Virtual Memory Virtual Memory is created to solve difficult memory management problems Data fragmentation in physical memory: Reuses blocks of memory that would otherwise be unused when programs free up data. Physical memory size constraints are also solved by using alternate forms of data storage to act like memory. Implemented via Paging or Segmentation Problem: Thrashing reduces preformance

Virtual MemoryOverview

Techniques used to give an application program logical access to contiguous working memory In reality the data is fragmented in primary and secondary memory.

Virtual MemoryMemory Fragmentation

Not just using disk space to extend the physical memory.

Uses real physical memory more efficiently.

Tricks programs into thinking they are using large blocks of contiguous addresses.

Reuses fragments in memory.

1950s: Primary memory

Magnetic Core Memory(non-volatile)

Virtual Memory: A History

Virtual Memory: A History

1950s: Secondary memory

Drum Memory (non-volatile)(BSD Unix: /dev/drum is default name for swap)

Virtual MemoryOverlays

Before virtual memory, programs that were too big for the size of physical memory used Overlays

Overlays still popular in embedded systems that require cheap hardware

Overlays require the programmer to manage memory for each program.

Virtual MemoryOverlays

Program manually divided into self-contained code blocks called Overlays

Size of Overlay limited by memory constraints (different for different systems)

Programmer had to use specific programming languages or assembly language to have control over the size of the program and the size of the overlay.

Virtual MemoryA history

Virtual memory was developed in approximately 1959–1962, at the University of Manchester for the Atlas Computer, completed in 1962.

In 1961, Burroughs released the B5000, the first commercial computer with virtual memory. It used segmentation rather than paging.

Virtual Memory was controversial and required many theories, models, and experiments before it was adopted.

"the machine that everyone loves, and nobody buys"- Brian Randell

Virtual MemoryA history

Dynamic address translation required a specialized, expensive, and hard to build hardware

Worries that new system-wide algorithms of utilizing secondary storage would be far less effective than application-specific ones.

In 1969 an IBM research team led by David Sayre showed that the virtual memory overlay system consistently worked better than the best manually controlled systems.

Virtual MemoryPages

Virtual memory address space is divided into pages. A page is a block of contiguous virtual memory

addresses that are at least 4096 bytes in size. Pages are managed by Page Tables. Page Tables are kept in memory and translate

virtual addresses into physical addresses for use by the hardware.

A Page Directory manages Page Tables. There is one page directory per operating system.

Visualization of Paging System in Windows NT (32 bit)

Page Directory Entry Offset (Per

Process)

Page Table Entry Offset (Table +

Frame)

Memory byte offset (4096 bytes

in frame)

Virtual MemoryPaging

“Swapping” is the act of swapping data from physical memory into the virtual memory portion of the hard drive.

“Paging” is the act of reading data from the hard drive (not the virtual memory portion) and writing data permanently back to the hard drive.

Some Operating Systems use “Paging” to describe both (Windows and its “pagefile” vs Unix and its “swap”)

Virtual MemoryPage Fault

A Page Fault occurs when a program attempts to access a page that is not current in main memory.

The operating system takes over and does the following: 1. Determines the location of the data in secondary storage.

2. Creates an empty page frame in main memory.

3. Loads the data into the empty page frame.

4. Updates the Page Table to show the new data.

5. Return control to the program, retrying the last instruction which caused the page fault.

Virtual MemoryPage Fault

If main memory is full in step 2: Swap out page in memory using an algorithm (most commonly used is discarding based on Least Recently Used)

A Page is “Dirty” if it has been modified since it was read from secondary storage.

If Page is “Dirty”, the OS writes the changes back to secondary storage. Else, the OS just discards the page.

When Page Fault swapping occurs, OS attempts to predict which pages will be used later and load those at the same time.

Virtual MemoryPaging Strategies

Demand Paging: No pages are loaded into RAM unless specifically requested by the program. Program starts with NO pages loaded into RAM. Pages of program of the program that are not run will never be loaded.

Virtual MemoryPaging Strategies

Loader Paging: Attempts to predict which parts of the program will be loaded. If possible, every page of the program will be loaded into memory at the start of execution.

Virtual MemoryPaging Strategies

Anticipatory Paging: Uses the locality of reference to pre-load pages from the program that are most likely to be executed. The goal is to minimize the number of page faults.

Virtual MemoryPaging Strategies

Swap Prefetch: A type of anticipatory paging which specifically pre-loads pages

Pre-loads pages from program that caused last pagefault.

Pre-loads pages that have been swapped out back into memory when a large program releases it's memory in anticipation of the user needing to use the other programs instead.

Virtual MemoryPaging Strategies

Pre-cleaning: Writes “Dirty” pages back into secondary storage periodically as a syncing mechanism, before page needs to be swapped out.

Makes starting a new program much faster as pages that were “Dirty” and would have to be written back to secondary storage are now clean and are just discarded.

Virtual MemoryPaging Replacement Algorithms

Theoretical Optimal Paging Algorithm First In, First Out Second Chance (Special case of FIFO) Clock (Similar to Second Chance) Least Recently Used Not Frequently Used Aging (Combines LRU with NFU) Random

Virtual MemoryThrashing

Thrashing is when the Operating System spends more and more time swapping pages to and from virtual memory on the hard drive than running the program

Occurs when the sum of localities from running processes cannot all fit into main memory.

When a computer is using more and more resources for a

decreasing amount of work done, it is said to be thrashing.

Virtual MemoryThrashing

Solutions for Thrashing: Increase size of main memory Reduce number of concurrent processes

running Reduce the size of concurrent processes

running