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Operating Systems/Virtual Memory.md

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+---
+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