vault backup: 2025-03-17 09:59:23

.obsidian/workspace.json

@@ -34,9 +34,9 @@
               "type": "pdf",
               "state": {
                 "file": "Concurrent Systems/slides/class 4.pdf",
-                "page": 14,
+                "page": 16,
                 "left": -27,
-                "top": 94,
+                "top": 130,
                 "zoom": 0.5754156769596199
               },
               "icon": "lucide-file-text",

Concurrent Systems/notes/4b -.md

@@ -88,4 +88,72 @@ operation end_write() :=
 	else
 		CW.signal()
 
-```
+```
+
+#### Readers/Writers problem, strong priority to Writers
+```
+monitor RW_READERS :=
+	AR, WR, AW, WW init at 0
+	condition CR, CW
+
+operation begin_read() :=
+	if WW + AW != 0 then
+		CR.wait()
+		CR.signal()
+	AR++
+
+operation end_read() :=
+	AR--
+	if AR = 0 then
+		CW.signal()
+
+operation begin_write() :=
+	WW++
+	if (AR+AW != 0) then
+		CW.wait()
+	AW++
+	WW--
+
+operation end_write() :=
+	AW--
+	if WW > 0 then
+		CW.signal()
+	else
+		CR.signal()
+```
+
+#### Readers/Writers problem, a fair solution
+- after a write, all waiting readers are enabled
+- during a read, new readers must wait if writers are waiting
+```
+monitor RW_READERS :=
+	AR, WR, AW, WW init at 0
+	condition CR, CW
+
+operation begin_read() :=
+	WR++
+	if WW + AW != 0 then
+		CR.wait()
+		CR.signal()
+	AR++
+	WR--
+
+operation end_read() :=
+	AR--
+	if AR = 0 then
+		CW.signal()
+
+operation begin_write() :=
+	WW++
+	if (AR+AW != 0) then
+		CW.wait()
+	AW++
+	WW--
+
+operation end_write() :=
+	AW--
+	if WW > 0 then
+		CW.signal()
+	else
+		CR.signal()
+```