vault backup: 2025-05-05 04:53:58

.obsidian/workspace.json

@@ -11,29 +11,42 @@
             "id": "46800597ab6eb156",
             "type": "leaf",
             "state": {
-              "type": "markdown",
+              "type": "image",
               "state": {
-                "file": "Operating Systems/Memory Management.md",
+                "file": "Operating Systems/assets/Pasted image 20250505042548.png"
-                "mode": "source",
-                "source": false
               },
-              "icon": "lucide-file",
+              "icon": "lucide-image",
-              "title": "Memory Management"
+              "title": "Pasted image 20250505042548"
             }
           },
           {
             "id": "33dc0f1ebcbf4c21",
             "type": "leaf",
             "state": {
-              "type": "release-notes",
+              "type": "image",
               "state": {
-                "currentVersion": "1.8.10"
+                "file": "Operating Systems/assets/Pasted image 20250505042548.png"
               },
-              "icon": "lucide-book-up",
+              "icon": "lucide-image",
-              "title": "Release Notes 1.8.10"
+              "title": "Pasted image 20250505042548"
+            }
+          },
+          {
+            "id": "565b9e04e8704d81",
+            "type": "leaf",
+            "state": {
+              "type": "markdown",
+              "state": {
+                "file": "Operating Systems/Virtual Memory.md",
+                "mode": "source",
+                "source": false
+              },
+              "icon": "lucide-file",
+              "title": "Virtual Memory"
             }
           }
-        ]
+        ],
+        "currentTab": 2
       }
     ],
     "direction": "vertical"
@@ -210,13 +223,17 @@
       "omnisearch:Omnisearch": false
     }
   },
-  "active": "46800597ab6eb156",
+  "active": "565b9e04e8704d81",
   "lastOpenFiles": [
-    "Operating Systems/Overview.md",
-    "Operating Systems/Processes and Threads.md",
     "Operating Systems/Scheduling.md",
-    "Operating Systems/Inter-Process Communication.md",
+    "Operating Systems/Processes and Threads.md",
     "Operating Systems/Memory Management.md",
+    "Operating Systems/Inter-Process Communication.md",
+    "Operating Systems/Virtual Memory.md",
+    "Operating Systems/assets/Pasted image 20250505042548.png",
+    "Operating Systems/assets/Pasted image 20250505042419.png",
+    "Operating Systems/assets/Pasted image 20250204103541.png",
+    "Operating Systems/Overview.md",
     "Operating Systems/assets/Pasted image 20250502183523.png",
     "Operating Systems/assets/Pasted image 20250502183221.png",
     "Operating Systems/assets/Pasted image 20250502183310.png",
@@ -225,12 +242,9 @@
     "Operating Systems/assets/Pasted image 20250502183002.png",
     "Operating Systems/assets/Pasted image 20250502182934.png",
     "README.md",
-    "Pasted image 20250502182936.png",
     "Untitled.canvas",
     "Discrete Structures/Midterm/attempt 2.md",
     "Discrete Structures/Mathematical Data Structures.md",
-    "Operating Systems/assets/image.png",
-    "Operating Systems/assets/Pasted image 20250502181012.png",
     "unicef.org.md",
     "Linear Algebra/Matrices.md",
     "Languages & Machines/Regular languages.md",
@@ -248,7 +262,6 @@
     "Functional Programming/Recursion.md",
     "Functional Programming/Lists.md",
     "Functional Programming/Eq and Num.md",
-    "Functional Programming/Introduction to Functional Programming.md",
     "Languages & Machines/assets",
     "Languages & Machines",
     "Extracurricular/Misc/Proposed Routine Plan.canvas",

Operating Systems/Virtual Memory.md

@@ -0,0 +1,98 @@
+---
+type: theoretical
+backlinks:
+  - "[[Memory Management]]"
+---
+Separating logical memory from physical memory.
+- Allows **very large** virtual memory, when only a smaller physical memory is available
+The **virtual address space** of a process refers to the logical view of how a process is stored
+
+## Implementation
+Maintain a copy of the process (including code,data heap and stack segments) in a special area of the HDD named **virtual memory**
+
+> [!IMPORTANT]- Refresher - code, data, heap and stack segments
+> ![](Pasted%20image%2020250505042419.png)
+
+>[!NOTE]- Another refresher - stack vs. heap allocation
+> ![](Pasted%20image%2020250505042548.png)
+
+- Process is divided into pages
+- If a page is referred to, it is brought into main memory (**DEMAND PAGING**)
+
+## Page faults
+When a page is referenced but not present in memory - a main memory access is required.
+
+## Page replacement
+The act of replacing a frame in memory with another one (which we need) from the main memory.
+
+
+### Basic Page Replacement
+Linear search for free frame, if none -> select victim page via **a page replacement algorithm** and do your thing!
+
+
+## Typical pages
+Contain:
+- A P (or V for valid) bit -> page is in RAM (else page fault)
+- M (dirty) bit - page in RAM modified
+- R bit - referenced since last time R was cleared
+
+## Belady’s Anomaly
+
+Increasing the number of page frames (or memory pages) allocated to a process can lead to an increase in the number of page faults, rather than a decrease.
+## Frame-allocation algorithms
+How many frames to give to each process
+
+### Fixed
+Each process receives the same number of frames -> **Equal Allocation**
+
+### Proportional
+Allocate according to size of process.
+
+## Page-replacement algorithms
+Which page to replace, optimizing for lowest page fault rate.
+
+### FIFO
+Read title.
+### Least Recently Used (LRU)
+Replace page that has not been used recently.
+
+**Counter**:
+- Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter.
+- When a page is to be changed, find smallest value in counters.
+**Stack**:
+- Keep a stack of page numbers in a double link form[^1]
+- On page reference - move to top
+
+### LRU Approximation
+Since LRU is slow as shit, we need an approximation.
+
+We introduce a **Reference bit**:
+- Each page has `R = 0` at first
+- Whenever referenced `R = 1`
+- Replace any page with `R = 0`
+
+#### Second-chance (Clock)
+Do the above. It's called a clock cuz it's like a `itertools.cycle()`. It holds the state of last accessed page (clock hand) and moves on from there on next iteration.
+
+## The Working Set Model
+We want to have the set of pages "in use" in RAM!
+- Working set changes over time
+- Usually approximated by the set of pages used by some amount of most recent references
+
+## Thrashing
+When a process is busy swapping pages. 
+Multiprogramming[^2] is high, CPU utilization[^3] is low.
+
+### Solving it
+Give the process(es) more memory by:
+- Taking it from another process
+- Swap out shit until demand is lower (scheduler)
+
+## Prepaging
+
+
+
+---
+[^1]: Doubly-linked list where we can go `prev` and `next`
+[^2]: Many processes wanting resources
+[^3]: Shit actually happening