Lecture9:DemandPagingCSE120:PrinciplesofOpera>ngSystems

UCSanDiego:SummerSessionI,2009FrankUyeda

Announcements

•  Project2isdueonTonight•  Homework3isdueMonday.

•  FinalExam:3p‐6ponSaturday,August1

2

PeerWise

•  Processesareloadedandthenremovedfrommemory,crea>ngfreememoryspacebrokenintoliTlepieces.Whattypeoffragmenta>onisthis?–  Internal– External

3

PeerWise

•  Whatisthedifferencebetween"Segmen>on"and"Paging"?(Choosetheonewiththehighesttruthvalue)–  A.Lehastheanswer.–  B.Segmenta>onisnon‐con>nousalloca>onofpagesof

memory,wherepagingiscon>nousalloca>onofpagesofmemory.

–  C.Segmenta>onistheuseofvariablepagesizes,andpagingistheuseofdiscretepagesizes.

–  D.C–  E.D&A

•  Really?!–  Noneoftheanswersarecorrect…except,maybeA.

4

Recap:Paging

55

Page1

Page2

Page3

PageP

PhysicalMemoryPage1

Page2

Process1’sVAS

…..

0x00000000

0x00040000

0x00000400

...

PageN

..

PageM

Page3

…..

Page1

Page2

Process2’sVAS

PageQ

Page3

MMU

TLB

PageTable

V PageFrame

1 0x04

1 0x01

1 0x05

1 0x07

V PageFrame

1 0x04

1 0x01

1 0x05

1 0x07

Recap:VirtualMemory

•  MemoryManagementUnit(MMU)–  Hardwareunitthattranslatesavirtualaddresstoaphysicaladdress

•  Transla>onTable–  Storedinmainmemory–  EachPageTableEntrystorespermissionandbookkeepingbits.

•  Transla>onLookasideBuffer(TLB)–  HardwarecachefortheMMU’svirtual‐to‐physicaltransla>onstable

6

CPUTransla>on

TableMMU

Memory

VirtualAddress

PhysicalAddress

TLB

PageFrameNumberProtVRM

Recap:PageTables

7

Page1

Page2

Page3

PageN

PhysicalMemory…..

Pageframe Offset

PhysicalAddress

Pagenumber Offset

VirtualAddress

Pagetableentry

PageTable

0xFFFFFFFF

0x00000000

Problem:PageTablescanbelarge!

Recap:2LevelPageTable

8

Page1

Page2

Page3

PageN

PhysicalMemory…..

Pageframe Offset

PhysicalAddress

Secondary Offset

VirtualAddress

SecondaryPageTable(s)

0xFFFFFFFF

0x00000000

Master

Pagetableentry

MasterPageTable

Pageframe(PTE)Pageframe(PTE)

InvertedPageTable

9

Page1

Page2

Page3

PageN

PhysicalMemory…..

index Offset

PhysicalAddress

VPN Offset

VirtualAddress

0xFFFFFFFF

0x00000000

PID

<PID,VPN>

InvertedPageTable

index

InvertedPageTable

•  Oneglobalpagetable–  Onepagetableentryperphysicalpage.–  EntrieskeyedbyPIDandvirtualpagenumber.–  Physicalframenumber=indexinpagetable.

•  Advantages–  Boundedamountofmemoryforpagetable(s).

•  32‐bitaddressspace,4Kpages,4GBRAM,4BperPTE•  1‐levelpagetable:_____x#processes.•  InvertedTable:______MB

•  Disadvantages–  Costlytransla>on

•  Usinghashingcanhelp

10

Recap:Segmenta>on

11

Heap

Text

Data

Stack

Recap:Segmenta>on

12

PhysicalMemorySegmentTable

limit baseVirtualAddress

Segment# Offset

< +Yes?

No?

Protec>onFault

GoalsforToday

•  PagingTricks– DemandPaging

– CopyonWrite

•  PageReplacementPolicies– Whichpagesshouldwekeepinmemory?– Howmanypagesshouldeachprocessget?

13

PagedVirtualMemory•  Whatifthereisn’tenoughphysicalmemoryavailableto

loadaprogram?–  Alargepor>onofprogram’scodemaybeunused–  Alargepor>onofprogram’scodemaybeusedinfrequently

•  Idea:loadapageonlyifyouneeditProcess’sVirtualAddressSpace Machine’sPhysicalAddressSpace

(availableRAM)

14

DemandPaging

•  We’vemen>onedbeforethatpagescanbemovedbetweenmemoryanddisk–  Thisprocessiscalleddemandpaging

•  OSusesmainmemoryasapagecacheofallthedataallocatedbyprocessesinthesystem–  Ini>ally,pagesareallocatedfrommemory– Whenmemoryfillsup,alloca>ngapageinmemoryrequiressomeotherpagetoevictedfrommemory

•  Whyphysicalmemorypagesarecalled“frames”–  Evictedpagesgotodisk(where?Theswapfilepar>>on)–  ThemovementofpagesbetweenmemoryanddiskisdonebytheOS,andistransparenttotheapplica>on

15

PagedVirtualMemory•  Aprogram’spagesloadedondemand•  Wherearepagesotherwise?

–  Ondisk(executable)–  InOSswapfile(anactualfileondisk)–  Shared(morelater)

Process’sVirtualAddressSpace

Machine’sPhysicalAddressSpace(availableRAM)

/swap~/program.coff

Disk

16

DemandPagingExampleProcess’sVirtualAddressSpace PhysicalMemory

/swap~/program.coff

Disk

PC

…

…

PC

0x00 1 0x000x01 0 ‐‐‐0x02 0 ‐‐‐0x03 0 ‐‐‐

0x11 0 ‐‐‐0x10 0 ‐‐‐

…

VPN V PPNPageTable

0x01 1 0x01

17

DemandPagingExampleProcess’sVirtualAddressSpace PhysicalMemory

/swap~/program.coff

Disk

…

…

PC

0x00 1 0x000x01 0 ‐‐‐0x02 1 0x020x03 1 0x03

0x11 0 ‐‐‐0x10 1 0x09

…

VPN V PPNPageTable

0x01 1 0x01

0x10 0 0x00??

0x04 1 0x09

0x00

0x090x0A

18

PageFaults•  Whathappenswhenaprocessaccessesapagethathasbeen

evicted(orisotherwisenotinmemory)?–  1.Whenitevictsapage,theOSsetsthePTEasinvalidand

storestheloca>onofthepageintheswapfileinthePTE–  2.Whenaprocessaccessesthepage,theinvalidPTEwill

causeatrap(pagefault)–  3.ThetrapwillruntheOSpagefaulthandler–  4.HandlerusestheinvalidPTEtolocatepageinswapfile–  5.Readspageintoaphysicalframe,updatesPTEtopointtoit–  6.Restartsprocess

•  Butwheredoesitputit?Hastoevictsomethingelse–  OSusuallykeepsapooloffreepagesaroundsothatalloca>onsdonot

alwayscauseevic>ons–  Replaceanotherpage

•  Evictedpagemaygoinswapfile.WhichpagesneedtobewriTenouttoswap?

19

Recap:PageLookups

20

CPU

PageTable

Memory

VirtualAddress

PhysicalAddress

TLB

Disk

Recap:PageLookups

21

CPU

PageTable

Memory

VirtualAddress

PhysicalAddress

TLB

Disk

SecondaryPageTable

.....

Recap:PageLookups

22

CPU

PageTable

Memory

VirtualAddress

PhysicalAddress

TLB

Disk

SecondaryPageTable

Case1:VirtualAddress–to‐PhysicalAddressMappingisinTLB

VPN V R M RO PFN

0x000 1 1 0 1 0x100

0x121 1 0 0 0 0x111

0x071 1 1 0 0 0x212

Howmanymemoryreferences?TLB_hit:1memoryreference

(1)

Recap:PageLookups

23

CPU

PageTable

Memory

VirtualAddress

PhysicalAddress

TLB

Disk

SecondaryPageTable

Case2:VirtualAddress–to‐PhysicalAddressMappingisnotinTLBbutisinMemory

VPN V R M RO PFN

0x000 1 1 0 1 0x100

0x121 1 0 0 0 0x111

0x071 1 1 0 0 0x212

Howmanymemoryreferences?Mem_hit:1pagetablelookup +1memoryreferenceTwo‐levelPagetable?

(1)

(2)

Recap:PageLookups

24

CPU

PageTable

Memory

VirtualAddress

PhysicalAddress

TLB

Disk

SecondaryPageTable

Case3:VirtualAddress–to‐PhysicalAddressMappingisnotinTLBandnotinMemory

VPN V R M RO PFN

0x000 1 1 0 1 0x100

0x121 0 0 0 0 0x111

0x071 1 1 0 0 0x212

Howmanymemoryreferences?Mem_miss:1pagetablelookup +1pagetableupdate(s)* +1memoryreference

Howmuch6me?diskretrievalinvolvedtoo

(1)

(2)

(3?)

(4?)

MemoryAccesses

•  TLBHit:fast–  1TLBaccess+1memoryaccess

•  MemoryHit:medium–  1TLBaccess+2+memoryaccess

•  MemoryMiss:slow–  1TLBaccess+3+memoryaccess+1diskaccess

•  Wantthingsfastaspossible;minimize>mespentresolvingphysicaladdress.Howdoyoudothis?–  Minimizenumberofmemoryreferences(TLBpolicy)–  Minimizenumberofpagefaults(Pagereplacementpolicy)

25

Observa>on

•  OSesspendalotof>mecopyingdata– Systemcallargumentsbetweenuser/kernelspace

– En>readdressspacestoimplementfork()

•  Howcanwereducethiscost?

26

PageSharingEmacsbuffer#1

PhysicalMemory

0x00 1 0x03

0x01 0 ‐‐‐0x10 1 0x120x11 1 0x15

VPN V PPN

0x03 1 0x11

0x00 1 0x03

0x01 0 ‐‐‐0x10 1 0x140x11 1 0x16

VPN V PPN

0x03 1 0x13

Emacsbuffer#2

..

Whatifsharedpageismodified?

0x04

27

Copy‐On‐Write

•  OSesspendalotof>mecopyingdata–  Systemcallargumentsbetweenuser/kernelspace–  En>readdressspacestoimplementfork()

•  UseCopy‐on‐Write(CoW)todeferlargecopiesaslongaspossible,hopingtoavoidthemaltogether–  Insteadofcopyingpages,createsharedmappingsofparentpagesinchildvirtualaddressspace

–  Sharedpagesareprotectedasread‐onlyinchild•  Readshappenasusual•  Writesgenerateaprotec>onfault,traptoOS,copypage,changepagemappinginclientpagetable,restartwriteinstruc>on

•  Howdoesthishelpfork()?

28

PageSharing:CoWEmacsbuffer#1

PhysicalMemory

0x00 1 0x03

0x01 0 ‐‐‐0x10 1 0x120x11 1 0x15

VPN V PPN

0x03 1 0x11

Emacsbuffer#2

..

0M

000

0x00 1 0x03

0x01 0 ‐‐‐0x10 1 0x120x11 1 0x16

VPN V PPN

0x03 1 0x13

0M

000

BuffersshareVPN0x12Processwritestopage

1

29

PageSharing:CoWEmacsbuffer#1

PhysicalMemory

0x00 1 0x03

0x01 0 ‐‐‐0x10 1 0x120x11 1 0x15

VPN V PPN

0x03 1 0x11

Emacsbuffer#2

..

0M

000

0x00 1 0x03

0x01 0 ‐‐‐0x10 1 0x140x11 1 0x16

VPN V PPN

0x03 1 0x13

0M

000

BuffersshareVPN0x12ProcesswritestopageNewpageisallocated

30

PagingTricksSummary

•  DemandPaging– Observa>on:LimitedavailableRAM,notallpagesused

–  Idea:Loadpagesonlywhenneeded•  PageSharing

– Observa>on:Ozen>mes,processeshavesimilarvirtualpages

–  Idea:Copy‐on‐writeforefficientpagesharing

31

ActualMemorySta>s>cs3.7GBused,4.1GBavailable=>90+%memoryused2.1GBswapallocated

32

ActualMemorySta>s>cs~10%‐50%ofmemoryissharedperprocess=>~370MB–1.85GBmemsaved

33

ActualMemorySta>s>csWhichpagestoevict?Howmuchphysicalmemoryshouldeachprocessget?

34

PageReplacement

•  Problem:Memoryisscarce–  Idea:let’suseonlypagesthatweneed

•  Pagereplacementpolicy:– Howdowedecidewhichpagestokickoutwhenmemoryisfullornearfull?

– Howmuchmemorydoweallocatetoeachprocess?

35

PageReplacementPolicy

•  Wanttokeepmostac>vepagesinmemory– Conversely,wanttokickoutinac6vepages

•  Whatcanwedo?– Swapping:suspendanen>reprocessandputsallofitspagestodisk/swap

– Paging:removeunusedorinfrequentlyusedpagesforanyprocesstodisk/swap

36

Goal:Evic>ngtheBestPage

•  Thegoalofthereplacementalgorithmistoreducethepagefaultrate–  Selectthebestvic>mpagetoremove

•  Thebestpagetoevictistheonenevertouchedagain–  Processwillneveraskforit,soitwillneverfaultonit

•  Howdoweknowthatthereabestpage?•  Neverisalong>me,sopickingthepageclosestto

“never”isthenextbestthing–  Evic>ngthepagethatwon’tbeusedforthelongestperiodof

>meminimizesthenumberofpagefaults–  ProvedbyBelady

37

Locality

•  Allpagingschemesdependonlocality–  ProcessesreferencepagesinlocalizedpaTerns

•  Temporallocality–  Loca>onsreferencedrecentlylikelytobereferencedagain

•  Spa>allocality–  Loca>onsnearrecentlyreferencedloca>onsarelikelytobereferencedsoon

•  Althoughthecostofpagingishigh,ifitisinfrequentenoughitisacceptable–  Processesusuallyexhibitbothkindsoflocalityduringtheirexecu>on,makingpagingprac>cal

38

Goal:Evic>ngtheBestPage

•  Thegoalofthereplacementalgorithmistoreducethepagefaultrate–  Selectthebestvic>mpagetoremove

•  Thebestpagetoevictistheonenevertouchedagain–  Processwillneveraskforit,soitwillneverfaultonit

•  Howdoweknowthatthereabestpage?Locality•  Neverisalong>me,sopickingthepageclosestto

“never”isthenextbestthing–  Evic>ngthepagethatwon’tbeusedforthelongestperiodof

>meminimizesthenumberofpagefaults–  ProvedbyBelady

39

Belady’sAlgorithm

•  Belady’salgorithmisknownastheop>malpagereplacementalgorithm–  Lowestfaultrateforanypagereferencestream–  Idea:replacethepagethatwillnotbeusedforthelongest>me

inthefuture–  Problem:havetopredictthefuture

•  WhyisBelady’susefulthen?Useitasayards>ck–  Compareimplementa>onsofpagereplacementalgorithmswith

theop>maltogaugeroomforimprovement–  Ifop>malisnotmuchbeTer,thenalgorithmispreTygood–  Ifop>malismuchbeTer,thenalgorithmcouldusesomework

•  Randomreplacementisozenthelowerbound•  Butrandomisnotnecessarilytheworst…..whatwouldbe?

Pagereferences:

1 2 3 4 1 2 5 1 2 3 4 5

1 1 1 1 1 1 1 1 2 3 4 4

2 3 4 4 2 5 5 5 5 5 52PagesRAM

9pagefaults

40

Belady’sAlgorithm

•  Belady’salgorithmisknownastheop>malpagereplacementalgorithm–  Lowestfaultrateforanypagereferencestream–  Idea:replacethepagethatwillnotbeusedforthelongest>me

inthefuture–  Problem:havetopredictthefuture

•  WhyisBelady’susefulthen?Useitasayards>ck–  Compareimplementa>onsofpagereplacementalgorithmswith

theop>maltogaugeroomforimprovement–  Ifop>malisnotmuchbeTer,thenalgorithmispreTygood–  Ifop>malismuchbeTer,thenalgorithmcouldusesomework

•  Randomreplacementisozenthelowerbound•  Butrandomisnotnecessarilytheworst…..whatwouldbe?

41

First‐InFirst‐Out(FIFO)

•  FIFOisanobviousalgorithmandsimpletoimplement– Maintainalistofpagesinorderinwhichtheywerepagedin

–  Onreplacement,evicttheonebroughtinlongest>meago

•  Whymightthisbegood?– Maybetheonebroughtinthelongestagoisnotbeingused

•  Whymightthisbebad?–  Thenagain,maybeit’snot

•  Wedon’thaveanyinforma>ontosayonewayoranother

42

FIFOPageReplacementExamplePagereferences:

1 2 3 4 1 2 5 1 2 3 4 5

1 1 3 3 1 1 5 5 2 2 4 4

2 2 4 4 2 2 1 1 3 3 52PagesRAM

3PagesRAM

4PagesRAM

12pagefaults

43

FIFOPageReplacementExamplePagereferences:

1 2 3 4 1 2 5 1 2 3 4 5

1 1 3 3 1 1 5 5 2 2 4 4

2 2 4 4 2 2 1 1 3 3 5

1 1 1 4 4 4 5 5 5 5 5 5

2 2 2 1 1 1 1 1 3 3 3

3 3 3 2 2 2 2 2 4 4

2PagesRAM

3PagesRAM

4PagesRAM

12pagefaults

9pagefaults

44

FIFOPageReplacementExamplePagereferences:

1 2 3 4 1 2 5 1 2 3 4 5

1 1 3 3 1 1 5 5 2 2 4 4

2 2 4 4 2 2 1 1 3 3 5

1 1 1 4 4 4 5 5 5 5 5 5

2 2 2 1 1 1 1 1 3 3 3

3 3 3 2 2 2 2 2 4 4

1 1 1 1 1 1 5 5 5 5 4 4

2 2 2 2 2 2 1 1 1 1 5

3 3 3 3 3 3 2 2 2 2

4 4 4 4 4 4 3 3 3

2PagesRAM

3PagesRAM

4PagesRAM

12pagefaults

9pagefaults

10pagefaults!

45

First‐InFirst‐Out(FIFO)

•  FIFOsuffersfrom“Belady’sAnomaly”– Thefaultratemightactuallyincreasewhenthealgorithmisgivenmorememory(verybad)

46

LeastRecentlyUsed(LRU)

•  LRUusesreferenceinforma>ontomakeamoreinformedreplacementdecision–  Idea:Wecan’tpredictthefuture,butwecanmakeaguessbaseduponpastexperience

– Onreplacement,evictthepagethathasnotbeenusedforthelongest>meinthepast(Belady’s:future)

– WhendoesLRUdowell?WhendoesLRUdopoorly?•  Implementa>on

–  Tobeperfect,needto>mestampeveryreference(ormaintainastack)–muchtoocostly

–  Soweneedtoapproximateit

47

LRUPageReplacementExamplePagereferences:

1 2 3 4 1 2 5 1 2 3 4 5

1 1 3 3 1 1 5 5 2 2 4 4

2 2 4 4 2 2 1 1 3 3 52PagesRAM

3PagesRAM

4PagesRAM

12pagefaults

48

LRUPageReplacementExamplePagereferences:

1 2 3 4 1 2 5 1 2 3 4 5

1 1 3 3 1 1 5 5 2 2 4 4

2 2 4 4 2 2 1 1 3 3 5

1 1 1 4 4 4 5 5 5 3 3 3

2 2 2 1 1 1 1 1 1 4 4

3 3 3 2 2 2 2 2 2 5

1 1 1 1 1 1 1 1 1 1 1 5

2 2 2 2 2 2 2 2 2 2 2

3 3 3 3 5 5 5 5 4 4

4 4 4 4 4 4 3 3 3

2PagesRAM

3PagesRAM

4PagesRAM

12pagefaults

10pagefaults

8pagefaults

49

LeastRecentlyUsed(LRU)

•  LRUusesreferenceinforma>ontomakeamoreinformedreplacementdecision–  Idea:Wecan’tpredictthefuture,butwecanmakeaguessbaseduponpastexperience

– Onreplacement,evictthepagethathasnotbeenusedforthelongest>meinthepast(Belady’s:future)

– WhendoesLRUdowell?WhendoesLRUdopoorly?•  Implementa>on

–  Tobeperfect,needto>mestampeveryreference(ormaintainastack)–muchtoocostly

–  Soweneedtoapproximateit

50

Approxima>ngLRU

•  LRUapproxima>onsusethePTEreferencebit–  Keepacounterforeachpage–  Atregularintervals,foreverypagedo:

•  Ifrefbit=0,incrementcounter

•  Ifrefbit=1,zerothecounterandthereferencebit–  Thecounterwillcontainthenumberofintervalssincethelastreferencetothepage

–  Thepagewiththelargestcounteristheleastrecentlyused•  Somearchitecturesdon’thaveareferencebit

–  Cansimulatereferencebitusingthevalidbittoinducefaults–  Whathappenswhenwemakeapageinvalid?

51

Approxima>ngLRU

VPN PFN CNTRR

0x00

0x01

0x02

0x03

0x04

0x05

0xA1

0xA2

0xA0

0x10

0x03

0x04

0

0

0

0

0

0

0000

0000

0000

0000

0000

0000

PageTable

CNTR

0001

0001

0001

0001

0001

0001

Interval1

CNTR

0002

0000

0000

0002

0002

0000

Interval2

CNTR

0003

0001

0001

0003

0003

0001

Interval3

CNTR

0004

0000

0000

0004

0004

0002

Interval4

0

1

1

0

0

1

0

0

0

0

0

0

0

1

1

0

0

0

Whichpage(s)wouldbeevictednextbasedonApproximateLRU?Whatdoesagoodapproxima>ondependupon?

52

Approxima>ngLRU

•  LRUapproxima>onsusethePTEreferencebit–  Keepacounterforeachpage–  Atregularintervals,foreverypagedo:

•  Ifrefbit=0,incrementcounter

•  Ifrefbit=1,zerothecounterandthereferencebit–  Thecounterwillcontainthenumberofintervalssincethelastreferencetothepage

–  Thepagewiththelargestcounteristheleastrecentlyused•  Somearchitecturesdon’thaveareferencebit

–  Cansimulatereferencebitusingthevalidbittoinducefaults–  Whathappenswhenwemakeapageinvalid?

53

Approxima>ngLRU:Clock

•  NotRecentlyUsed(NRU)–UsedbyUnix–  Replacepagethatis“oldenough”–  Arrangeallofphysicalpageframesinabigcircle(clock)–  AclockhandisusedtoselectagoodLRUcandidate

•  Sweepthroughthepagesincircularorderlikeaclock•  Iftherefbitisoff,ithasn’tbeenusedrecently

–  Whatistheminimum“age”ifreferencebitisoff?

•  Ifthereferencebitison,turnitoffandgotonextpage–  Armmovesquicklywhenpagesareneeded–  Lowoverheadwhenplentyofmemory–  Ifmemoryislarge,“accuracy”ofinforma>ondegrades

•  Useaddi>onalhands

54

Approxima>ngLRU:Clock

VPN PFN R

0x00

0x01

0x02

0x03

0xA1

0xA2

0xA0

0x10

1

1

1

1

PageTable

ClockHand

(1)

VPN PFN R

0x00

0x01

0x02

0x03

0xA1

0xA2

0xA0

0x10

0

1

1

1

(2)

VPN PFN R

0x00

0x01

0x02

0x03

0xA1

0xA2

0xA0

0x10

0

0

1

1

(3)

VPN PFN R

0x00

0x01

0x02

0x03

0xA1

0xA2

0xA0

0x10

0

0

0

1

(4)

VPN PFN R

0x00

0x01

0x02

0x03

0xA1

0xA2

0xA0

0x10

0

0

0

0

(5)

Evictpage!

JustlikeFIFO,dependingonclockhand?55

Approxima>ngLRU:Clock

VPN PFN R

0x00

0x01

0x02

0x03

0xA1

0xA2

0xA0

0x10

1

0

0

1

PageTable

ClockHand

(1)

VPN PFN R

0x00

0x01

0x02

0x03

0xA1

0xA2

0xA0

0x10

0

0

1

1

(2)

Evictpage!

Whatdoyouneedforthisalgorithmtoworkwell?Whatifmemoryislarge?

56

Approxima>ngLRU:Clock

•  NotRecentlyUsed(NRU)–UsedbyUnix–  Replacepagethatis“oldenough”–  Arrangeallofphysicalpageframesinabigcircle(clock)–  AclockhandisusedtoselectagoodLRUcandidate

•  Sweepthroughthepagesincircularorderlikeaclock•  Iftherefbitisoff,ithasn’tbeenusedrecently

–  Whatistheminimum“age”ifreferencebitisoff?

•  Ifthereferencebitison,turnitoffandgotonextpage–  Armmovesquicklywhenpagesareneeded–  Lowoverheadwhenplentyofmemory–  Ifmemoryislarge,“accuracy”ofinforma>ondegrades

•  Useaddi>onalhands

57

Today’slecture:PageReplacement(2)

•  Problem:Memoryisscarce–  Idea:let’suseonlypagesthatweneed

•  Pagereplacementpolicy:– Howdowedecidewhichpagestokickoutwhenmemoryisfullornearfull?

– Howmuchmemorydoweallocatetoeachprocess?

58

PageReplacementPolicies

•  Inamul>programmingsystem,weneedawaytoallocatememorytocompe>ngprocesses

•  Problem:Howtodeterminehowmuchmemorytogivetoeachprocess?–  Fixedspacealgorithms

•  Eachprocessisgivenalimitofpagesitcanuse•  Whenitreachesthelimit,itreplacesfromitsownpages•  Localreplacement

–  Someprocessesmaydowellwhileotherssuffer

–  Variablespacealgorithms•  Process’setofpagesgrowsandshrinksdynamically•  Globalreplacement

–  Oneprocesscanruinitfortherest

59

WorkingSetModel

•  Aworkingsetofaprocessisusedtomodelthedynamiclocalityofitsmemoryusage–  DefinedbyPeterDenninginthe60s

•  Defini>on–  WS(t,w) = { pages P s.t. P was referenced in the time

interval (t,t+w) } –  t = time, w = working-set window (measuredinpagereferences)

•  ApageisintheworkingsetWSonlyifitwasreferencedinthelastwreferences

60

WorkingSetSize

•  Workingsetsizeisthenumberofpagesintheworkingset–  Thenumberofpagesreferencedintheinterval(t,t+w)

•  Theworkingsetsizechangeswithprogramlocality–  Duringperiodsofpoorlocality,youreferencemorepages–  Withinthatperiodof>me,theworkingsetsizeislarger

•  Intui>vely,wanttheworkingsettobethesetofpagesaprocessneedsinmemorytopreventheavyfaul>ng–  Eachprocesshasaparameterwthatdeterminesaworkingset

withfewfaults–  Denning:Don’trunaprocessunlessworkingsetisinmemory

61

WorkingSetProblems

•  Problems– Howdowedeterminew?– Howdoweknowwhentheworkingsetchanges?

•  Toohardtoanswer–  So,workingsetisnotusedinprac>ceasapagereplacementalgorithm

•  However,itiss>llusedasanabstrac>on–  Theintui>oniss>llvalid– Whenpeopleask,“HowmuchmemorydoesiTunesneed?”,theyareineffectaskingforthesizeofitsworkingset.

62

PageFaultFrequency(PFF)

•  PageFaultFrequency(PFF)isavariablespacealgorithmthatusesamoread‐hocapproach–  1.Monitorthefaultrateforeachprocess–  2.Ifthefaultrateisaboveahighthreshold:giveitmorememory

•  Hopethatitfaultsless•  Notalwaystrue(FIFO+Belady’sAnomaly)

–  3.Elseifthefaultrateisbelowalowthreshold:takeawaymemory

•  Hopethatits>lldoesn’tfaultmuch•  Mightnotfaultmoreatall

•  HardtousePFFtodis>nguishbetweenchangesinlocalityandchangesinsizeofworkingset

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PFFandMul>programming•  Whatdoesthepagefaultfrequencyofprocessestellyou?

Degreeofmul>programming(#concurrentprocesses)

Canthishappen?

System“goodput”(#ofprograms’instruc>onsexecuted)

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Thrashing

•  Pagereplacementalgorithmsavoidthrashing– Whenmostofthe>meisspendbytheOSinpagingdatabackandforthfromthedisk

–  No>mespentdoingusefulwork(makingprogress)–  Inhissitua>on,thesystemisovercommiTed

•  Noideawhichpagesshouldbeinmemorytoreducefaults•  Couldjustbethatthereisn’tenoughphysicalmemoryforalloftheprocessesinthesystem

•  Ex:RunningWindowsXPwith64MBmemory…(orVistawith1GB)

–  Possiblesolu>ons•  Swapping–writeoutallpagesofaprocess•  Buymorememory

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PageReplacementSummary•  Whichpage(s)tokickoutwhenmemoryisunderconstraint?

–  Pagereplacementalgorithms•  Belady’s–op>malreplacement(min#faults)•  FIFO–replacepageloadedfurthestinpast•  LRU–replacepagereferencedfurthestinpast•  LRUClock–replacepagethatis“oldenough”•  PageFaultFrequency–grow/shrinkpagesetasafunc>onoffaultrate

•  Howmanypagesperprocess?–  Globalvslocalreplacementpolicies

•  Shouldaprocessreplaceitsownpage,orthatofanother?–  WorkingSet–keepthesetofpagesinmemorythathasminimalfault

rate(the“workingset”)•  PageFaultFrequency–grow/shrinkpagesetasafunc>onoffaultratetoavoid

thrashing

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NextTime

•  ReadChapter10&11.1‐11.8•  HomeworkdueonMonday.

•  CheckWebsiteforcourseannouncements–  hTp://www.cs.ucsd.edu/classes/su09/cse120

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