Unit 5.1 Memory Management_Device Driver_IPC.ppt

This is for learning purpose.

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Computer Engineering
(NBA Accredited)
Prof. A. V. Brahmane
Assistant Professor
E-mail : [email protected]
Contact No: 91301 91301 Ext :145, 9922827812
Subject-Operating System and Administration (CO2013)
Unit V-Memory Management Policies, I/O Subsystem
and IPC

2. Content
•Swapping,
•DemandPaging,
•Driverinterface,
•diskdrivers,
•ProcessTracing,
•Networkcommunication,
•Sockets
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3. Memory Management Policies
•Historically,UNIXsystemstransferredentireprocessesbetweenprimarymemoryand
theswapdevice.
•Suchamemorymanagementpolicyiscalledswapping.
•ItmadesensetoimplementsuchapolicyonthePDP11,wherethemaximumprocess
sizewas64K.
•TheBSDsystem(release4.0)wasthefirstmajorimplementationofademand
pagingpolicy,transferringmemorypagesinsteadofprocessestoandfromasecondary
device;laterreleasesofUNIXsystemValsosupportdemandpaging.
•Theentireprocessdoesnothavetoresideinmainmemorytoexecute,andthekernel
loadspagesforaprocessondemandwhentheprocessreferencesthepages.
•Thispermitsgreaterflexibilityinmappingthevirtualaddressspaceofaprocessintothe
physicalmemoryofamachine,usuallyallowingthesizeofaprocesstobegreaterthan
theamountofavailablephysicalmemoryandallowingmoreprocessestofit
simultaneouslyinmainmemory.Swappingpolicyiseasiertoimplementandhasless
overhead.
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4. Swapping
•Swappinghas3partstoit:
•managingspaceontheswapdevice
•swappingprocessesoutofmainmemory
•swappingprocessesintomainmemory
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5. Allocation of Swap Space
•Theswapdeviceisablockdeviceinaconfigurablesectionofadisk.
•Thespaceonswapdeviceisallocatedintheformofcontiguousblocks(asopposedto
singleblockallocationforfiles).
•Theallocationofspaceontheswapdeviceistransitory(asopposedtolonglastingfile
allocation).
•Aprocessthatresidesontheswapdevicewilleventuallymigratebacktomainmemory.
•SincespeediscriticalandthesystemcandoI/Ofasterinonemulti-blockoperationthan
inseveralsingleblockoperations,thekernelallocatescontiguousspaceontheswap
devicewithoutregardforfragmentation(asopposedtotheflexibleallocationforfiles
whichresultsintominimumfragmentation).
•Theallocationschemeforswapspaceallocationdiffersfromtheschemeusedforfile
systems.
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6. •Therefore,thedatastructuresalsodiffer.
•Thekernelmaintainsthefreespacefortheswapdeviceinanin-coretable,
calledmap.
•Amapisanarraywhereeachentryconsistsofanaddressofanallocatable
resourceandthenumberofresourceunitsavailablethere;thekernelinterprets
theaddressandunitsaccordingtothetypeofmap.
•Initially,amapcontainsoneentrythatindicatestheaddressandthetotal
numberofresources
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7. Initial Swap Map
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8. The algorithm to allocate map space,malloc
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9. Allocation of Swap Space
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10. Allocation of Swap Space
•The following figures show the state of the map after allocating 100 units, 50
units, then 100 units again:
•Whenfreeingresources,thekernelfindstheirproperpositioninthemapby
address.Threecasesarepossiblehere:
•Thefreedresourcescompletelyfillaholeinthemap.Inthiscase,thekernel
combinesthenewlyfreedresourcesandtheexisting(two)entriesintooneentry
inthemap.
•Thefreedresourcespartiallyfillahole.Inthiscase,thekerneladjuststheaddress
andunitsfieldsofanappropriateentrytoaccountfortheresourcesjustfreed.
•Thefreedresourcespartiallyfillaholebutarenotcontiguoustoanyresourcesin
themap.Thenanewentryiscreatedandinsertedintheproperposition.
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11. •Consideringthepreviousexample,ifthekernelfrees50unitsofswapspace
startingataddress101,theswapmapwillcontainanewentry(case3).Ifthe
kernelthenfrees100unitsstartingataddress1,itadjuststhefirstentryofthe
swapmap(case2).Supposethekernelnowrequests200unitsofswapspace,
sincethefirstentrycannotsatisfytherequirement,thesecondentryismodified
andspaceisgivenfromaddress251to450,bothinclusive.
•Theseactionsareshownbelowonnextslide:
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12. Allocating Swap Space from Secondary Entry in Map
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13. Swapping Processes Out of Main Memory
•Aprocessisswappedoutifthekernelneedsspaceinmemory.Itneedsspacein
memoryunderfollowingsituations:
•Theforksystemcallmustallocatespaceforachildprocess.
•Thebrksystemcallincreasesthesizeofaprocess.
•Aprocessbecomeslargerbythenaturalgrowthofitsstack.
•Thekernelwantstofreespaceinmemoryforprocessesithadpreviously
swappedoutandshouldnowswapin.
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14. •Whenthekerneldecidesthataprocessiseligibleforswappingfrommainmemory,it
decrementsthereferencecountofeachregionintheprocessandswapstheregionout
ifitsreferencecountdropsto0.
•Thekernelallocatesspaceonaswapdeviceandlockstheprocessinmemory(forcases
1-3),preventingswapperfromswappingitoutwhilethecurrentswapoperationisin
progress.
•Thekernelsavestheswapaddressoftheregionintheregiontableentry.
•Thebuffercacheisbypassedwhenthekernelswapsdatabetweentheswapdeviceand
useraddressspace.
•Ifmemoryisorganizedinpages,thedatatobeswappedoutislikelytobediscontiguous
inphysicalmemory.
•Thekernelmustgatherthepageaddressesofdatatobeswappedout,andthedisk
drivermayusethecollectionofpageaddressestosetuptheI/O.Theswapperwaitsfor
eachI/Ooperationtocompletebeforeswappingoutotherdata.
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15. •Itisnotnecessarythatthekernelwritetheentirevirtualaddressspaceofaprocess
toaswapdevice.
•Instead,itcopiesthephysicalmemoryassignedtoaprocesstotheallocatedspace
ontheswapdevice,ignoringunassignedvirtualaddresses.
•Whenthekernelswapstheprocessbackintomemory,itknowsthevirtualaddress
mapoftheprocess,soitcanreassigntheprocesstothecorrectvirtualaddresses.
•Thekerneleliminatesanextracopyfromadatabuffertophysicalmemoryby
readingthedataintothephysicalmemorylocationsthatwerepreviouslysetupto
conformtothevirtualaddresslocations.
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16. Mapping process Space onto the Swap Device
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17. Swapping Processes Into the Main Memory
•Theswapperprocess(process0),istheonlyprocessthatswapsprocessesinto
memoryfromswapdevices.
•Aftersysteminitialization,theswappergoesintoaninfiniteloop,whereitsonly
taskistodoprocessswapping.
•Theswappersleepsifthereisnoworkforit.Swapperexecutesonlyinkernel
mode.
•Whenswapperwakesuptoswapprocessesin,itselectstheprocesseswhichhas
spentthelongesttimeinthestate"readytorunbutswappedout".
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18. The algorithmswapper
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19. •Theswappertriestoswapoutsleepingprocessesratherthanready-to-run
processes,becauseready-to-runprocessesmightgetscheduledearlierthansleeping
processes.
•Aready-to-runprocessmustbecoreresidentforatleast2secondsbeforebeing
swappedout,andaprocesstobeswappedinmusthavebeenswappedoutforat
least2seconds.
•Iftheswappercannotfindaprocessestoswapoutorifneithertheprocesstobe
swappedinnortheprocesstobeswappedouthaveaccumulatedmorethan2
secondsresidencetimeintheirenvironment,thenswappersleepsontheeventthat
itwantstoswapaprocessintomemorybutcannotfindroomforit.
•Theclockwillawakentheswapperonceasecondinthatstate.Thekernelalso
awakensswapperifanotherprocessgoestosleep,sinceitmaybemoreeligiblefor
swappingoutthantheprocessespreviouslyconsideredbytheswapper.Inanycase,
theswapperwakesupandbeginsexecutionfromthebeginning,attemptingtoswap
ineligibleprocesses
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20. •HereisanexampleofprocessschedulinginasystemwhereA,B,C,Daresame
sizedprocessesandonly2processescanstayinmainmemory.
•Thisistheschedulingtimeline:
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21. Sequence of Swapping operations
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22. •Theswapperchoosesprocessestoswapinbasedontheamountoftimethe
processeshadbeenswappedout.
•Anothercriterioncouldhavebeentoswapinthehighest-priorityprocessthatis
readytorun,sincesuchprocessesdeserveabetterchancetoexecute.
•Ithasbeendemonstratedthatsuchapolicyresultsin"slightly"betterthroughput
underheavysystemload.
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23. •Thealgorithmforchoosingaprocesstoswapouthassomeseriousflaws:
•Theswapperswapsoutaprocessbasedonitspriority,memory-residencetime,
andnicevalue.
•Althoughitswapsoutaprocessonlytomakeroomforaprocessbeingswapped
in,itmayswapoutaprocessthatdoesnotprovideenoughmemoryforthe
incomingprocess.
•Forinstance,iftheswapperattemptstoswapinaprocessthatoccupies1
megabyteofmemoryandthesystemcontainsnofreememory,itisfutiletoswap
outaprocessthatoccupiesonly2Kbytesofmemory.
•Analternativestrategywouldbetoswapoutgroupsofprocessesonlyifthey
provideenoughmemoryfortheincomingprocess.
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24. •Iftheswappersleepsbecauseitcouldnotfindenoughmemorytoswapina
process,itsearchesagainforaprocesstoswapinalthoughithadpreviously
chosenone.
•Thereasonisthatotherswappedprocessesmayhaveawakenedinthemeantime
andtheymaybemoreeligibleforswappinginthanthepreviouslychosen
process.
•Butthatissmallsolacetotheoriginalprocessstilltryingtobeswappedin.
•Insomeimplementations,theswappertriestoswapoutmanysmallerprocesses
tomakeroomforthebigprocesstobeswappedinbeforesearchingforanother
processtoswapin;thisistherevisionintheswapperalgorithmshownbythe
commentsinthealgorithmgivenabove.
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25. •Iftheswapperchoosesa"ready-to-run"processtoswapout,itispossiblethat
theprocesshadnotexecutedsinceitwaspreviouslyswappedin.Thefollowing
figureshowssuchacase,wherethekernelswapsinprocessDatthe2-second
mark,schedulesprocessC,andthenswapsoutprocessDatthe3-secondmarkin
favorofprocessE(becauseoftheinteractionofthenicevalue)eventhough
processDhadneverrun.Suchthrashingisclearlyundesirable:
•Iftheswapperattemptstoswapoutaprocessbutcannotfindspaceontheswap
device,asystemdeadlockcouldariseifthefollowingfourconditionsaremet:
•Alltheprocessesinmainmemoryareasleep
•Alltheready-to-runprocessesareswappedout
•Thereisnoroomontheswapdevicefornewprocesses
•Thereisnoroominmainmemoryforincomingprocesses
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26. Demand Paging
•MachineswhosememoryarchitecturesisbasedonpagesandwhoseCPUhas
restartableinstructionscansupportakernelthatimplementsademandpaging
algorithm,swappingpagesofmemorybetweenmainmemoryandaswapdevice.
•Demandpagingsystemsfreeprocessesfromsizelimitationsotherwiseimposed
bytheamountofphysicalmemoryavailableonamachine.
•Thekernelstillhasalimitonthevirtualsizeofaprocess,dependentonthe
amountofvirtualmemorythemachinecanaddress.
•Demandpagingistransparenttotheuserexceptforthevirtualsizepermissible
toaprocess.
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27. •Theportionofthetextsectionwhichcontainsthefrequentlycalledsubroutines
andprogramloopsisasmallsubsetofthetotalspaceoftheprocess.Thisis
knownastheprincipleof"locality".
•Theworkingsetofaprocessisthesetofpagesthattheprocesshasreferencedin
itslastnmemoryreferences;thenumberniscalledthewindowoftheworking
set.
•Iftheworkingsetstaysinthememory,swappingcouldbereduced.
•Whenaprocessaddressesapagethatisnotinitsworkingset,itincursapage
fault;inhandlingthefault,thekernelupdatestheworkingset,readinginpages
fromasecondarydeviceifnecessary.
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28. Working set a process
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29. •Asshowninthediagram,workingsetswithlargerwindowsizesaremore
efficient,implyingthattheprocesswillnotfaultoftenasoften.
•Itisexpensivetoremembertheorderofpagereferences.Instead,systems
approximateaworkingsetmodelbysettingareferencebitwheneveraprocess
accessesapageandbysamplingmemoryreferencesperiodically:Ifapagewas
recentlyreferenced,itispartofaworkingset;otherwise,it"ages"inmemory
untilitiseligibleforswapping.
•Whenaprocessaccessesapagethatisnotpartofitsworkingset,itincurs
avaliditypagefault.Thekernelsuspendsexecutionoftheprocessuntilitreads
thepageintomemoryandmakesitaccessibletotheprocess.Whenthepageis
loadedinmemory,theprocessrestartstheinstructionitwasexecutingwhenit
incurredthefault.Thus,theimplementationofapagingsubsystemhastwo
parts:swappingrarelyusedpagestoaswappingdeviceandhandlingpagefaults.
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30. Data Structures for Demand Paging
•Thekernelcontains4majordatastructurestosupportlow-levelmemory
managementfunctionsanddemandpaging:
•pagetableentries
•diskblockdescriptors
•pageframedatatable(calledpfdataforshort)
•swap-usetable
•pfdatatableisallocatedonceforthelifetimeofthesystem,butallocationsfor
otherstructurearedynamic.
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31. •Asseenpreviously,theregioncontainspagetablestoaccessphysicalmemory.
Eachentryofapagetablecontainsthephysicaladdressofthepage,protection
bitsindicatingwhetherprocessescanread,writeorexecutefromthepage,and
thefollowingbitfieldstosupportdemandpaging:
•Valid
•Reference
•Modify
•Copyonwrite
•Age
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32. •Thekernelsetsthevalidbittoindicatethatthecontentsofapagearelegal,but
thepagereferenceisnotnecessarilyillegalifthevalidbitisnotset.
•Thereferencebitindicateswhetheraprocessrecentlyreferencedapage,and
themodifybitindicateswhetheraprocessrecentlymodifiedthecontentsofa
page.
•Thecopyonwritebit,usedintheforksystemcall,indicatesthatthekernelmust
createanewcopyofthepagewhenaprocessmodifiesitscontents.
•Thekernelmanipulatestheagebitstoindicatehowlongapagehasbeena
memberoftheworkingsetofaprocess.
•Assumethekernelmanipulatesthevalid,copyonwrite,andagebits,andthe
hardwaresetsthereferenceandmodifybitsofthepagetableentry(later,a
hardwarethatdoesnothavethesecapabilitiesisconsidered).
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33. The structure of region
•
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34. •Eachpagetableentryisassociatedwithadiskblockdescriptor,whichdescribes
thediskcopyofthevirtualpage.
•Processesthatsharearegionthereforeaccesscommonpagetableentriesand
diskblockdescriptors.
•Thecontentsofavirtualpagesareeitherinaparticularblockonaswapdevice,
inanexecutable,ornotonaswapdevice.Ifthepageisonaswapdevice,the
diskblockdescriptorcontainsthelogicaldevicenumberandblocknumber
containingthepagecontents.
•Ifthepageiscontainedinanexecutablefile,thediskblockdescriptorcontains
thelogicalblocknumberinthefilethatcontainsthepage;thekernelcanquickly
mapthisnumberintoitsdiskaddress.
•Thediskblockdescriptoralsoindicatestwospecialconditionssetduringexec:
thatapageis"demandfill"or"demandzero",studiedlater.
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35. Pfdata table
•Thepfdatatabledescribeseachpageofphysicalmemoryandisindexedbypage
number.Itcontainsthesefields:
•Stateofthepage,indicatingthatthepageisonaswapdeviceorexecutablefile,
thatDMAiscurrentlyunderwayforthepage(readingdatafromaswapdevice),
orthatthepagecanbereassigned.
•Thenumberofprocessesthatreferencethepage.Thereferencecountequalsthe
numberofvalidpagetableentriesthatreferencethepage.Itmaydifferfromthe
numberofprocessesthatshareregionscontainingthepage,aswillbedescribed
belowwhenreconsideringthealgorithmfork.
•Thelogicaldevice(swaporfilesystem)andblocknumberthatcontainsacopyof
thepage.
•Pointerstootherpfdatatableentriesonalistoffreepagesandonahashqueue
ofpages.
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36. Relationship between page table entries, disk block descriptors, pfdata table
entries, and the swap-use count table.
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37. Fork in Paging System
•Inswappingsystems,thekernelcopiestheaddressspaceoftheparent.Butin
pagingsystem,forsharedregions,kerneljustincrementsthereferencecountin
theregiontableentrytosharetheregion.Andforprivateregionssuchasdata
andstack,thekernelallocatesanewregiontableentryandpagetable.Thenit
examinesthepagesontheparentprocessandiftheyarevalid,incrementsthe
referencecountinpfdatatableentry,sharingthepage(asopposedtosharingthe
region).
•Thekernelsetsthe"copy-on-write"bitinfork,forthesharedpagesintheprivate
regions.Ifanyprocesswritesonsuchsharedpage,itincursthe"protectionfault".
Inthefaulthandler,thekernelmakesacopyofpageandthentheprocesscan
writeonitsprivatecopy.Sothecopyingisdiffereduntilanactuallywrite
operationisdone.
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38. The diagram below shows the kernel data structures after
theforksystem call:
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39. Exec in a Paging System
•Onademandpagedsystem,theexecutablefilemaybetoolargetofitinthe
availablemainmemory.Therefore,thekerneldoesnotpreassignmemoryto
executablefilebut"faults"itin,assigningmemoryasneeded.Itfirstassignsthe
pagetablesanddiskblockdescriptorsfortheexecutablefile,markingthepage
tableentries"demandfill"(fornon-bssdata)or"demandzero"(forbssdata).
Followingavariantofreadalgorithmforreadingthefileintomemory,the
processincursavalidityfaultasitreadseachpage.Thefaulthandlernotes
whetherthepageis"demandfill",meaningitscontentswillimmediatelybe
overwrittenwiththecontentsoftheexecutablefilesoitneednotbecleared,or
thatitis"demandzero",meaningthatitscontentsshouldbecleared.Ifthe
processcannotfitintomemory,thepage-stealerprocessperiodicallyswaps
pagesfrommemory,makingroomfortheincomingfile.
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40. •Therearesomeinefficienciesinthisscheme:
•Aprocessincursapagefaultwhenreadingeachpageoftheexecutablefile,eventhoughitmay
neveraccessthepage.
•Thepagestealermayswappagesfrommemorybeforetheexecisdone,resultingintwoextra
swapoperationsperpageiftheprocessneedsthepageearly.
•Tomakeexecmoreefficient,thekernelcandemandpagedirectlyfromtheexecutablefileifthe
dataisproperlyaligned,asindicatedbyaspecialmagicnumber.However,useofstandard
algorithms(suchasbmap)toaccessafilewouldmakeitexpensivetodemandpagefromindirect
blocksbecauseofthemultiplebuffercacheaccessesnecessarytoreadablock.
•Consistencyproblemscouldarisebecausebmapisnotreentrant.ThekernelsetsvariousI/O
parametersintheu-areaduringthereadsystemcall.Ifaprocessincursapagefaultduring
areadsystemcallwhenattemptingtocopydatatouserspace,itwouldoverwritethesefieldsin
theu-areatoreadthepagefromthefilesystem.Therefore,thekernelcannotusetheregular
algorithmstofaultinpagesfromthefilesystem.Thealgorithmsare,ofcourse,reentrantin
regularcases,becauseeachprocesshasseparateu-areaandaprocesscannotsimultaneously
executemultiplesystemcalls.
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41. The Page-Stealer Process
•Thepagestealerisakernelprocessthatswapsoutmemorypagesthatareno
longerpartoftheworkingsetofaprocess.
•Itiscreatedonsysteminitializationandisinvokedthroughoutthesystemlifetime
whensystemislowonfreepages.Itexamineseveryactive,unlockedregionand
incrementstheagefieldofallvalidpages.
•Thekernellocksaregionwhenaprocessfaultsonapageintheregion,sothat
thepagestealercannotstealthepagebeingfaultedin.
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42. •Therearetwopagingstatesforapageinmemory:thepageisagingandisnotyet
eligibleforswapping,orthepageiseligiblefroswappingandisavailablefor
reassignmenttoothervirtualpages.
•Thefirststateindicatesthataprocessrecentlyaccessedthepage.Some
machinessetareferencebitwhentheyreferenceapage,butsoftwaremethods
canbesubstitutedifthehardwaredoesnothavethisfeature.
•Thepagestealerturnsoffthereferencebitforsuchpagesbutremembershow
manyexaminationshavepassedsincethepagewaslastreferenced.
•Thefirststatethusconsistsofseveralsubstates,correspondingtothenumberof
passesthepagestealermakesbeforethepageiseligibleforswapping.
•Whenthenumberexceedsathresholdvalue,thekernelputsthepageintothe
secondstate,readytobeswapped.
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43. State diagram for Page Aging
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44. Page Faults
•Therearetwotypesofpagefaults:validityfaultsandprotectionfaults.
•Thefaulthandlersmayhavetoreadapagefromdisktomemoryandsleep
duringtheI/Ooperation,therefore,theyareanexceptiontothegeneralrulethat
interrupthandlerscannotsleep.
•Validity Fault Handler
•If a process attempts to access a page whosevalidbit is not set, it incurs a validity
fault and then the kernel invokes the validity fault handler.
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45. Protection Fault Handler
•Ifaprocessaccessesavalidpagebutitspermissionbitsdonotpermitaccessto
it,theprocessincursaprotectionfault.Aprocessalsoincursaprotectionfault
whenitattemptstowriteapagewhosecopyonwritebitwassetduring
theforksystemcall.Thekernelmustdeterminewhetherpermissionwasdenied
becausethepagerequiresacopyonwriteorwhethersomethingtrulyillegal
happened.
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46. A Hybrid System with Swapping and Demand Paging
•Demandpagingsystemstreatmemorymoreflexiblythanswappingsystems.But
therecouldbesituationswherethepagestealerandvalidityfaulthandlerthrash
(spendmoretimeonswappingpagesratherthanprocessexecution)becauseof
shortageofmemory.Ifthesumoftheworkingsetsofallprocessesisgreater
thanthephysicalmemoryonamachine,thefaulthandlerwillusuallysleep,
becauseitcannotallocatepagesforaprocess.Thepagestealerwillnotbeable
tostealpagesfastenough,becauseallpagesareinaworkingset.System
throughputwillbeaffectedbadly.
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47. •ThesystemVkernelrunsswappinganddemandpagingbothtoavoid
thrashing.Whenthekernelisnotabletoallocatepagesforaprocess,
itwakesuptheswapperandputsthecallingprocessinastate
equivalentto"readytorunbutswapped".Theswapperswapsout
entireprocessesuntilavailablememoryexceedsthehigh-watermark.
Foreachprocessswappedout,itmakesone"ready-to-runbut
swapped"processreadytorun.Itdoesnotswapthoseprocessesin
viathenormalswappingalgorithmbutletsthemfaultinpagesas
needed.Thismethodslowsdownthesystemfaultrateandreduces
thrashing.
•
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48. •Thank you….
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