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Lecture 8: Virtual Memory

Operating SystemFall 2006

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Two characteristics of paging and segmentation Memory references are dynamically translated

into physical addresses at run time A process may be swapped in and out of main

memory such that it occupies different regions A process may be broken up into pieces that

do not need to located contiguously in main memory

All pieces of a process do not need to be loaded in main memory during execution

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Virtual Memory It is not necessary that all of the pages or all of the

segments of a process be in main memory during execution. As long as the piece holding the next instruction and the data to be accessed are in main memory, then execution may proceed.

Use page table to do address translation. If the page is not in memory, it generates a page fault interrupt, the OS will bring the page from disk into main memory. When this is done, resume execution.

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Advantages of Virtual Memory More processes may be maintained in main

memory Only load in some of the pieces of each process With so many processes in main memory, it is very

likely a process will be in the Ready state at any particular time

A process may be larger than all of main memory. Programs become portable across different platforms.

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Types of Memory Real memory

Physical Main memory Virtual memory

Programmer perceived memory Memory on disk Allows for effective multiprogramming and

relieves the user of tight constraints of main memory

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Principle of Locality Program and data references within a process

tend to cluster Only a few pieces of a process will be needed

over a short period of time Possible to make intelligent guesses about

which pieces will be needed in the future This suggests that virtual memory may work

efficiently

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Support Needed for Virtual Memory Hardware must support paging and

segmentation Operating system must be able to

management the movement of pages and/or segments between secondary memory and main memory

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Paging Each process has its own page table Each page table entry contains the frame number

of the corresponding page in main memory Presence Bit: A bit is needed to indicate whether

the page is in main memory or not Modify Bit:

Another bit is needed to indicate if the page has been altered since it was last loaded into main memory

If no change has been made, the page does not have to be written to the disk when it needs to be swapped out

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Page Table Entries

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Translation Lookaside Buffer Contains page table entries that have been

most recently used Functions same way as a memory cache

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Paging Hardware With TLB

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Structure of the Page Table Hierarchical Paging

Hashed Page Tables

Inverted Page Tables

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Hierarchical Page Tables Break up the logical address space into

multiple page tables

A simple technique is a two-level page table

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Two-Level Page-Table Scheme

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Address-Translation Scheme

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Memory Protection Memory protection implemented by associating

protection bit with each frame

Valid-invalid bit attached to each entry in the page table:

“valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page

“invalid” indicates that the page is not in the process’ logical address space

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Valid (v) or Invalid (i) Bit In A Page Table

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Segmentation May be unequal, dynamic size Simplifies handling of growing data structures Allows programs to be altered and recompiled

independently Lends itself to sharing data among processes Lends itself to protection

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Segment Tables corresponding segment in main memory Each entry contains the length of the segment A bit is needed to determine if segment is

already in main memory Another bit is needed to determine if the

segment has been modified since it was loaded in main memory

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Segment Table Entries

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Segmentation Hardware

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Combined Paging and Segmentation Paging is transparent to the programmer Paging eliminates external fragmentation Segmentation is visible to the programmer Segmentation allows for growing data

structures, modularity, and support for sharing and protection

Each segment is broken into fixed-size pages

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Combined Segmentation and Paging

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OS Supports for Virtual Memory Virtual Memory: not all pages of a

process are in main memory OS needs to decide on the following

issues: Fetch Policy Placement Policy Replacement Policy

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Fetch Policy Fetch Policy

Determines when a page should be brought into memory Demand paging – bring pages into main memory only

when it is needed Many page faults when process first started Less I/O needed Less memory needed Faster response More users

Prepaging – brings in more pages then needed even though it is not needed now.

Faster to bring in several pages than one at a time More efficient to bring in pages that reside contiguously on

the disk

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Placement Policy Decides where a process piece reside in

main memory For paging system, it is a trivial issue For segmentation system, use first-fit or

best-fit to look for a hole.

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Replacement Policy Determines which page to replace when a new

page needs to be brought in and there is no empty page frame around

Page removed should be the page least likely to be referenced in the near future

Most policies predict the future behavior on the basis of past behavior

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Replacement Policy Frame Locking

If frame is locked, it may not be replaced Kernel of the operating system Control structures I/O buffers Associate a lock bit with each frame

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Replacement Algorithms Belady’s Optimal Algorithm Least Recently Used Algorithm (LRU) First-in-first-out Algorithm (FIFO) Clock (approximation of LRU)

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Belady’s Optimal Algorithm Optimal policy

Selects for replacement that page for which the time to the next reference is the longest

Impossible to have perfect knowledge of future events

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Least Recently Used (LRU) Replaces the page that has not been

referenced for the longest time By the principle of locality, this should be the

page least likely to be referenced in the near future

Each page could be tagged with the time of last reference. This would require a great deal of overhead.

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First-in, first-out (FIFO) Treats page frames allocated to a process as a

circular buffer Pages are removed in round-robin style Simplest replacement policy to implement Page that has been in memory the longest is

replaced These pages may be needed again very soon LRU performs better than FIFO, but difficult to

implement.

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Clock Policy Additional bit called a use bit When a page is first loaded in memory, the

use bit is set to 0 When the page is referenced, the use bit is set

to 1 When it is time to replace a page, the first

frame encountered with the use bit set to 0 is replaced.

During the search for replacement, each use bit set to 1 is changed to 0

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Further Refinement for Clock Policy Two bits:

Used bit: u bit=1 when used Modify bit: m bit=1 when the page is written

Four states: u=0;m=0 – not used, not modified u=1;m=0 – used, not modified u=0;m=1 – not used, modified u=1;m=1 – used, modified

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Further Refinement for Clock Policy Algorithm:

1. Beginning at the current position of the ptr, scan the frame buffer. During this scan, make no changes to the use bit. The first frame encountered with (u=0;m=0) is selected for replacement.

2. If step 1 fails, scan again, looking for the frame with (u=0;m=1). The first such frame encountered is selected for replacement. During this scan, set the use bit to 0 on each frame that is bypassed.

3. If step 2 fails, the ptr should have returned to its original position and all of the frames in the set will have a use bit of 0. Repeat step 1, and if necessary, step 2. This time, a frame will be found for the replacement.

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End of lecture 8

Thank you!