Dining Philosophers Problem

by

The “dining philosophers problem” is a classical problem in computer science that demonstrates challenges related to resource allocation and synchronization in a concurrent computing environment. It is often used to illustrate the difficulties encountered in designing and implementing concurrent systems.

The problem is framed around a scenario where five philosophers are seated around a dining table, and there is a bowl of rice and a single chopstick between each pair of adjacent philosophers. Each philosopher has two actions they can perform: “think” or “eat.” To eat, a philosopher needs both the chopstick to their left and the chopstick to their right.

The challenge arises when multiple philosophers simultaneously attempt to eat, potentially leading to a deadlock or resource starvation. Deadlock occurs when each philosopher picks up the chopstick on their left, waiting indefinitely for the chopstick on their right to become available. Resource starvation occurs when a philosopher is unable to eat indefinitely due to the actions of others, even if a deadlock doesn’t occur.

Various solutions have been proposed to address the dining philosophers problem. One commonly used solution is called the “resource hierarchy” approach. In this approach, each philosopher is assigned a unique number from 1 to 5, and they are only allowed to pick up the chopstick with the lower-numbered philosopher first. This prevents circular wait conditions and guarantees progress, as at least one philosopher can always eat.

Another solution is the “waiter” approach, where a waiter is introduced to control the allocation of chopsticks. The waiter acts as a mediator and enforces a policy that allows a maximum of four philosophers to be seated at the table simultaneously. Philosophers must request permission from the waiter before attempting to eat. This approach ensures that deadlock and resource starvation are avoided, but it introduces a central point of control and potential bottleneck.

Additionally, techniques such as semaphores, monitors, and other synchronization primitives can be employed to address the dining philosophers problem. These mechanisms help manage access to shared resources (i.e., chopsticks) and ensure that philosophers can eat without deadlocks or resource starvation.

The dining philosophers problem serves as a fundamental example in concurrent programming and highlights the complexities involved in resource allocation and synchronization. It demonstrates the importance of careful design and synchronization mechanisms to maintain the integrity and efficiency of concurrent systems.

Algorithm for solving the dining philosophers problem using the resource hierarchy approach:

  1. Initialize an array of chopsticks, where each chopstick is initially set to “available.”
  2. Define a Philosopher class with the following methods:
    • think(): Represents the philosopher thinking.
    • pickUpChopsticks(): Tries to acquire the two adjacent chopsticks required for eating.
    • eat(): Represents the philosopher eating.
    • putDownChopsticks(): Releases the chopsticks after eating.
  3. Instantiate an array of Philosopher objects, each with a unique identifier from 1 to 5.
  4. Implement the pickUpChopsticks() method in the Philosopher class:
    • Acquire the chopstick with the lower identifier first.
    • If the lower chopstick is not available, wait until it becomes available.
    • Acquire the higher chopstick.
    • If the higher chopstick is not available, release the lower chopstick and wait until both chopsticks are available.
  5. Implement the putDownChopsticks() method in the Philosopher class:
    • Release both chopsticks, setting them back to “available.”
  6. Implement the main algorithm:
    • Create a loop that cycles through the philosophers indefinitely.
    • Each philosopher, in order of their identifier, performs the following steps:
      1. Think for a random amount of time.
      2. Call pickUpChopsticks() to acquire the chopsticks.
      3. If successful, eat for a random amount of time.
      4. Call putDownChopsticks() to release the chopsticks.

By following this algorithm, the philosophers will take turns thinking and eating, ensuring that at least one philosopher can always eat without causing a deadlock.

It’s important to note that this algorithm assumes an infinite loop for the philosophers to continue their actions indefinitely. The exact implementation and synchronization mechanisms used may vary depending on the programming language and concurrency framework you choose to employ.

Another solution algorithm for the dining philosophers problem using the waiter approach:

  1. Initialize a semaphore called waiter with an initial value of 4. This semaphore represents the waiter who controls access to the table.
  2. Initialize an array of semaphores called chopsticks, where each semaphore is initially set to 1. This represents the availability of each chopstick.
  3. Define a Philosopher class with the following methods:
    • think(): Represents the philosopher thinking.
    • pickUpChopsticks(): Tries to acquire both chopsticks required for eating.
    • eat(): Represents the philosopher eating.
    • putDownChopsticks(): Releases both chopsticks.
  4. Implement the pickUpChopsticks() method in the Philosopher class:
    • Acquire the waiter semaphore.
    • Acquire the chopstick on the left by decrementing its semaphore value.
    • Acquire the chopstick on the right by decrementing its semaphore value.
  5. Implement the putDownChopsticks() method in the Philosopher class:
    • Release the chopstick on the left by incrementing its semaphore value.
    • Release the chopstick on the right by incrementing its semaphore value.
    • Release the waiter semaphore.
  6. Implement the main algorithm:
    • Create a loop that cycles through the philosophers indefinitely.
    • Each philosopher, in order of their identifier, performs the following steps:
      1. Think for a random amount of time.
      2. Call pickUpChopsticks() to acquire the chopsticks.
      3. If successful, eat for a random amount of time.
      4. Call putDownChopsticks() to release the chopsticks.

By using the waiter semaphore, the algorithm ensures that a maximum of four philosophers can sit at the table simultaneously, preventing deadlock and resource starvation.

Again, please note that the exact implementation details may vary depending on the programming language and synchronization mechanisms you choose to utilize.

Leave a Comment