vault backup: 2022-11-01 13:49:33

content/notes/22-virtual-memory.md

@@ -67,12 +67,39 @@ This means an extra step in the operating system’s page-servicing routine.
 
 FIFO
 - replace the oldest page
+- ![example](https://i.imgur.com/EM09dtl.png)
 - pro
 	- simple to understand/implement
 - con
 	- maybe first page to be ref'd is often being referenced
-	- belady's anomaly
+	- belady's anomaly: possible to increase numbe of page faults when increasing the number of frames in memory
+
+Optimal page replacement
+- replace the page that will not be used for the longest time
+- ![example](https://i.imgur.com/RiNi1JI.png)
+- pro: optimal
+- con: can predict future
+
+LRU (least recently used)
+- replace page than has not been used for the longest time
+- ![example](https://i.imgur.com/VpMGeXY.png)
+- pro: optimal if you look back in time
+- con: time consuming to keep record of LRU
+- future memory references resemble past ones
 
 # frame allocation
+how many frames to allocate to a process?
+
+how to distribute frames among n processes in a multitasking scenario
+- equal allocation: all created equal: so each get equal number of frames
+- proportional allocation: allocate depending on size of process
+- local vs global replacement: 
+	- local: steal from your own frames
+	- global: can steal from "victim" processes
 
 # thrashing
+when a proces has too lttle frames it will have a lot of page faults.
+
+a thrashing process spending more time paging than executing
+
+the working set of a process is the number of pages a process needs in orde