virtual memory os notes

8/13/2019 Virtual Memory OS notes

1/51

Virtual Memory

Gordon College

Stephen Brinton


2/51

Virtual Memory

Background Demand Paging

Process Creation

Page Replacement Allocation of Frames

Thrashing

Demand Segmentation Operating System Examples


3/51

Background Virtual memoryseparation of user logical

memory from physical memory. Only part of the program needed

Logical address space > physical addressspace.

(easier for programmer) shared by several processes.

efficient process creation.

Less I/O to swap processes

Virtual memory can be implemented via: Demand paging

Demand segmentation


4/51

Larger Than Physical Memory


5/51

Shared Library Using Virtual

Memory


6/51

Demand Paging

Bring a page into memory only when itis needed

Less I/O needed

Less memory needed Faster response

More users (processes) able to execute

Page is needed reference to it invalid reference abort

not-in-memory bring to memory


7/51

Valid-Invalid Bit With each page table

entry a validinvalid bit isassociated

(1 in-memory, 0 not-

in-memory)

Initially validinvalid bit isset to 0 on all entries

During address

translation, if validinvalid

bit in page table entry is 0

page-fault trap

1

11

1

0

0

0

Frame # valid-invalid bit

page table

Example of a pagetable snapshot:


8/51

Page Table: Some Pages Are Not in

Main Memory


9/51

Page-Fault Trap1. Reference to a page with invalid bit set - trap to

OS page fault2. Must decide???:

Invalid reference abort.

Just not in memory.

3. Get empty frame.4. Swap page into frame.

5. Reset tables, validation bit = 1.

6. Restart instruction:

what happens if it is in the middle of aninstruction?


10/51

Steps in Handling a Page

Fault


11/51

What happens if there is no free frame?

Page replacementfind somepage in memory, but not really in

use, swap it out

algorithm performancewant an algorithm

which will result in minimum number

of page faults

Same page may be brought into

memory several times


12/51

Performance of Demand Paging

Page Fault Rate: 0p1 (probability of pagefault)

ifp= 0 no page faults

ifp= 1, every reference is a fault

Effective Access Time (EAT)

EAT = (1p) x memory access

+p(page fault overhead

+ [swap page out ]

+ swap page in

+ restart overhead)


13/51

Demand Paging Example

Memory access time = 1 microsecond

50% of the time the page that is being replaced hasbeen modified and therefore needs to be swapped

out

Page-switch time: around 8 ms.

EAT = (1p) x (200) + p(8 ms)

= (1p) x (200) + p(8,000,000)= 200 + 7,999,800p

220 > 200 + 7,999,800p

p < .0000025


14/51

Process Creation

Virtual memory allows other benefits

during process creation:

- Copy-on-Write

- Memory-Mapped Files


15/51

Copy-on-Write

Copy-on-Write (COW) allows both parent and childprocesses to initially sharethe same pages inmemory

If either process modifies a shared page, only then isthe page copied

COW allows more efficient process creation as onlymodified pages are copied

Free pages are allocated from a poolof zeroed-outpages


16/51

Need: Page Replacement Prevent over-

allocation ofmemory bymodifying page-fault serviceroutine to includepage

replacement

Use modify(dirty) bit toreduce overheadof page transfers

only modifiedpages arewritten to disk


17/51

Basic Page Replacement


18/51

Page Replacement Algorithms

GOAL: lowest page-fault rate

Evaluate algorithm by running it

on a particular string of memory

references (reference string) andcomputing the number of page

faults on that string

Example string: 1,4,1,6,1,6,1,6,1,6,1


19/51


20/51

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 perprocess)

4 frames

FIFO ReplacementBeladys Anomaly

more frames more 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

2

4

5 10 page faults

44 3


21/51

FIFO Page Replacement


22/51

FIFO Illustrating Beladys Anomaly


23/51

Optimal Algorithm Goal: 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:

Well, is it at least 4% as good as Optimal Algorithm?

1

2

3

4

6 page faults

4 5


24/51

Optimal Page Replacement

Optimal: 9 faultsFIFO: 15 faults67% increase over the optimal


25/51

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 isreferenced through this entry: counter = clock

When a page needs to be changed, look at the counters todetermine which are to change

1

2

3 4

4 3

5

5


26/51

LRU Page Replacement


27/51

LRU Algorithm (Cont.)

Stack implementationkeep a stack of pagenumbers in a double link form:

Page referenced:

move it to the top (most recently used)

Worst case: 6 pointers to be changed No search for replacement

A B CHead

Tail

B A CHead

Tail

1

2

3 5

4

Also Bs previous link


28/51

Use Of A Stack to Record The Most

Recent Page References


29/51

LRU Approximation Algorithms

Reference bit

With each page associate a bit, initially =0

When page is referenced bit set to 1

Replacement: choose something with 0(if one exists). We do not know the order,however.

Second chance (Clock replacement)

Need reference bit

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


30/51

Second-Chance (clock) Page-Replacement Algorithm


31/51

Counting Algorithms

Keep a counter of the number ofreferences that have been madeto each page

LFU Algorithm: replaces pagewith smallest count- indicates an actively used page

MFU Algorithm: based on theargument that the page with thesmallest count was probably justbrought in and has yet to be used


32/51

Allocation of Frames

Each process needs minimumnumber of pages: depends oncomputer architecture

Example: IBM 3706 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


33/51

Fixed Allocation Equal allocationFor example, if there are 100

frames and 5 processes, give each process 20frames.

Proportional allocationAllocate according to the

size of process

m

S

spa

m

sS

ps

iii

i

ii

forallocation

framesofnumbertotal

processofsize

5964137

127

564137

10

127

10

64

2

1

2

a

a

s

s

m

i


34/51

Global vs. Local Allocation

Global replacementprocessselects a replacement frame fromthe set of all frames - one

process can take a frame from

another Con: Process is unable to control its own

page-fault rate.

Pro: makes available pages that are less

used pages of memory

Local replacementeachprocess selects from only its own

set of allocated frames


35/51

Thrashing

Number of frames < minimumrequired for architecturemust

suspend process Swap-in, swap-out level of intermediate CPU

scheduling

Thrashinga process is busy

swapping pages in and out


36/51

Thrashing

Consider this:

CPU utilization lowincrease processes

A process needs morepagesgets them from

other processes Other process must

swap intherefore wait

Ready queue shrinks

therefore system thinksit needs moreprocesses


37/51

Demand Paging and Thrashing

Why does demand paging work?

Locality model

as a process executes it moves from

locality to locality Localities may overlap

Why does thrashing occur?

size of locality > total memory sizeall localities added together

Locality In


38/51

Locality In

A Memory-

ReferencePattern


39/51

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

page referencesExample: 10,000 instruction

WSSi(working set of Process Pi) =total number of pages referenced in the mostrecent (varies in time)

if too small will not encompass entirelocality

if too large will encompass several localities

if = will encompass entire program

D= WSSitotal demand frames

if D> mThrashing

Policy if D> m, then suspend one of theprocesses


40/51

Working-set model


41/51

Keeping Track of the Working Set

Approximate WS with interval timer + a reference bit

Example: = 10,000 references

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 timeunits


42/51

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


43/51

Memory-Mapped Files Memory-mapped file I/O allows file I/O to be treated as

routine memory access by mappinga disk block to a page

in memory

How?

A file is initially read using demand paging. A page-

sized portion of the file is read from the file system into a

physical page.

Subsequent reads/writes to/from the file are treated as

ordinary memory accesses.

Simplifies file access by treating file I/O through memory

rather than read()write()system calls (less overhead)

Sharing: Also allows several processes to map the same fileallowing the pages in memory to be shared


44/51

Memory Mapped Files


45/51

WIN32 API

Steps:1. Create a file mapping for the file

2. Establish a view of the mapped file in theprocesss virtual address space

A second process can the open andcreate a view of the mapped file in its

virtual address space

O


46/51

Other Issues -- Prepaging

Prepaging

To reduce the large number of page faults thatoccurs at process startup

Prepage all or some of the pages a process willneed, before they are referenced

But if prepaged pages are unused, I/O andmemory was wasted

Assume spages are prepaged and of thepages is used

Is cost of s * save pages faults > or < thanthe cost of prepagings * (1- ) unnecessary pages?

near zero prepaging loses


47/51

Other IssuesPage Size

Page size selection must take

into consideration:

fragmentation

table size

I/O overhead locality


48/51

Other IssuesProgram Structure

Program structure

Int[128,128] data;

Each row is stored in one page

Program 1

for (j = 0; j


49/51

Other IssuesI/O interlock

I/O InterlockPages mustsometimes be locked intomemory

Consider I/O. Pages that areused for copying a file from adevice must be locked from

being selected for eviction by apage replacement algorithm.

Reason Why Frames Used For I/O


50/51

Reason Why Frames Used For I/O

Must Be In Memory


51/51

Other IssuesTLB Reach

TLB Reach - The amount of memory accessible fromthe TLB

TLB Reach = (TLB Size) X (Page Size)

Ideally, the working set of each process is stored in the

TLB. Otherwise there is a high degree of page faults.

Increase the Page Size. This may lead to an increase infragmentation as not all applications require a large

page size

Provide Multiple Page Sizes. This allows applications

that require larger page sizes the opportunity to use

them without an increase in fragmentation.

virtual memory os notes

Documents