'''
The dining philosophers_ problem is an example problem often used in
concurrent algorithm design to illustrate synchronisation issues and
techniques for resolving them.
The problem
===================
Five silent philosophers sit at a round table with each a bowl of spaghetti.
A fork ``f`` is placed between each pair of adjacent philosophers ``P``::
P P
O f O
f f
P O O P
f f
O
P
Each philosopher ``P`` must alternately think and eat from his bowl ``O``.
Eating is not limited by the amount of spaghetti left: assume an infinite
supply.
However, a philosopher can only eat while holding both the fork ``f`` to
the left and the fork to the right.
Each philosopher can pick up an adjacent fork, when available, and put it down,
when holding it. These are separate actions: forks must be picked up and put
down one by one.
This implementation will just work. No starvation or dead-lock.
There are two parameters:
* Average eating period, the higher the more time is spend eating.
* Average waiting period, the higher the more frequent philosophers
get a chance to eat.
To run the example, type::
pulsar manage.py
Implementation
=====================
.. autoclass:: DiningPhilosophers
:members:
:member-order: bysource
.. _philosophers: http://en.wikipedia.org/wiki/Dining_philosophers_problem
'''
import random
from asyncio import ensure_future
import pulsar
from pulsar import command
###########################################################################
# EXTRA COMMAND LINE PARAMETERS
class PhilosophersSetting(pulsar.Setting):
virtual = True
app = 'philosophers'
section = "Socket Servers"
class EatingPeriod(PhilosophersSetting):
flags = ["--eating-period"]
validator = pulsar.validate_pos_float
default = 2
desc = """The average period of eating for a philosopher."""
class WaitingPeriod(PhilosophersSetting):
flags = ["--waiting-period"]
validator = pulsar.validate_pos_float
default = 2
desc = """The average period of waiting for a missing fork."""
###########################################################################
# PULSAR COMMANDS FOR DINING PHILOSOPHERS
@command(ack=False)
def putdown_fork(request, fork):
self = request.actor.app
try:
self.not_available_forks.remove(fork)
except KeyError:
self.logger.error('Putting down a fork which was already available')
@command()
def pickup_fork(request, fork_right):
self = request.actor.app
num_philosophers = self.cfg.workers
fork_left = fork_right - 1
if fork_left == 0:
fork_left = num_philosophers
for fork in (fork_right, fork_left):
if fork not in self.not_available_forks:
# Fork is available, send it to the philosopher
self.not_available_forks.add(fork)
return fork
############################################################################
# DINING PHILOSOPHERS APP
[docs]class DiningPhilosophers(pulsar.Application):
description = ('Dining philosophers sit at a table around a bowl of '
'spaghetti and waits for available forks.')
cfg = pulsar.Config(workers=5, apps=['philosophers'])
def monitor_start(self, monitor):
self.not_available_forks = set()
def worker_start(self, philosopher, exc=None):
self._loop = philosopher._loop
self.eaten = 0
self.thinking = 0
self.started_waiting = 0
self.forks = []
philosopher._loop.call_soon(self.take_action, philosopher)
[docs] def worker_info(self, philosopher, info=None):
'''Override :meth:`~.Application.worker_info` to provide
information about the philosopher.'''
info['philosopher'] = {'number': philosopher.number,
'eaten': self.eaten}
[docs] def take_action(self, philosopher):
'''The ``philosopher`` performs one of these two actions:
* eat, if he has both forks and then :meth:`release_forks`.
* try to :meth:`pickup_fork`, if he has fewer than 2 forks.
'''
loop = philosopher._loop
forks = self.forks
if forks:
#
# Two forks. Eat!
if len(forks) == 2:
self.thinking = 0
self.eaten += 1
philosopher.logger.info("eating... So far %s times",
self.eaten)
eat_time = 2*self.cfg.eating_period*random.random()
return loop.call_later(eat_time, self.release_forks,
philosopher)
#
# One fork only! release fork or try to pick one up
elif len(forks) == 1:
waiting_period = 2*self.cfg.waiting_period*random.random()
if self.started_waiting == 0:
self.started_waiting = loop.time()
elif loop.time() - self.started_waiting > waiting_period:
philosopher.logger.debug("tired of waiting")
return self.release_forks(philosopher)
#
# this should never happen
elif len(forks) > 2: # pragma nocover
philosopher.logger.critical('more than 2 forks!!!')
return self.release_forks(philosopher)
else:
if not self.thinking:
philosopher.logger.warning('%s thinking...', philosopher.name)
self.thinking += 1
self.pickup_fork(philosopher)
[docs] def pickup_fork(self, philosopher):
"""Pick up a fork
"""
ensure_future(self._pickup_fork(philosopher), loop=philosopher._loop)
[docs] def release_forks(self, philosopher):
'''The ``philosopher`` has just eaten and is ready to release both
forks.
This method releases them, one by one, by sending the ``put_down``
action to the monitor.
'''
forks = self.forks
self.forks = []
self.started_waiting = 0
for fork in forks:
philosopher.logger.debug('Putting down fork %s', fork)
philosopher.send('monitor', 'putdown_fork', fork)
philosopher._loop.call_later(self.cfg.waiting_period, self.take_action,
philosopher)
def actorparams(self, monitor, params):
avail = set(range(1, monitor.cfg.workers+1))
for philosopher in monitor.managed_actors.values():
info = philosopher.info
if info:
avail.discard(info['philosopher']['number'])
else:
avail = None
break
number = min(avail) if avail else len(monitor.managed_actors) + 1
params.update({'name': 'Philosopher %s' % number, 'number': number})
async def _pickup_fork(self, philosopher):
fork = await philosopher.send(philosopher.monitor, 'pickup_fork',
philosopher.number)
if fork:
forks = self.forks
if fork in forks:
philosopher.logger.error('Got fork %s. I already have it',
fork)
else:
philosopher.logger.debug('Got fork %s.', fork)
forks.append(fork)
philosopher._loop.call_soon(self.take_action, philosopher)
if __name__ == '__main__': # pragma nocover
DiningPhilosophers().start()
