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.. _dining-philosophers-sample:

Dining Philosophers
###################

Overview
********

An implementation of a solution to the Dining Philosophers problem (a classic
multi-thread synchronization problem).  This particular implementation
demonstrates the usage of multiple preemptible and cooperative threads of
differing priorities, as well as dynamic mutexes and thread sleeping.

The philosopher always tries to get the lowest fork first (f1 then f2).  When
done, he will give back the forks in the reverse order (f2 then f1).  If he
gets two forks, he is EATING.  Otherwise, he is THINKING. Transitional states
are shown as well, such as STARVING when the philosopher is hungry but the
forks are not available, and HOLDING ONE FORK when a philosopher is waiting
for the second fork to be available.

Each Philosopher will randomly alternate between the EATING and THINKING state.

It is possible to run the demo in coop-only or preempt-only mode. To achieve
this, set these values for CONFIG_NUM_COOP_PRIORITIES and
CONFIG_NUM_PREEMPT_PRIORITIES in prj.conf:

preempt-only:

  CONFIG_NUM_PREEMPT_PRIORITIES 6
  CONFIG_NUM_COOP_PRIORITIES 0

coop-only:

  CONFIG_NUM_PREEMPT_PRIORITIES 0
  CONFIG_NUM_COOP_PRIORITIES 6

In these cases, the philosopher threads will run with priorities 0 to 5
(preempt-only) and -7 to -2 (coop-only).

Building and Running
********************

This project outputs to the console.  It can be built and executed
on QEMU as follows:

.. zephyr-app-commands::
   :zephyr-app: samples/philosophers
   :host-os: unix
   :board: qemu_x86
   :goals: run
   :compact:

Sample Output
=============

.. code-block:: console

   Philosopher 0 [P: 3]  HOLDING ONE FORK
   Philosopher 1 [P: 2]  HOLDING ONE FORK
   Philosopher 2 [P: 1]  EATING  [ 1900 ms ]
   Philosopher 3 [P: 0]  THINKING [ 2500 ms ]
   Philosopher 4 [C:-1]  THINKING [ 2200 ms ]
   Philosopher 5 [C:-2]  THINKING [ 1700 ms ]