paging - swarthmore college solution is paging. paging vocabulary • for each process, the virtual...

Paging11/10/16

RecallfromTuesday

Oursolutiontofragmentationistosplitupaprocess’saddressspaceintosmallerchunks.

Process1

OS

Process2

Process1

Process3

Process2

PhysicalMemory

OS:Place

Process3

Process3

Process3

Process3

RecallfromTuesday

Wesupportvirtualaddressingbytranslatingaddressesatruntime.

Wecan’tachievethisusingbaseandboundregistersunlesseachprocess’smemoryisallinoneblock. P2

0

N2-1

P2

P1

P3

0

N-1

Base +

<

Bound

y/n?

Howcanwedoboth?

• Wewanttosupporttranslationofvirtualaddressestophysicaladdresseson-the-fly.

• Wewanttosplitupeachprocess’saddressspacetousephysicalmemorymoreefficiently.

Thesolutionispaging.

PagingVocabulary

• Foreachprocess,thevirtual addressspaceisdividedintofixed-sizepages.

• Forthesystem,thephysical memoryisdividedintofixed-sizeframes.

• Thesizeofapageisequaltothatofaframe.• Often4KBinpractice.

MainIdea

• ANYvirtualpagecanbestoredinanyavailableframe.• Makesfindinganappropriately-sizedmemorygapveryeasy– they’reallthesamesize.

• Foreachprocess,OSkeepsatablemappingeachvirtualpagetophysicalframe.

MainIdea

• ANYvirtualpagecanbestoredinanyavailableframe.• Makesfindinganappropriately-sizedmemorygapveryeasy– they’reallthesamesize.

PhysicalMemory

VirtualMemory

(OSMapping)Implicationsforfragmentation?

External:goesaway.Nomoreawkwardly-sized,unusablegaps.

Internal:Aboutthesame.Processcanalwaysrequestmemoryandnotuseit.

Addressing

• Likewedidwithcaching,we’regoingtochopupmemoryaddressesintopartitions.

• Virtualaddresses:• High-orderbits:page#• Low-orderbits:offsetwithinthepage

• Physicaladdresses:• High-orderbits:frame#• Low-orderbits:offsetwithintheframe

Example:32-bitvirtualaddresses

• Supposewehave8-KB(8192-byte)pages.• Weneedenoughbitstoindividuallyaddresseachbyteinthepage.• Howmanybitsdoweneedtoaddress8192items?


• Supposewehave8-KB(8192-byte)pages.• Weneedenoughbitstoindividuallyaddresseachbyteinthepage.• Howmanybitsdoweneedtoaddress8192items?• 213 =8192,soweneed13bits.• Lowest13bits:offsetwithinpage.


• Supposewehave8-KB(8192-byte)pages.• Weneedenoughbitstoindividuallyaddresseachbyteinthepage.• Howmanybitsdoweneedtoaddress8192items?• 213 =8192,soweneed13bits.• Lowest13bits:offsetwithinpage.

• Remaining19bits:pagenumber.

We’llcallthesebitsp. We’llcallthesebitsi.

We’llcallthesebitsp. We’llcallthesebitsi.

OSPageTableForProcess

Virtualaddress:

Physicaladdress:

We’ll(still)callthesebitsi.We’llcallthesebitsf.

Whereisthispageinphysicalmemory?(Inwhichframe?)

Oncewe’vefoundtheframe,whichbyte(s)dowewanttoaccess?

AddressPartitioning

PhysicalAddress

LogicalAddressPagep Offseti

FrameV Perm …R D

PhysicalMemory

PageTable

AddressTranslation

PageTable• Onetableperprocess• Tableentryelements• V:validbit• R:referencedbit• D:dirtybit• Frame:locationinphy mem• Perm:accesspermissions

• Tableparametersinmemory• Pagetablebaseregister• Pagetablesizeregister

FrameV Perm …PTBRPTSR

R D

• Virtualaddress=p+i• Physicaladdress=f+i• First,doaseriesofchecks

LogicalAddressPagep Offseti

PhysicalAddress

FrameV Perm …R D

AddressTranslation

CheckifPagep isWithinRangeLogicalAddress

Pagep

PTBRPTSR

p <PTSR

Offseti

PhysicalAddress

FrameV Perm …R D

CheckifPageTableEntryp isValidLogicalAddress

Pagep

PTBRPTSR

V==1

Offseti

PhysicalAddress

FrameV Perm …R D

CheckifOperationisPermittedLogicalAddress

Pagep

PTBRPTSR

Perm(op)

Offseti

PhysicalAddress

FrameV Perm …R D

TranslateAddressLogicalAddress

Pagep

PTBRPTSR

Offseti

PhysicalAddress

FrameV Perm …R D

concat

PhysicalAddressbyConcatenation

LogicalAddressPagep

PTBRPTSR

Offseti

FrameV Perm …R D

PhysicalAddressFramef Offseti

SizingthePageTableLogicalAddress

Pagep Offseti

Numberofbitsnspecifiesmaxsizeoftable,wherenumberofentries=2n

Numberofbitsneededtoaddressphysicalmemoryinunitsofframes

Numberofbitsspecifiespagesize

FrameV Perm …R D

ExampleofSizingthePageTable

• 32bitvirtualaddresses,1GBphysicalmemory• Addresspartition:20bitpagenumber,12bitoffset

Pagep:20bits Offseti:12bits

…

FrameV Perm …R D

Howmanyentries(rows)willtherebeinthispagetable?A. 212,becausethat’showmanytheoffsetfieldcan

addressB. 220,becausethat’showmanythepagefieldcan

addressC. 230,becausethat’showmanyweneedtoaddress1

GBD. 232,becausethat’sthesizeoftheentireaddress

space


Howbigisaframe?


20bitstoaddress220=1Mentries

…

FrameV Perm …R D


Whatwillbetheframesize,inbytes?A. 212,becausethat’showmanybytestheoffset

fieldcanaddressB. 220,becausethat’showmanybytesthepagefield

canaddressC. 230,becausethat’showmanybytesweneedto

address1GBD. 232,becausethat’sthesizeoftheentireaddress

space

ExampleofSizingthePageTablePagep:20bits Offseti:12bits


Pagesize=framesize=212 =4096bytes

…

FrameV Perm …R D


Howmanybitsdoweneedtostoretheframenumber?


A:12 B:18C:20D:30E:32



?


…

FrameV Perm …R D





18bitstoaddress230/212 frames


…

Sizeofanentry?

FrameV Perm …R D

Howbigisanentry,inbytes?(Roundtoapoweroftwobytes.)


A:1B:2 C:4 D:8E:16





…

Sizeofanentry?

FrameV Perm …R D







…

4bytesneededtocontain24(1+1+1+18+3+…)bits

FrameV Perm …R D

Totaltablesize?







…

4bytesneededtocontain24(1+1+1+18+3+…)bits Tablesize=

1Mx4=4MB

FrameV Perm …R D

Concerns

• 4MBofbookkeepingforeveryprocess?• 200processes->800MBjusttostorepagetables…

• We’regoingtoneedatonofmemoryjustforpagetables…

• Weneedtodoalookupinourpagetable,whichisinmemory,everytimeaprocessaccessesmemory.• Isn’tthatslowingdownmemorybyafactorof2?

Multi-LevelPageTables(You’renotresponsibleforthis.TakeanOSclassforthedetails.)

LogicalAddress1st-levelPaged Offseti

FrameV …R D

2nd-levelPagep

FrameV …R DPointsto(base)framecontaining2nd-levelpagetable

concat

PhysicalAddressReducesmemoryusageSIGNIFICANTLY:onlyallocatepagetablespacewhenweneedit.Morememoryaccessesthough…

Cachingthepagetable

• Eachlookupcostsanothermemoryreference• Foreachreference,additionalreferencesrequired• Slowsmachinedownbyfactorof2ormore

• Takeadvantageoflocality• Mostreferencesaretoasmallnumberofpages• Keeptranslationsoftheseinhigh-speedmemory(acacheforpagetranslation)

VMImplications

• Notallpiecesneedtobeinmemory• Needonlypiecebeingreferenced• Otherpiecescanbeondisk• Bringpiecesinonlywhenneeded

• Illusion:thereismuchmorememory• What’sneededtosupportthisidea?• Awaytoidentifywhetherapieceisinmemory• Awaytobringinpieces(fromwhere,towhere?)• Relocation(whichwehave)

SampleContentsofPageTableEntry

• Valid:isentryvalid(pageinphysicalmemory)?• Ref:hasthispagebeenreferencedyet?• Dirty:hasthispagebeenmodified?• Frame:whatframeisthispagein?• Protection:whataretheallowableoperations?• read/write/execute

FramenumberValid Ref Dirty Prot:rwx

Pagefaults

Apagefaultoccurswhenwetrytoaccessavirtualaddressthathasnocorrespondingphysicaladdress.

mechanismforhandlingapagefault:1. readinthevirtualpagefromdisk

• Locationkeptinkerneldatastructure.2. storeitinaphysicalmemoryframe

• Mayneedtokicksomethingelseout.3. UpdatePTEwithframenum &validbit=14. Restartinstructionthatcausedthepagefault

PageFaultsareExpensive

• Disk:5-6ordersmagnitudeslowerthanRAM• Veryexpensive;butifveryrare,tolerable

• Example• RAMaccesstime:100nsec• Diskaccesstime:10msec• p =pagefaultprobability• Effectiveaccesstime:100+p × 10,000,000nsec• Ifp =0.1%,effectiveaccesstime=10,100nsec !

Handingfaultsfromdiskseemsveryexpensive.Howcanwegetawaywiththisinpractice?

A. Wehavelotsofmemory,anditisn’tusuallyfull.

B. Weusespecialhardwaretospeedthingsup.

C. Wetendtousethesamepagesoverandover.

D. Thisistoocommon&expensivetodoinpractice!

PrincipleofLocality

• Notallpiecesreferenceduniformlyovertime• Makesuremostreferencedpiecesinmemory• Ifnot,thrashing:constantfetchingofpieces

• Referencesclusterintime/space• Willbetosameorneighboringareas• Allowspredictionbasedonpast

PageReplacement

• Goal:removepage(s)notexhibitinglocality

• Pagereplacementisabout• whichpage(s)toremove• whentoremovethem

• Howtodoitinthecheapestwaypossible• Leastamountofadditionalhardware• Leastamountofsoftwareoverhead

BasicPageReplacementAlgorithms

• FIFO:selectpagethatisoldest• Simple:useframeordering• Doesn’tperformverywell(oldestmaybepopular)

• OPT:selectpagetobeusedfurthestinfuture• Optimal,butrequiresfutureknowledge• Establishesbestcase,goodforcomparisons

• LRU:selectpagethatwasleastrecentlyused• Predictfuturebasedonpast;worksgivenlocality• Costly:time-stamppageseachaccess,findleast

• Goal:minimizereplacements(maximizelocality)

Summary• Wegiveeachprocessavirtualaddressspacetosimplifyprocessexecution.

• OSmaintainsmappingofvirtualaddresstophysicalmemorylocationsinapagetable.• Onepagetableforeveryprocess

• Providestheabstractionofverylargememory:notallpagesneedberesidentinmemory• Bringpagesinfromdiskondemand

WorksheetexampleStepthroughthestreamofVirtualAddressesfromtheCPU:

• ShowhowthebitsofeachaddressareusedforeachVirtualAddress&itsPhysicalAddressmapping• TranslateeachVAtoitsPAusingtheappropriatePTE• UpdatePTEsappropriatelyasyougo• ShowthehistoryofthecontentsofRAMastheseaddressesareaccessed• Whichvirtualpageofwhichprocessdoesitstore

• ImplementaFIFOpagereplacementpolicyforRAM

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paging - swarthmore college solution is paging. paging vocabulary • for each process, the virtual...

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