vault backup: 2025-03-12 11:04:27

Concurrent Systems/notes/4 - Semaphores.md

@@ -93,4 +93,23 @@ B.consume() :=
 > Reading from / writing into the buffer can be very expensive!
 
 #### (Multiple) Producers/Consumers
-**Accessing BUF in MUTEX slows down the implementation**
+**Accessing BUF in MUTEX slows down the implementation**, we would like to have the possibility to read and write from different cells **in parallel**.
+- we use two arrays FULL and EMPTY of atomic boolean registers, initialized at ff (all zeros) and tt (all ones), respectively
+- we have two extra semaphores SP and SC, both initialized at 1
+
+```
+B.produce(v) :=
+	FREE.down()
+	SP.down()
+	while not EMPTY[in] do
+		IN <- (IN+1) mod k
+	i <- IN
+	EMPTY[IN] <- ff
+	SP.up()
+	BUF[i] <- v
+	FULL[i] <- tt
+	BUSY.up()
+	return
+
+
+```