Monitors

  • Invented by Tony Hoare ...

Monitors

  • Invented by Tony Hoare in 1974
  • Like a C++ class

    • Consists of vars and procedures
      1. Only one thread in a monitor at a time (automatic mutual exclusion)
      2. Specifal type of variable, called condition variable
        • wait
        • signal
        • broadcast
      3. No public variables allowed (must call procedures to access variables)
  • A high-level abstraction that provides a convenient and effective mechanism for process synchronization

  • Only one process may be active within the monitor at a time

  • Condition Variables

    1. Automatic unlock and lock for mutual exclusion
    2. cond.wait () - Thread is put on queue for “cond”, goes to sleep.
    3. cond.signal () - If queue for “cond” not empty, wake up on thread
    4. cond.broadcast() - Wake up all threads waiting on queue for “cond”
  • Semantics of Signal

    • Signal and Wait (Hoare-style)
      • Signaler passes lock, CPU to waiter; waiter runs immediately
      • Waiter gives lock, CPU back to signaler when
        1. It exits critical section
        2. Or, it waits again
    • Signal and Continue (Mesa-style)
      • invented by Xerox company
      • signaler continues executing
      • waiter put on ready queue
      • when waiter actually gets to run
        1. May have to wait for lock again
        2. State may have changed! Use “while”, not “if”
      • Used in Java, Pthread
    • Monitor types
  • Bounded Buffer by Monitor
    In Bounded Buffer,

Enqueue (int v)  
{  
        while( size == MAX_SIZE) full.wait();  

        BUFFER[tail] = v;  
        tail = (tail+1) % MAX_SIZE;  
        size++;  

        if (size ==1) empty.signal();  
}  
Deque (int v)  
{  
        while(size==0) empty.wait();  

        int i = head;  
        head = (head+1) % MAX_SIZE;  
        size--;  

        if ( size == MAX_SIZE-1) full.signal();  
        return BUFFER[i];  
}  
  • Hoare Style
    • Monitor Implementation (using semaphores)
      • Need mutual exclusion semaphore mutex (init to 1) so that only one process is active within monitor
      • Need a semaphore next (next to exit) for the signaling process to suspend itself
        • initialized to zero
      • next_count is number of processes blocked on next
      • Before exiting a procedure, process must either:
        1. Signal other waiting processes in monitor next before exiting
        2. Signal mutex and exit
    • Monitor Implementation
    Procedure F:  
                wait(mutex);  
                ...  
                body of F  
                ...  
                if (next_count > 0) signal(next);  
                else signal(mutex);  
    end;  
+ Condition Variable Implementation

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