vault backup: 2025-03-17 10:09:23

Concurrent Systems/notes/4c - Dining Philosophers.md

@@ -5,3 +5,25 @@ The first real practical example of a concurrent system.
 - a philosopher must pick up first one chopstick, then the second one, not both at once
 ![[Pasted image 20250317100456.png|100]]
 
+**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