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| title | aliases | tags | ||
|---|---|---|---|---|
| 22-virtual-memory |
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Swapping and virtual memory
swapping
- if there is not enough physical memory we need to sawp processes out of the main ememory to the secondary storage e.g., disk
- when a process is ready, it is swapped into the main memory
- allows more processes to multitask
partially loaded proceses
- dynamic loading
- load a potion of code when it is called as some code may not need to be executed, e.g., code for handling errors
- pros
- process not limited by amount of avilable memory
- more processes multitasking
- quicker to swap than entire process
virtual memory
- idea that processes dont need to be fully in memory to run
extends main memory to secondary storage, and allows dynamic loading of processes while they execute
- programmer deals with vmem just like paging scheme
- mem manager in OS kernel controls loading pages of the process into main mem from secondary storage
Demand paging
dont load a page into mem until it is referenced by CPU
implementing
- in paging scheme there are extra bits in the table to provide more information
- valid/invlid bit
- protection bits
- in demand paging the mem manager uses valid/invalid bit to tell if a page is loaded
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
page faults
- "trap" occurs when trying to access and invalid page
- what the os does:
- check if caused by invalid memory access or unavailable page frame
- if nvalid mem access
- terminate process
- else
- find a free frame
- read the page from the disk to the free frame
- modify the page table
- restart the instruction
- VMAs are searched to find valid areas or memory access
page replacement algorithms
If main memory is full when a page fault is generated, one of the pages currently being held needs to be replaced.
This means an extra step in the operating system’s page-servicing routine.
- Find the desired page on the backing store (secondary storage).
- Find a free frame of memory
- If there’s a free frame in memory, use it
- Otherwise: Select a frame to swap out
- Save it to the backing store (in case it’s changed)
- Proceed as before.
FIFO
- replace the oldest page
- pro
- simple to understand/implement
- con
- maybe first page to be ref'd is often being referenced
- 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
- pro: optimal
- con: can predict future
LRU (least recently used)
- replace page than has not been used for the longest time
- 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