Introduction to Dining Philosophers Problem
The Dining Philosophers Problem is a classic synchronization problem in computer science and concurrent programming. It demonstrates the challenges of allocating limited resources among multiple processes in a deadlock-free and starvation-free manner.
Problem Setup
- Imagine five philosophers sitting around a circular dining table.
- Each philosopher alternates between thinking and eating.
- In front of each philosopher is a plate of food, and between each pair of philosophers is a single fork (or chopstick).
- To eat, a philosopher needs two forks: the one on their left and the one on their right.
Goals
- No Deadlock: Philosophers shouldn’t all wait indefinitely for resources.
- No Starvation: Every philosopher should eventually get a chance to eat.
Challenges
- Deadlock: If all philosophers pick up their left fork at the same time, they’ll wait indefinitely for the right fork.
- Starvation: A philosopher may be perpetually unable to eat if others keep eating continuously.
Solutions
Several strategies can solve or mitigate the problem:
1. Resource Ordering
Assign an ordering to the forks (e.g., numbered 1 to 5). Philosophers pick up the lower-numbered fork first, then the higher-numbered fork. This avoids circular wait and prevents deadlock.
2. Allow One Less Philosopher
Allow at most four philosophers to sit and attempt to eat simultaneously. This ensures at least one fork is always free, breaking the deadlock condition.
3. Asymmetric Pickup
Make an odd-numbered philosopher pick up the left fork first and an even-numbered philosopher pick up the right fork first. This prevents all philosophers from picking up the same fork simultaneously.
4. Using a Semaphore
Introduce a semaphore to limit the number of philosophers who can attempt to eat simultaneously. For example, set the semaphore count to 4, allowing only four philosophers to try eating at the same time.
5. Chandy/Misra Solution
This approach assigns priorities to the forks and uses a token-based system. Philosophers communicate with each other to request and release forks, ensuring synchronization and fairness.
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Key Takeaways
- The Dining Philosophers Problem is a teaching tool for understanding synchronization, deadlock, and concurrency control.
- Solutions often involve avoiding circular dependencies, limiting resource usage, or coordinating access via communication.