vault backup: 2025-03-17 09:29:23

Concurrent Systems/notes/4b -.md

@@ -6,5 +6,24 @@ It guarantees mutual exclusion.
 Inter-process synchronization is done through *conditions*, which are objects that provide the following operations:
 - ***wait*:** the invoking process suspends, enters into the condition's queue, and releases the mutex on the monitor
 - ***signal*:** if no process is in the condition's queue, the nothing happens. Otherwise
-	- it reactivates the first suspended process, suspends the signaling process that however has a priority to re-enter the monitor (*Hoare semantics*)
-	- 
+	- reactivates the first suspended process, suspends the signaling process that however has a priority to re-enter the monitor (*Hoare semantics*)
+	- completes its task and the first process in the condition's queue has the priority to enter the monitor (after that the signaling one terminates or suspends) (*Mesa semantics*).
+
+#### A very typical use: Rendez-vous
+A soon as a process arrives at a barrier it needs to suspend and wait for every other process to reach the barrier.
+
+```
+monitor RNDV :=
+	cnt ∈ {0,…,m} init at 0
+	condition B
+	operation barrier() :=
+		cnt++
+		if cnt < m then
+			B.wait()
+		else
+			cnt <- 0
+		B.signal()
+		return
+```
+The last process wakes up one of the others... Then he wakes up another one etc...
+