors in POSIX

• Attributes include
  • Semantics for trying to lock a mutex
  • Sharing of mutexes and condition variables
  • Priority ceiling
  • Clock used for timeouts

Object-Orientation and Synchronisation

• SInce mutual exclusion and condition synchronisation schemes must consider all involved methods and guards, new behaviour cannot be added without re-evalutating the class

• Re-use through inheritance does not translate to synchronised classes

• Parent class might need to be adapted in order to suit the global synchronisation scheme

Nested Monitor Calls

• Monitors are not composible easily

• Assuming a thread in a monitor is calling an operation in another monitor and is suspended at a conditional variable there

  • Called monitor is aware of the suspension and allows other threads to enter
  • Monitor is possible not aware of the suspension and keeps its lock
  • Unjustified locked calling reduces system performance and leads to potential deadlocks

Criticism of Monitors

• Mutual exclusion solved elegantly and safely
• All criticism about semaphorse applies to monitors
• Mixture of low and high-level synchronisation constructs