UNIT – IV

VIRTUAL MEMORY MANAGEMENT

Handled by

K. Venkatesh & Razia Sultana

Virtual Memory

• Background

• Demand Paging

• Performance of Demand Paging

• Page Replacement

• Page-Replacement Algorithms

• Allocation of Frames

• Thrashing

Background

• Virtual memory – separation of user logical memory from physical memory.– Only part of the program needs to be in memory

for execution.– Logical address space can therefore be much

larger than physical address space.– Need to allow pages to be swapped in and out.

• Virtual memory can be implemented via:– Demand paging – Demand segmentation

Demand Paging

• Bring a page into memory only when it is needed.– Less I/O needed– Less memory needed – Faster response– More users

• Page is needed reference to it– invalid reference abort– not-in-memory bring to memory

Valid-Invalid Bit• With each page table entry a valid–invalid bit is associated

(1 in-memory, 0 not-in-memory)• Initially valid–invalid but is set to 0 on all entries.• Example of a page table snapshot.• During address translation, if valid–invalid bit in page table entry is 0

page fault.

11110

00

Frame # valid-invalid bit

page table

Page Fault• If there is ever a reference to a page, first reference will trap to

OS page fault• OS looks at another table to decide:

– Invalid reference abort.– Just not in memory.

• Get empty frame.• Swap page into frame.• Reset tables, validation bit = 1.• Restart instruction: Least Recently Used

– block move

– auto increment/decrement location

What happens if no frame is free?• Page replacement – find some page in

memory, but not really in use, swap it out.– algorithm– performance – want an algorithm which will

result in minimum number of page faults.

• Same page may be brought into memory several times.

Performance of Demand Paging• Page Fault Rate 0 p 1.0

– if p = 0 no page faults – if p = 1, every reference is a fault

• Effective Access Time (EAT)EAT = (1 – p) x memory access

+ p (page fault overhead+ [swap page out ]+ swap page in+ restart overhead)

Demand Paging Example• Memory access time = 1 microsecond

• 50% of the time the page that is being replaced has been modified and therefore needs to be swapped out.

• Swap Page Time = 10 msec = 10,000 msecEAT = (1 – p) x 1 + p (15000)

1 + 15000P (in msec)

Page Replacement

• Prevent over-allocation of memory by modifying page-fault service routine to include page replacement.

• Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk.

• Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory.

Page-Replacement Algorithms• Want lowest page-fault rate.

• Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string.

• In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5.

First-In-First-Out (FIFO) Algorithm• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

• 3 frames (3 pages can be in memory at a time per process)

• 4 frames

• FIFO Replacement – Belady’s Anomaly– more frames less page faults

1

2

3

1

2

3

4

1

2

5

3

4

9 page faults

1

2

3

1

2

3

5

1

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4

510 page faults

44 3

Optimal Algorithm• Replace page that will not be used for longest period of time.

• 4 frames example 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

• How do you know this?

• Used for measuring how well your algorithm performs.

1

2

3

4

6 page faults

4 5

Least Recently Used (LRU) Algorithm• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

• Counter implementation– Every page entry has a counter; every time page is

referenced through this entry, copy the clock into the counter.

– When a page needs to be changed, look at the counters to determine which are to change.

1

2

3

5

4

4 3

5

LRU Algorithm (Cont.)

• Stack implementation – keep a stack of page numbers in a double link form:– Page referenced:

• move it to the top• requires 6 pointers to be changed

– No search for replacement

LRU Approximation Algorithms• Reference bit

– With each page associate a bit, initially -= 0– When page is referenced bit set to 1.– Replace the one which is 0 (if one exists). We do not

know the order, however.

• Second chance– Need reference bit.– Clock replacement.– If page to be replaced (in clock order) has reference

bit = 1. then:• set reference bit 0.• leave page in memory.• replace next page (in clock order), subject to same rules.

Counting Algorithms

• Keep a counter of the number of references that have been made to each page.

• LFU Algorithm: replaces page with smallest count.

• MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used.

Allocation of Frames

• Each process needs minimum number of pages.

• Example: IBM 370 – 6 pages to handle SS MOVE instruction:– instruction is 6 bytes, might span 2 pages.– 2 pages to handle from.– 2 pages to handle to.

• Two major allocation schemes.– fixed allocation– priority allocation

Fixed Allocation• Equal allocation – e.g., if 100 frames and 5

processes, give each 20 pages.

• Proportional allocation – Allocate according to the size of process.

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ps

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for allocation

frames of number total

process of size

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Priority Allocation

• Use a proportional allocation scheme using priorities rather than size.

• If process Pi generates a page fault,

– select for replacement one of its frames.– select for replacement a frame from a process

with lower priority number.

Global vs. Local Allocation• Global replacement – process selects a

replacement frame from the set of all frames; one process can take a frame from another.

• Local replacement – each process selects from only its own set of allocated frames.

Thrashing

• If a process does not have “enough” pages, the page-fault rate is very high. This leads to:– low CPU utilization.

– operating system thinks that it needs to increase the degree of multiprogramming.

– another process added to the system.

• Thrashing a process is busy swapping pages in and out.

Thrashing Diagram

• Why does paging work?Locality model– Process migrates from one locality to another.– Localities may overlap.

• Why does thrashing occur? size of locality > total memory size

Working-Set Model working-set window a fixed number of page references

Example: 10,000 instruction

• WSSi (working set of Process Pi) =total number of pages referenced in the most recent (varies in time)– if too small will not encompass entire locality.– if too large will encompass several localities.– if = will encompass entire program.

• D = WSSi total demand frames

• if D > m Thrashing

• Policy if D > m, then suspend one of the processes.

Keeping Track of the Working Set• Approximate with interval timer + a reference bit

• Example: = 10,000– Timer interrupts after every 5000 time units.– Keep in memory 2 bits for each page.– Whenever a timer interrupts copy and sets the values of all

reference bits to 0.– If one of the bits in memory = 1 page in working set.

• Why is this not completely accurate?

• Improvement = 10 bits and interrupt every 1000 time units.

Page-Fault Frequency Scheme

• Establish “acceptable” page-fault rate.– If actual rate too low, process loses frame.– If actual rate too high, process gains frame.

Other Considerations

• Preparing

• Page size selection– fragmentation– table size – I/O overhead– locality

Other Consideration (Cont.)• Program structure

– Array A[1024, 1024] of integer– Each row is stored in one page– One frame – Program 1 for j := 1 to 1024 do

for i := 1 to 1024 doA[i,j] := 0;

1024 x 1024 page faults – Program 2 for i := 1 to 1024 do

for j := 1 to 1024 doA[i,j] := 0;

1024 page faults

• I/O interlock and addressing

Demand Segmentation• Used when insufficient hardware to

implement demand paging.

• OS/2 allocates memory in segments, which it keeps track of through segment descriptors

• Segment descriptor contains a valid bit to indicate whether the segment is currently in memory.– If segment is in main memory, access continues,– If not in memory, segment fault.

File-System Interface

• File Concept

• Access Methods

• Directory Structure

• Protection

• Consistency Semantics

File Concept

• Contiguous logical address space

• Types: – Data

• numeric• character• binary

– Program

File Structure• None - sequence of words, bytes• Simple record structure

– Lines – Fixed length– Variable length

• Complex Structures– Formatted document– Relocatable load file

• Can simulate last two with first method by inserting appropriate control characters.

• Who decides:– Operating system– Program

File Attributes• Name – only information kept in human-readable form.

• Type – needed for systems that support different types.

• Location – pointer to file location on device.

• Size – current file size.

• Protection – controls who can do reading, writing, executing.

• Time, date, and user identification – data for protection, security, and usage monitoring.

• Information about files are kept in the directory structure, which is maintained on the disk.

File Operations

• create• write• read• reposition within file – file seek• delete• truncate• open(Fi) – search the directory structure on disk

for entry Fi, and move the content of entry to memory.

• close (Fi) – move the content of entry Fi in memory to directory structure on disk.

File Types – name, extension

Executable exe, com, bin ornone

ready-to-run machine-language program

Object obj, o complied, machinelanguage, not linked

Source code c, p, pas, 177,asm, a

source code in variouslanguages

Batch bat, sh commands to thecommand interpreter

Text txt, doc textual data documents

Word processor wp, tex, rrf, etc. various word-processorformats

Library lib, a libraries of routines

Print or view ps, dvi, gif ASCII or binary file

Archive arc, zip, tar related files groupedinto one file, sometimescompressed.

File Type Usual extension Function

Access Methods• Sequential Access

read nextwrite next resetno read after last write

(rewrite)• Direct Access

read nwrite nposition to n

read nextwrite next

rewrite nn = relative block number

Directory Structure• A collection of nodes containing information about all files.

F 1 F 2 F 3F 4

F n

Directory

Files

• Both the directory structure and the files reside on disk.

• Backups of these two structures are kept on tapes.

Information in a Device Directory• Name • Type• Address • Current length• Maximum length• Date last accessed (for archival)• Date last updated (for dump)• Owner ID (who pays)• Protection information (discuss later)

Operations Performed on Directory• Search for a file

• Create a file

• Delete a file

• List a directory

• Rename a file

• Traverse the file system

Organize the Directory (Logically) to Obtain

• Efficiency – locating a file quickly.

• Naming – convenient to users.– Two users can have same name for different

files.– The same file can have several different names.

• Grouping – logical grouping of files by properties, (e.g., all Pascal programs, all games, …)

Single-Level Directory

• A single directory for all users.

• Naming problem

• Grouping problem

Two-Level Directory• Separate directory for each user.

• Path name

• Can have the saem file name for different user

• Efficient searching

• No grouping capability

Tree-Structured Directories

Tree-Structured Directories (Cont.)

• Efficient searching

• Grouping Capability

• Current directory (working directory)– cd /spell/mail/prog– type list

Tree-Structured Directories (Cont.)• Absolute or relative path name• Creating a new file is done in current directory.• Delete a file

rm <file-name>• Creating a new subdirectory is done in current

directory.mkdir <dir-name>

Example: if in current directory /spell/mailmkdir count

mail

prog copy prt expcount

• Deleting “mail” deleting the entire subtree rooted by “mail”.

Acyclic-Graph Directories• Have shared subdirectories and files.

Acyclic-Graph Directories (Cont.)

• Two different names (aliasing)

• If dict deletes list dangling pointer.

Solutions:– Backpointers, so we can delete all pointers.

Variable size records a problem.– Backpointers using a daisy chain

organization.– Entry-hold-count solution.

General Graph Directory

General Graph Directory (Cont.)• How do we guarantee no cycles?

– Allow only links to file not subdirectories.– Garbage collection.– Every time a new link is added use a cycle

detectionalgorithm to determine whether it is OK.

Protection

• File owner/creator should be able to control:– what can be done– by whom

• Types of access– Read– Write– Execute– Append– Delete– List

Lets test …• Define virtual memory?• Define demand paging?• Differentiate demand paging from pure demand paging?• How is the effective access time calculated?• What are memory mapped files• What is victim frame?• What is the use of modify bit?• What is frame allocation algorithm?• What is the disadvantage of FIFO page replacement algorithm?• Define optimal page replacement algorithm• What are the types of LRU approximation page replacement algorithm?• What are four possible cases in enhanced second chance algorithm?• Name the types of counting based page replacement algorithm?• Define thrashing?• Name the file attributes?• What operations can be implemented on files?• Differentiate single level directory from two level directory?• Define mount point?• Define controlled access?• What is ACL?• Differentiate absolute path name and relative path name?

Reference:1. “Operating System Concepts” by Abraham Silberschatz, Peter Baer Galvin and Greg Gagne, Sixth Edition, John Wiley & Sons (ASIA) Pvt. Ltd, 2003.