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110 lines
3.8 KiB
Markdown
110 lines
3.8 KiB
Markdown
---
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title: "22-virtual-memory"
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aliases:
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tags:
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- cosc204
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- lecture
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sr-due: 2022-11-04
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sr-interval: 3
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sr-ease: 250
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---
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# Swapping and virtual memory
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swapping
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- if there is not enough physical memory we need to sawp processes out of the main ememory to the secondary storage e.g., disk
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- 
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- when a process is ready, it is swapped into the main memory
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- allows more processes to multitask
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partially loaded proceses
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- dynamic loading
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- load a potion of code when it is called as some code may not need to be executed, e.g., code for handling errors
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- pros
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- process not limited by amount of avilable memory
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- more processes multitasking
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- quicker to swap than entire process
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virtual memory
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- idea that processes dont need to be fully in memory to run
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extends main memory to secondary storage, and allows dynamic loading of processes while they execute
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- programmer deals with vmem just like paging scheme
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- mem manager in OS kernel controls loading pages of the process into main mem from secondary storage
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# Demand paging
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dont load a page into mem until it is referenced by CPU
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implementing
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- in paging scheme there are extra bits in the table to provide more information
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- valid/invlid bit
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- protection bits
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- in demand paging the mem manager uses valid/invalid bit to tell if a page is loaded
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note: each page can be stored twice: loaded in main memory and in the backing store - these two copies need to remain consistent. i.e.., changes to one must be reflected in the other
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page faults
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- "trap" occurs when trying to access an invalid page
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- what the os does:
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- check if caused by invalid memory access or unavailable page frame
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- if nvalid mem access
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- terminate process
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- else
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- find a free frame
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- read the page from the disk to the free frame
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- modify the page table
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- restart the instruction
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- VMAs are searched to find valid areas or memory access
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# page replacement algorithms
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If main memory is full when a page fault is generated, one of the pages currently being held needs to be replaced.
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This means an extra step in the operating system’s page-servicing routine.
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- Find the desired page on the backing store (secondary storage).
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- Find a free frame of memory
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- If there’s a free frame in memory, use it
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- Otherwise: Select a frame to swap out
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- Save it to the backing store (in case it’s changed)
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- Proceed as before.
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FIFO
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- replace the oldest page
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- 
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- pro
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- simple to understand/implement
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- con
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- maybe first page to be ref'd is often being referenced
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- belady's anomaly: possible to increase numbe of page faults when increasing the number of frames in memory
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Optimal page replacement
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- replace the page that will not be used for the longest time
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- 
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- pro: optimal
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- con: can predict future
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LRU (least recently used)
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- replace page than has not been used for the longest time
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- 
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- pro: optimal if you look back in time
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- con: time consuming to keep record of LRU
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- future memory references resemble past ones
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# frame allocation
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how many frames to allocate to a process?
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how to distribute frames among n processes in a multitasking scenario
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- equal allocation: all created equal: so each get equal number of frames
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- proportional allocation: allocate depending on size of process
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- local vs global replacement:
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- local: steal from your own frames
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- global: can steal from "victim" processes
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# thrashing
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when a proces has too lttle frames it will have a lot of page faults.
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a thrashing process spending more time paging than executing
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the working set of a process is the number of pages a process needs in order to execute without causing too many page faults
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this is how much memory we allocate to a process to avoid thrashing |