1.2 KiB
1.2 KiB
The first real practical example of a concurrent system.
Nphilosophers seated around a circular table- one chopstick between each pair of philosophers
- a philosophers must pick up its two nearest chopsticks in order to eat
- a philosopher must pick up first one chopstick, then the second one, not both at once !Pasted image 20250317100456.png
PROBLEM: Devise a deadlock-free algorithm for allocating these limited resources (chopsticks) among several processes (philosophers).
A wrong solution
each chopstick is governed by a mutual exclusion semaphore that prevents any other philosopher from picking up the chopstick when it is already in use by another philosopher
semaphore chopstick[5] initialized to 1
Philosopher(i) :=
while(1) do
chopstick[i].down()
chopstick[(i+1)%N].down()
// eat
chopstick[(i+1)%N].up()
chopstick[i].up()
No two neighbors can eat simultaneously, but we can have a deadlock if all philosophers grab their right chopstick simultaneously.
Solution 1
Give a number to all forks and always try with the smaller. All philosophers must first pick left and then right, except for the last one that first picks right and then left.
So there will be c