monitors - TarisMajor/5143-OpSystems GitHub Wiki

Monitors are high-level synchronization constructs used to manage access to shared resources in concurrent programming. They provide a mechanism for ensuring mutual exclusion and synchronization by encapsulating shared resources and the operations that manipulate them, along with the synchronization code. Monitors are designed to simplify the process of writing correct and efficient concurrent programs by handling the complexity of synchronization internally.

1. Encapsulation: Monitors encapsulate shared resources and the operations that manipulate them, ensuring that only one thread can access the critical section at a time.
2. Mutual Exclusion: Monitors ensure that only one thread can be active within the monitor at any given time, preventing race conditions.
3. Condition Variables: Monitors provide condition variables for threads to wait for certain conditions to be met before proceeding, facilitating synchronization.

1. Shared Resources: The data structures or resources that multiple threads need to access and manipulate.
2. Operations: The procedures or methods that operate on the shared resources.
3. Condition Variables: Variables used for thread synchronization, allowing threads to wait and be signaled based on specific conditions.

1. Simplified Synchronization: Monitors abstract the complexity of synchronization, making it easier for developers to write correct concurrent programs.
2. Safety: By ensuring mutual exclusion and providing built-in synchronization mechanisms, monitors help prevent race conditions and data corruption.
3. Modularity: Monitors promote modular design by encapsulating shared resources and their associated operations, improving code organization and maintainability.

1. Limited Flexibility: Monitors may impose restrictions on the order and manner in which threads access shared resources, reducing flexibility in some cases.
2. Performance Overhead: The built-in synchronization mechanisms of monitors can introduce performance overhead, especially in systems with high concurrency levels.
3. Deadlock Risk: Improper use of condition variables and synchronization within monitors can lead to deadlocks if not carefully managed.

1. Operating Systems: Monitors are used in operating systems to manage access to critical system resources and ensure synchronization between kernel threads.
2. Multithreaded Applications: Monitors are widely used in multithreaded applications to coordinate access to shared data structures and prevent race conditions.
3. Real-Time Systems: In real-time systems, monitors help manage concurrent tasks and ensure timely access to shared resources.

Consider a monitor for a bounded buffer, where producers add items to the buffer and consumers remove items from the buffer. The monitor ensures mutual exclusion and synchronization between producers and consumers.

Pseudocode:

class BoundedBuffer {
    private final int[] buffer;
    private int count, in, out;
    private final Condition notEmpty, notFull;
    
    public BoundedBuffer(int size) {
        buffer = new int[size];
        count = in = out = 0;
        notEmpty = new Condition();
        notFull = new Condition();
    }
    
    public synchronized void insert(int item) {
        while (count == buffer.length) {
            notFull.await(); // Wait if buffer is full
        }
        buffer[in] = item;
        in = (in + 1) % buffer.length;
        count++;
        notEmpty.signal(); // Signal that buffer is not empty
    }
    
    public synchronized int remove() {
        while (count == 0) {
            notEmpty.await(); // Wait if buffer is empty
        }
        int item = buffer[out];
        out = (out + 1) % buffer.length;
        count--;
        notFull.signal(); // Signal that buffer is not full
        return item;
    }
}

In this example, the BoundedBuffer monitor ensures that producers and consumers can safely add and remove items from the buffer, using condition variables to synchronize their access based on buffer states.

• Operating System Concepts by Abraham Silberschatz, Peter B. Galvin, and Greg Gagne
• Monitors - GeeksforGeeks
• Modern Operating Systems by Andrew S. Tanenbaum
• Wikipedia - Monitor (Synchronization)
• ChatGPT