Python Implementation of Dining-Philosphers Solution using Semaphore














































Python Implementation of Dining-Philosphers Solution using Semaphore



Python Implementation of Dining-Philosphers Solution using Semaphore

Description: Five philosophers, spend their time thinking and eating spaghetti. They eat at a round table with five individual seats. For eating each philosopher needs two forks (the resources). There are five forks on the table, one left and one right of each seat. When a philosopher cannot grab both forks it sits and waits. Eating takes random time, then the philosopher puts the forks down and leaves the dining room. After spending some random time thinking he again becomes hungry, and the circle repeats itself.

Algorithm:
wait(fork[i]); // pick left
if right fork is free:
wait( fork[ (i+1) % 5] ); // pick right if free
else
signal(fork[i]) // put left
. .
. EATING
.

//put left and right after eating
signal( fork[i] );
signal( fork[ (i+1) % 5] );
.
. THINKING

continue loop

Program:
import threading
import random
import time

#inheriting threading class in Thread module
class Philosopher(threading.Thread):
running = True #used to check if everyone is finished eating

#Since the subclass overrides the constructor, it must make sure to invoke the base class constructor (Thread.__init__()) before doing anything else to the thread.
def __init__(self, index, forkOnLeft, forkOnRight):
threading.Thread.__init__(self)
self.index = index
self.forkOnLeft = forkOnLeft
self.forkOnRight = forkOnRight

def run(self):
while(self.running):
# Philosopher is thinking (but really is sleeping).
time.sleep(30)
print ('Philosopher %s is hungry.' % self.index)
self.dine()

def dine(self):
# if both the semaphores(forks) are free, then philosopher will eat
fork1, fork2 = self.forkOnLeft, self.forkOnRight
while self.running:
fork1.acquire() # wait operation on left fork
locked = fork2.acquire(False)
if locked: break #if right fork is not available leave left fork
fork1.release()
print ('Philosopher %s swaps forks.' % self.index)
fork1, fork2 = fork2, fork1
else:
return
self.dining()
#release both the fork after dining
fork2.release()
fork1.release()

def dining(self):
print ('Philosopher %s starts eating. '% self.index)
time.sleep(30)
print ('Philosopher %s finishes eating and leaves to think.' % self.index)

def main():
forks = [threading.Semaphore() for n in range(5)] #initialising array of semaphore i.e forks

#here (i+1)%5 is used to get right and left forks circularly between 1-5
philosophers= [Philosopher(i, forks[i%5], forks[(i+1)%5])
for i in range(5)]

Philosopher.running = True
for p in philosophers: p.start()
time.sleep(100)
Philosopher.running = False
print ("Now we're finishing.")


if __name__ == "__main__":
main()


Output:

Philosopher 0 is hungry.
Philosopher 1 is hungry.
Philosopher 0 starts eating.
Philosopher 2 is hungry.
Philosopher 2 starts eating.
Philosopher 4 is hungry.
Philosopher 3 is hungry.
Philosopher 4 swaps forks.
Philosopher 0 finishes eating and leaves to think.
Philosopher 1 swaps forks.
Philosopher 4 starts eating.
Philosopher 2 finishes eating and leaves to think.
Philosopher 3 swaps forks.
Philosopher 1 starts eating.
Philosopher 4 finishes eating and leaves to think.
Philosopher 0 is hungry.
Philosopher 3 starts eating.
Philosopher 0 swaps forks.
Philosopher 1 finishes eating and leaves to think.
Philosopher 2 is hungry.
Philosopher 0 starts eating.
Philosopher 2 swaps forks.
Now we're finishing.
Philosopher 3 finishes eating and leaves to think.
Philosopher 2 starts eating.
Philosopher 4 is hungry.
Philosopher 0 finishes eating and leaves to think.
Philosopher 1 is hungry.
Philosopher 2 finishes eating and leaves to think.
#KeyBoard Interrupt to exit

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