quartz/content/notes/22-virtual-memory.md

title	aliases	tags
22-virtual-memory		cosc204 lecture

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

frame allocation

thrashing