The Dining Philosophers Problem

The Dining Philosophers Problem

The Dining Philosophers Problem

Consider five philosophers who spend their lives thinking and eating. The philosophers share a common circular table surrounded by five chairs, each belonging to one philosopher. In the center of the table is a bowl of rice, and the table is laid with five single chopsticks. When a philosopher thinks, she does not interact with her colleagues. From time to time, a philosopher gets hungry and tries to pick up the two chopsticks that are closest to her (the chopsticks that are between her and her left and right neighbors). A philosopher may pick up only one chopstick at a time. Obviously, she cannot pick up a chopstick that is already in the hand of a neighbor. When a hungry philosopher has both her chopsticks at the same time, she eats without releasing her chopsticks. When she is finished eating, she puts down both of her chopsticks and starts thinking again.

The structure of philosopher i

do {
wait (chopstick[i]) ;
wait (chopstick[(i+1) % 5] ) ;
...
eat
. . .
signal (chopstick [i] ;
signal(chopstick[(i+1) % 5] ) ;
. . .
think
...
) while (1);

The dining-philosophers problem is considered a classic synchronization problem, neither because of its practical importance nor because computer scientists dislike philosophers, but because it is an example of a large class of concurrency-control problems. It is a simple representation of the need to allocate several resources among several processes in a deadlock- and starvation free manner.

One simple solution is to represent each chopstick by a semaphore. A philosopher tries to grab the chopstick by executing a wait operation on that semaphore; she releases her chopsticks by executing the signal operation on the appropriate semaphores. Thus, the shared data are

semaphore chopstick [5] ; where all the elements of chopstick are initialized to 1.

Although this solution guarantees that no two neighbors are eating simultaneously,

it nevertheless must be rejected because it has the possibility of creating a deadlock. Suppose that all five philosophers become hungry simultaneously, and each grabs her left chopstick. All the elements of chopstick will now be equal to 0. When each philosopher tries to grab her right chopstick, she will be delayed forever.

Use an asymmetric solution; that is, an odd philosopher picks up first her left chopstick and then her right chopstick, whereas an even philosopher picks up her right chopstick and then her left chopstick.Finally, any satisfactory solution to the dining-philosophers problem must guard against the possibility that one of the philosophers will starve to death.

 

Also Read :- What is semaphore

Also Read:- Semaphore Implementation

Also Read:- Binary Semaphore

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