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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Dining philosophers problem | 1/2 | https://en.wikipedia.org/wiki/Dining_philosophers_problem | reference | science, encyclopedia | 2026-05-05T11:01:47.089416+00:00 | kb-cron |
In computer science, the dining philosophers problem is an example problem often used in concurrent algorithm design to illustrate synchronization issues and techniques for resolving them. It was originally formulated in 1965 by Edsger Dijkstra as a student exam exercise, presented in terms of computers competing for access to tape drive peripherals. Soon after, Tony Hoare gave the problem its present form.
== Problem statement == Five philosophers dine together at the same table. Each philosopher has their own plate at the table. There is a fork between each pair of adjacent plates. The dish served is a kind of spaghetti which has to be eaten with two forks. Each philosopher can only alternately think and eat. Moreover, a philosopher can only eat their spaghetti when they have both a left and a right fork. Thus, two forks will only be available when their two nearest neighbors are thinking, not eating. After an individual philosopher finishes eating, they will put down both forks. The problem is how to design a regimen (a concurrent algorithm) such that any philosopher will not starve; i.e., each can forever continue to alternate between eating and thinking, assuming that no philosopher can know when others may want to eat or think (an issue of incomplete information).
=== Problems === The problem was designed to illustrate the challenges of avoiding deadlock, a system state in which no progress is possible. To see that a proper solution to this problem is not obvious, consider a proposal in which each philosopher is instructed to behave as follows:
think unless the left fork is available; when it is, pick it up; think unless the right fork is available; when it is, pick it up; when both forks are held, eat for a fixed amount of time; put the left fork down; put the right fork down; repeat from the beginning. With these instructions, the situation may arise where each philosopher holds the fork to their left; in that situation, they will all be stuck forever, waiting for the other fork to be available: it is a deadlock. Resource starvation, mutual exclusion, and livelock are other types of sequence and access problems.
== Solutions == These four conditions are necessary for a deadlock to occur:
mutual exclusion (no fork can be simultaneously used by multiple philosophers) resource holding (the philosophers hold a fork while waiting for the second) non-preemption (no philosopher can take a fork from another), and circular wait (each philosopher may be waiting on the philosopher to their left) A solution must negate at least one of those four conditions. In practice, negating mutual exclusion or non-preemption somehow can give a valid solution, but most theoretical treatments assume that those assumptions are non-negotiable, instead attacking resource holding or circular waiting (often both).
=== Dijkstra's solution === Dijkstra's solution negates resource holding; the philosophers atomically pick up both forks or wait, never holding exactly one fork outside of a critical section. To accomplish this, Dijkstra's solution uses one mutex, one semaphore per philosopher, and one state variable per philosopher. This solution is more complex than the resource hierarchy solution. This is a C++20 version of Dijkstra's solution with changes by Andrew S. Tanenbaum:
The function test() and its use in take_forks() and put_forks() make the Dijkstra solution deadlock-free.
=== Resource hierarchy solution === This solution negates circular waiting by assigning a partial order to the resources (the forks, in this case), and establishes the convention that all resources will be requested in order, and that no two resources unrelated by order will ever be used by a single unit of work at the same time. Here, the resources (forks) will be numbered 1 through 5, and each unit of work (philosopher) will always pick up the lower-numbered fork first, and then the higher-numbered fork, from among the two forks he plans to use. The order in which each philosopher puts down the forks does not matter. In this case, if four of the five philosophers simultaneously pick up their lower-numbered forks, only the highest-numbered fork will remain on the table, so the fifth philosopher will not be able to pick up any fork. Moreover, only one philosopher will have access to that highest-numbered fork, so he will be able to eat using two forks. This can intuitively be thought of as having one "left-handed" philosopher at the table, who – unlike all the other philosophers – takes his fork from the left first. While the resource hierarchy solution avoids deadlocks, it is not always practical, especially when the list of required resources is not completely known in advance. For example, if a unit of work holds resources 3 and 5 and then determines it needs resource 2, it must release 5, then 3, before acquiring 2, and then it must re-acquire 3 and 5 in that order. Computer programs that access large numbers of database records would not run efficiently if they were required to release all higher-numbered records before accessing a new record, making the method impractical for that purpose. The resource hierarchy solution is not fair. If philosopher 1 is slow to take a fork, and if philosopher 2 is quick to think and pick their forks back up, then philosopher 1 will never get to pick up both forks. A fair solution must guarantee that each philosopher will eventually eat, no matter how slowly that philosopher moves relative to the others. The following source code is a C++11 implementation of the resource hierarchy solution for five philosophers. The sleep_for() function simulates the time normally spent with business logic.
For GCC: compile with