Dining Philosophers Problem (DPP)

The Dining Philosophers Problem (DPP) is a classic synchronization problem in computer science that illustrates the challenges of deadlock and resource sharing in concurrent systems. The problem states that there are 5 philosophers sharing a circular table where they eat and think alternatively. There is a bowl of rice for each philosopher and 5 chopsticks placed between them. A philosopher needs both their right and left chopstick to eat. A hungry philosopher may only eat if both chopsticks are available, otherwise they put down any chopstick they hold and begin thinking again.

This problem demonstrates a large class of concurrency control issues including deadlock, starvation, and mutual exclusion in operating systems.

Solution Using Semaphores

A solution to the Dining Philosophers Problem uses a semaphore to represent each chopstick. A chopstick can be picked up by executing a wait() operation on the semaphore and released by executing a signal() operation.

The structure of the chopstick semaphores is shown below −

semaphore chopstick[5];

Initially, all elements of the chopstick array are initialized to 1, as the chopsticks are on the table and not picked up by any philosopher.

The structure of a random philosopher i is given as follows −

do {
    wait(chopstick[i]);
    wait(chopstick[(i+1) % 5]);
    
    // EATING THE RICE
    
    signal(chopstick[i]);
    signal(chopstick[(i+1) % 5]);
    
    // THINKING
    
} while(1);

In the above structure, the wait() operation is performed on chopstick[i] and chopstick[(i+1) % 5], meaning philosopher i picks up the chopsticks on both sides. After eating, the signal() operation releases both chopsticks, and the philosopher returns to thinking.

Deadlock Problem

The above solution ensures that no two neighboring philosophers can eat simultaneously. However, this solution can lead to a deadlock scenario. This occurs if all philosophers simultaneously pick up their left chopstick − then none can pick up their right chopstick, and the system becomes deadlocked.

Deadlock Prevention Solutions

Several approaches can prevent deadlock in the dining philosophers problem −

Asymmetric Solution Implementation

do {
    if (i % 2 == 0) {  // Even philosopher
        wait(chopstick[(i+1) % 5]);  // Right first
        wait(chopstick[i]);          // Left second
    } else {           // Odd philosopher
        wait(chopstick[i]);          // Left first
        wait(chopstick[(i+1) % 5]);  // Right second
    }
    
    // EATING
    
    signal(chopstick[i]);
    signal(chopstick[(i+1) % 5]);
    
    // THINKING
    
} while(1);

Conclusion

The Dining Philosophers Problem effectively demonstrates fundamental concurrency issues in operating systems, particularly deadlock scenarios in resource allocation. Various solutions like asymmetric ordering and resource limiting can prevent deadlock, making it a valuable model for understanding synchronization challenges.