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Deadlockisaconstantthreatinterminal-orientedsystems.Thiscomprehensivestudyofdeadlock-handlingtechniquesintroducesamethodforon-linedetectionindistributeddatabases.
TheDeadlockProblem:AnOverview
SreekaanthS.Isloor.
T.AnthonyMarslandUniversityofAlberta
Modernmultiprogrammingsystemsaredesigned
tosupportahighdegreeofparallelismbyensuringthat
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asmanysystemcomponentsaspossibleareoperatingconcurrently.Theincreasingtrendamongcommercial
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rmsforon-lineoperations,especiallythoseinvolvingintegrateddatabases,andtheconsequentneedbyactiveusersforresponsivesystemshaveplacedheavydemandson
operatingsystems.Compoundingthesedi culties,distributedprocessinghasarrivedasthesolution
toincrementalsystemgrowth.Suchcontemporarysystemsexhibitahighdegreeofresource
anddatasharing,asituationinwhichdeadlockisa
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constantthreat.Deadlocksarisewhenmembersofagroupofprocesseswhichholdresourcesareblockedinde
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nitelyfromaccesstoresourcesheldbyotherprocesseswithinthegroup.Whennomemberofthegroupwillrelinquishcontroloveritsresources
untilafterithascompleteditscurrentresourceacquisition,deadlockisinevitableand
canbebrokenonlybytheinvolvementofsomeexternalagency.
.NowatBellNorthernResearch,Ottawa.1
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Asetofprocessesbecomesdeadlockedasaconsequenceofexclusiveaccessandcircularwait.Thesimplestillustrationoftheseconditionsinvolvesonly
twoprocesses,eachholding,forexclusiveaccess,adi erentresourceandeachrequesting
accesstotheresourceheldbytheother.Theresultisacircular
waitwhichcannotbebrokenuntiloneoftheprocesses
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releasesitsresourceorcancelsitsrequest.
Inthisarticle,wesurveytheresourcemanagementproblemincomputersystemsbyreviewingthe
principles,techniques,andtoolsinvolvedinhandlingandavoidingdeadlocks;wealsopropose
anewmethodforhandlingdeadlockindistributeddatabases,closingwithsomecase
studiesontheresourcemanagementprobleminlargecomputersystems.
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Annotatedreferencestoworkscitedandacomprehensivebibliographyofsupplementarymaterialsareprovided.
Examplesofdeadlock
Thedeadlockproblem
occursinmanydi erentcontexts,andanalogiescanbemadetoreal-lifesituations,
providedoneinterpretstheprocessesandresourcesinvolvedappropriately.Forinstance,oneoften
hearsabout\deadlockedpeacetalksbetweentwowarringnations.In
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thatcontext,thepeace-negotiatingpartiesofthetwocountriesaretheprocesses,andtheoccupiedterritoriescouldrepresenttheresourcesoverwhichexclusivecontrol
issought.Thepeacetalkscouldbeblockedinde
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nitelyifbothpartiesrefusetogiveupanyoccupiedlandwhiledemandingthereturnofsomelandheldbytheiradversary.
Anothercommonexampleisthetra cdeadlock.Considerthesituation(Figure1)where
fourcars,A,B,C,andD,arriveatanintersectionatapproximately
thesametime.Topreventaccidentsatfour-waystopintersections,
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right-of-wayregulationsrequirethatAyieldtoB,BtoC,andsoon.AsfarasAisconcerned,partoftheintersection
belongstoBforexclusiveuse,soAmustwaituntilBpasses.
Acircularwaitisanessentialcomponentofthistra cdeadlock;ifall
fourvehiclesmoveforward,occupyingasmuchoftheintersection
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aspossible,alltra ccomestoastandstill.Inthisillustration,thecarsaretheprocesses,whilethefourquartersoftheintersectionare
theresourcesoverwhichexclusivecontrolisneeded.
Thedeadlocksituation
isdi erentdependingonwhetherthecarswanttoturnright,turnleft,
orgostraight.Assumingtheexitrouteforeachcar
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isclear,theapparentdeadlockdoesnotmaterializewhenallthecarswishtomakearightturn,sincethecarsrequireaccessto
onequarteroftheintersectionwhichnoothersdemand.Thingsaremoredi cult
whenthecarswanttoproceedaheadorturnleft.Togostraight,
carArequiresquarters1and2oftheintersection.
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Inthesecircumstances,adeadlockarisessinceeachcarneedsexclusiveaccesstoaquarteroftheintersectiontowhichthecaronits
rightsideholdsalegalright(controls).
Thetra cproblemcan
behandledthroughtheinterferenceofanexternalagency(e.g.,apoliceman)allocating
spacetooneofthevehicles.Alternatively,somebusycities
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\cross-hatchimportantintersections,andrequirethatnovehicleenterthatareaunlessitsexitrouteisclear.Thecross-hatchingtechniqueensuresthatnoprocess
willacquirearesource(occupytheintersection)whichitcannotsubsequentlyrelinquish(leave
byaclearexit).Asapracticalmatter,tra cdeadlockisusuallyresolved
byonedriveraggressivelyenteringtheintersection,preemptingitfrom
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theothers.Asanalternative,onemightforcevehiclestorollbackarandomdistanceandapproachthecrossroadsagain.Sinceeachdriverthus
delayshisentrybyadi erentamount,theproblemcausedbytheirsimultaneous
arrivaliseliminated.AsimilartechniqueisusedinEthernet1toresolveaccess
conictsonitscommunicationmediumandisalsousualon
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contention-modetelecommunicationcircuits.
Fordeadlocksthatmayoccurincomputeroperatinganddatabasesystems,wewillusetheterm\systemresourcequite
broadlytorefertostoragemedia(e.g.,primarymemory,tapes,disks,anddrums),
systemcomponents(e.g.,tapedrives,diskdrives,I/Ochannels,CPUs,readers,andprinters),
andinformation
2
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les,directories,programs,andsystemroutines).Considerasmallmultiprogrammingsystemwithasinglecardreaderandaprinter,inwhichtwouserjobs
shareuseoftheprinterandthecardreaderbymeansofrequest
andreleaseoperations,asgiveninstandardoperatingsystemstexts.2Dueto
independentschedulingofthejobs,requestandreleaseoperationscan
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beinterspersedinseveraldi erentorders.Someofthesesequencesleadtoa\deadlyembrace,adeadlockwheretwojobs,forexample,holdthe
printerandthecardreader,respectively,and|atthesametime|requesttheunitheld
bytheother.
Moregenerally,asetofwaitingprocessesforms
acircularchainwhereeachprocessholdsatleastone
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resourceandmakesaconictingaccessrequestforsomeresourceheldbythenextprocessinthechain.Suchacircularwaitconditioncan
arisewhenthefollowingnecessaryconditionshold:
processesrequestexclusive
controlofresources,
processesholdresourcesallocatedtothemwhilewaitingfor
additionalones,and
nopreemptionofaresourcefrom
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aprocesscanbedonewithoutabortingtheprocess.Inmanywaystheseconditionsarequitedesirable.Forconsistency,datarecordsshouldbeheld
untilupdateiscomplete.Similarly,preemption(thereclaimingofaresourcebythe
system)cannotbedonearbitrarilyand,especiallywhendataresourcesareinvolved,must
besupportedbyarollbackrecoverymechanism.Rollbackrestoresa
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processanditsresourcestoasuitablepreviousstatefromwhichtheprocesscaneventuallyrepeatitstransactions.
Forthedatabasecase
(Figure2),considertwoconcurrentlyexecutingprocessesPandQ,whichmodifyentities
MandN.Letusassumethatthedatabasecorrectness(consistency)assertionon
theentitiesisM=Nandthattheinitialvalues
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PP1:M=M+100P2:N=N+100QQ1:N=2*NQ2:M=2
*MPROCESSPPROCESSQSTEPACTIONSTEPACTIONP0REQUESTENTITY
MP1LOCKENTITYMP2READENTITYMP3WRITEENTITYM
P4REQUESTENTITYNP5LOCKENTITYNP6READ
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ENTITYNP7WRITEENTITYNP8UNLOCKENTITYMP9UNLOCKENTITYNQ0REQUESTENTITYNQ1LOCKENTITYNQ2READ
ENTITYNQ3WRITEENTITYNQ4REQUESTENTITYMQ5LOCKENTITY
MQ6READENTITYMQ7WRITEENTITYMQ8UNLOCKENTITYN
Q9UNLOCKENTITYM
Figure2.Consistentdatabase
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deadlock.Thesequenceofstepsleadingtodeadlockisp0q0p1q1p2p3q2q3p4q4.
sothatdatabasecorrectnessismaintained,concurrentoperationcanstillleadto
deadlock.Thereforeaconsistencyresulthasbeendevelopedwhichrequiresthattheprocesses
be\well-formedand\two-phase:"3
Awell-formedprocessisonewhichlocks
anentitybeforeactingonitfurtherandsubsequentlyunlocks
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it.Aprocessisthusrequiredtobedividedintogrowing(locking)andshrinking(unlocking)phases.The
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rstunlockactionsignalsthebeginningoftheshrinkingphase,afterwhichaprocesscannotissuealockrequestonanyentityinthe
databaseuntilallotherentitiesheldhavebeenreleased.Inthecontextof
Figure2,itisessentialtonotethattheprocessP(orQ)
cannotunlockentityM(orN)beforelockingentityN
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(orM),ifitistomaintaindatabasecorrectness.Notealsothatunderconcurrentoperationtheinterleavedsequenceofactionsp0q0p1q1
p2p3q2q3p4q4endsindeadlock.Severalotheraspectsof
concurrentoperationsuchastransaction-,lock-,log-,andrecovery-managementneednotconcernus
andhavebeendealtwiththoroughlyelsewhere.4
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Basicapproachestodeadlockhandling
One-basicstrategyforhandlingdeadlocksistoensureviolationofatleastoneofthethreeconditions
necessaryfordeadlock(exclusivecontrol,hold-wait,andnopreemption).Thismethodisusually
referredtoasdeadlockprevention,unlessitsprimaryaimistoavoiddeadlock
byusinginformationabouttheprocessesfutureintentionsregardingresource
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requirements.Atotallydi erentstrategyinterrogatestheprocess/resourcerelationshipsfromtimetotimeinordertoidentifytheexistenceofadeadlock.Thislatter
methodpresumesthatthesystemcansubsequentlydosomethingabouttheproblem.
Detectiontechniques.Thesetechniquesassumethatallresourcerequestswillbegranted
eventually.Aperiodicallyinvokedalgorithmexaminescurrentresourceallocationsand
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outstandingrequeststodetermineifanyprocessesorresourcesaredeadlocked.Ifadeadlockisdiscovered,thesystemmustrecoverasgracefully
4
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Sometimes,usingdetectionprinciplese ectivelyisdi cult|forexample,whenpreemptionofresourcessuchastapedrivesmightincurunacceptableoverhead.Nevertheless,detectiontechniques
havesomeadvantagessincetheschemesareinvokedintermittentlyandonlyessentialpreemptions
needbeperformed.
Inthedatabasecontext,detectionmethodsrelyon
themanagementsystemtoabort,rollbacktoaprevious
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checkpoint,andrestartatleastoneprocesstobreakthedeadlock.Here,theproblemofrollbackandrecoveryassumesgreatimportancefromtheviewpoint
ofmaintainingdatabaseconsistency.
Preventionmechanisms.Preventionistheprocessof
constrainingsystemuserssothatrequestsleadingtodeadlockneveroccur.Mostproposals
forpreventionrequireeachprocesstospecifyallneededresources
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beforetransactionsbegin.Deadlockscanbepreventedinseveralways,includingrequestingallresourcesatonce,preemptingresourcesheld,andorderingresources.
Thesimplestwayofpreventingdeadlockistooutlawconcurrency,butthisleads
toverypoorresourceutilizationandisnotconsistentwithcurrentsystemdesign
philosophies.Anothermethodrequiresthatallresourcesbeacquiredbefore
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processingstarts.Suchaschemeisine cient,sinceresourcesheldmaybeidleforprolongedperiods,butworkswellforprocesseswhichperforma
singleburstofactivity,suchasinput/outputdrivers,sincetheresourcecanbe
releasedimmediatelyaftereachuse.Forprocesseswithuctuatingrequirements,themethodcan
beimpractical.Inadatabaseenvironment,itmaybeimpossible
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needsmoremainmemorythaniscurrentlyavailable,itbecomesblocked.Subsequentlytheprocessisswappedtosecondarystoragebypreemptingitsmemoryfor
usebyanactiveprocess.Theblockedprocessisswappedbackonlywhen
theentire,largerquantityofmemoryisavailable.Forsomepeculiarsituationsin
databasesystems,thisuseofpreemptiontopreventdeadlocksis
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subjecttocyclicrestart,inwhichtwoormoreprocessesloopbycontinuallyblocking,aborting,andrestartingeachother.
5
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P1P2++ + + ++t/ / / /::::::P 2P1r w r w0 t1/ / / // / / /
P 2+ + +P2+:r+w+ + + +:: ::
t2rP 1+ +r+r wr ww:A A D D A A D D BB B B B
P1r D D r A wr BFigure 3. Cyclic restart.STARTS RESTARTS WAITSSTARTS WAITS RESTARTSREPETITIONTIME= PROCESS INITIATION OR RESTART
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+ = DATABASE READ OR WRITE OPERATION/ = PERIOD OF PROCESS BLOCKConsider two processes P1 and P2, checkpointed and starting at time t0, as shownin Figure 3. In the
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rst phase, processes P1 and P2 issue update requests fwA, wDg and fwBg, respectively, on entities fA, Dgand fBg. At time t1 > t0 process P1 requests access to entity B(rB) and gets blocked. Shortly afterwards,P2 requests access to D(rD), resulting in a deadlock. The deadlock arbitrator selects the blocked process P1for abortion and restart. In this case, the nature of the transactions are suchthat the situation at time t1is repeated at time t2. Such a cycle tends to be self-synchronizing, continuinginde
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nitely, notwithstandingminor system environment variations in timing.Havender5 developed a set of approaches which excludes a priori the possibilityof deadlock by restrictingthe way in which resources can be requested. In one approach, all the required resources must be requestedand granted before the process can proceed. In a second strategy, when a processholding certain resources isdenied a further request, the process must be capable of releasing all of its original resources and rerequestingthem, together with the additional ones.A more sophisticated form of prevention employs a unique ordering of the resources:5 A request for aspeci
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c resource is met only if all resources lower in rank, which are needed in the future, have also beenallocated. By this means, circular chains of blocked processes cannot occur, since each process requestsresources in the same orderly way. The feasibility of enforcing resource ordering by compile-time checksis a major advantage of the scheme. Restrictions on the allowable sequences of process requests forceknowledgeable use of the ordering rule. A process may request and hold some units of a resource rather earlyin the processing stage, in which case a later incremental request for the sameresource may be disallowed bythe ordering rule. Consequent recovery is possible only by preempting all the resources held by the process.In a database environment with data-driven processes ofuctuating needs, it is often impossible to orderthe data resources (records, entities, or
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elds), so this resource ordering method is seldom applicable.Avoidance schemes. In avoidance schemes, a resource request is granted only if at least one wayremains for all processes to complete execution. One basic scheme, referred to as the \banker's algorithm; "6manages multiple units of a single resource by requiring that the processes specify their total resource needsat initiation time. Furthermore, each process acquires or returns resource unitsone at a time. The algorithmdenies a request by any process whose remaining needs are in excess of the available resources. E ectively,6
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theschemeprojectsdetectionintothefuturetokeepthesystemfromcommittingitselftoanallocationwhicheventuallyleadstodeadlock.
Habermandevelopeda\maximumclaimsstrategy"7tocontrolthefutureresourcerequirements
foreachprocess.Thisgeneralizationofthebanker'salgorithmisapracticalexample
ofavoidancebutrequiresquantityinformation,intheformof
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referredtoasan\indi erentor\nostrategyapproach.Ithasasuper
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cialadvantageofsavingprocessortimeandspaceotherwiserequiredfordetection,prevention,oravoidancemethods.However,theonusofrecognizingdeadlocksisborne
byeitheracomputeroperatordiscoveringblockedprocessesorauserwaitingfor
ananswer,andignoringdeadlockshasdisastrouse ectsontheconsistencyofdata
inanydatabasesystem.
Approachescompared.Empiricalobservations
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havesuggestedthatdeadlockpreventionmechanismstendtoundercommitresourceswhiledetectiontechniquesgiveawayresourcessofreelythatprolongedblockingsituationsarisefrequently.
Avoidanceschemesfallsomewhereinbetween.However,obtaininggoodupperboundsisa
severetechnicaldi culty,sinceotherwiseresourcesareusedine ciently.
Ina
heavilyloadedsystem,Havender'spreventionmethodsprovideveryfewsafe
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preventionandavoidancecases,recoveryfromasystemimplementationerrorneedsarollbackmechanism.
Table1summarizesthebasicdeadlockdetection,prevention,
andavoidancetechniquesforoperatingsystems,alongwiththeirprimarymeritsandde
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ciencies.
Graph-theoreticmodelsfordeadlockdetection
Inconsideringthedeadlockproblem,therepresentationofprocessinteractionsduringresourceallocationphases
isimportant.Processesincomputersystemscanbedynamicinthesensethat
oneprocessmaycreateanother.Processesaresaidtointeractexplicitlywhenthey
communicateamongthemselves.Processinteractionsresultingfromcompetitionfor
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accesstophysicalobjectsaretermedimplicit.Blockingofprocessesmaybecausedbyeithertypeofinteraction.Holt8hasproposedthedistinctionof
reusableandconsumableresourcestomodelimplicitandexplicitinterations,respectively.Emphasizedis
thefactthatanarbitrarynumberofprocessescanwaitforaconsumable
resource,whereasonlya
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xednumberofreusableunitsareavailable.
Thephysicaldevicesofacomputersystem,suchastapedrives,disks,memory,andI/O
channels,arereusableresources.Thereisalwaysa
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xedtotalnumberofunitsoftheseresourcesinasystem.Anyunitofaparticularresourcecanbeheldbyoneprocessat
atime.Thus,eachunitofaresourceiseitherfreeforallocation
orisheldbyaprocess.Theallocationstrategiesspecifytheunitsize
oftheresource.For
7
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Table1.Summaryofdetection,prevention,andavoidanceapproachesforoperatingsystems.
PRINCIPLERESOURCEALLOCATIONPOLICYDIFFERENTSCHEMESMAJORADVANTAGESMAJOR
DISADVANTAGESDETECTIONVeryliberal;requestedresourcesaregrantedwherepossible.Invokeperiodicallyto
testfordeadlock.
Neverdelaysprocessinitiation.
Facilitateson-linehandling.
Inherentpreemptionlosses.PREVENTIONConservative;undercommitsresources.Requestingallresources
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atonce.Preemption.Resourceordering.
Workswellforprocessesthatperformasingleburstofactivity.
Nopreemptionnecessary.
Convenientwhen
appliedtoresourceswhosestatecanbesavedandrestoredeasily.
Feasible
toenforceviacompile-timechecks.
Needsnorun-timecomputationsinceproblemis
solvedinsystemdesign.
Ine cient.
Delaysprocessinitiation.
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Preemptsmoreoftenthannecessary.
Subjecttocyclicrestart.
Preemptswithoutmuchuse.
Disallowsincrementalresourcerequests.AVOIDANCESelectsmidway
betweenthatofdetectionandprevention.Manipulateto
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ndatleastonesafepath.
Nopreemptionnecessary.
Futureresourcerequirementsmustbeknown.
Processescanbeblockedforlong
periods.
example,memorymaybeallocatedbypages,disksmaybe
heldinunitsoftracks,cylinders,orentiredisks,anddatamaybe
assignedasrecords,
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elds,entities,or
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les.
Messagetextfromoperators,externalinterrupts,interprocesscommunications,addcardimagesproducedbyacardreaderareexamplesofconsumableresources.
Typically,thetotalnumberofresourceunitsisnot
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xed.Whenaconsumableresourceisacquiredbyaprocess,itceasestoexist.Aprocesswhichcreates(produces)aconsumableresourcemayrelease
anynumberofunitsatatimeandmustbetreatedasif
itwereholdingtheresource.Consumableresourcesarecreatedandreleasedbya
producingprocessandarerequested,acquired,andusedbyother
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processes.Ontheotherhand,reusableresourcesareassignedbytheresourcemanagertorequestingprocesses,whicheventuallyreturnthemtothemanager.
Manyphysicalresourcespermitonlyexclusiveusebyoneprocessata
time,butothers,likedataresourcesandread-onlyprograms,mayallowshareduse
byseveralprocesses.Anotherpropertyofaresourceisits
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abilitytoallowpreemption.Someresourcesmaybetakenbackbythesystemwithoutanyactionbythe
8
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RRRePePePeP1231132esses: shared access.e: exclusive access.RRR
ePePePeP1231132es
ses: shared access.e: exclusive access.Figure4.StategraphforExampleA.
process.Suchaprocessistheneitheraborted,rolledback
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(ifnecessary)andrestarted,orforcedtorerequestandthuswaitforthepreemptedresource.Thecostofabortingorrestartingtheprocessaccounts
forthepreemptionlosses.
Incertaincases,itispossibleto
suspendaprocessandpreemptoneofitsresources,yetpreservethecurrent
statesoftheprocessanditsuseofthatresource
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forlaterresumption.Typically,suchresumptiondoesnotloseprocessingtimealreadyspent.Forexample:
CPUinterrupts,inwhichtheresource
preemptedistheCPUandtheinformationthatmustbepreservedforlater
restorationisthestatusoftheprocess(forexample,registersandthe\program
statusword"),and
swapping,inwhichthepreemptedresource
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istheprimarymemoryandbackupisprovidedinsecondarystorage.Whenthesystemhasdi erentresourcetypesandmorethanoneresourceof
thesametype,thecomplexityofthedeadlockproblemincreases.Attemptstomodel
andformalizetheproblemhaveresultedintwomajor
8;9
proposals.Graph-theoreticmodelsoftheprocessinteractionshavebeendeveloped,
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anddeadlockshavebeenexpressedpreciselyintermsofgraphtopologies.
Stategraphmodel.Therelationshipsbetweenasetofprocessesand
asetofdistinctresourcesinusebytheseprocessescanbedescribed
byastategraph:9Thisisadirectedgraphwhosenodescorrespondto
theresourcesandwhoseedgesarede
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nedsothatifsomeprocessPhasaccesstoresourceRiandiswaitingforaccesstoresourceRj,thenthereexists
anedgedirectedfromnodeRitonodeRj.
Example
A.LetfP1;P2;P3}andfR1;R2;R3}betheprocessesandresourcesinasystem,
respectively.LetR1beheldforsharedaccessbyboth
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P1andP2,andletP3bewaitingforexclusiveaccesstoR1.AssumethatR2andR3areheldforexclusiveaccessby
P3andP1,respectively,whileP1andP2waitforexclusiveaccess.The
processinteractionsherecanberepresentedbythestategraphshowninFigure
4.Forclarityandbetterunderstanding,wehavelabeledthe
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edgesinthestategraph.Forinstance,theedgedirectedtoR3fromR1islabeledeP2sindicatingthatP2holdsR1forshared
(s)accessandawaitsexclusive(e)accesstoR3.Theexistenceofa
circuitinthestategraphmeansthatadeadlockexists.Acircuitis
acloseddirectedpathlinkingasubsetofthenodes
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inthegraph.ThusthepathR3R2R1R3linkingthreenodes,showsthatthreeprocessesareinvolvedindeadlock.
Inthegeneralcase,
involvingmultiple-unitresources,thestategraphjustde
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nedisinappropriate.Co man9hasproposedthattheresourcesbepartitionedintodi erenttypes,insuchawaythatresourcesof
9
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amethodneedsamoreelaboratestatedescriptionmechanism,supplementedwith\allocationand\requestmatricesandan"availableresourcesvector.Adetectionalgorithmmust
bedesignedwhichusesthesedatastructurestodiscoveradeadlockbysimply
investigatingeverypossibleallocationsequencefortheprocessesthatremain.
General
resourcegraphmodel.Holt'smodel8ofasystemofinteracting
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processesprovidesaversatilerepresentationforresourcemanagement.Theapproachischaracterizedbytheuseofa\generalresourcesystemwhichmodelsreusableas
wellasconsumableresources.Ageneralresourcegraphisde
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nedasa\bipartitegraph"10whosenodescorrespondtothesetofprocessesandthesetofresources(reusableandconsumable).Anavailableunits
vector,whoseelementsarepositivemeasuresofthequantityavailable,isassociatedwith
thesetofresources.Edgesdirectedfromaprocessnodetoaresource
nodearetermedrequestedges.Edgesdirectedfromreusableand
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consumableresourcenodestoprocessesarecalledassignmentandproduceredges,respectively.Aprocessisblockedifandonlyifthenumberofrequest
edgesfromtheprocesstoaparticularresourceexceedsthenumberofavailable
unitsoftheresource.Aprocessisdeadlockedwhenitisimpossibleto
gettheprocessoutoftheblockedstate.Holtintroduces
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agraphreductionmethodtocheckifaprocessisdeadlocked.Agraphreductioncorrespondstothebestsequenceofoperationsaparticularprocess
canexecutetohelpunblockotherprocesses.Thisisachievedbydeterminingif
thesuccessiveeliminationofallsinknodesproducespredecessorswhicharealsosinks.
Asinknodeisonewithnoedgesemanatingfrom
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theprocesswasnotdeadlocked.
Itcanbeshownthatforageneralresourcegraphinwhichallprocesseshavingrequestsare
blocked,theexistenceofacircuitisanecessaryandsu cientconditionfor
deadlock.AsimpleillustrationofageneralresourcegraphcorrespondingtoExampleA
isshowninFigure5.Itisevidentthatthe
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nallyP1waitsforP3toreleaseR2.Forthemultipleunitsofresourcecase,aweightisassociatedwitheachallocationedgeto
representthenumberofunitsheldbytheresource.
Deadlocksin
databases
Manydi erentdeadlock-handlingalgorithmsandapproachesdevelopedforoperatingsystems
havebeenpresented.Databaseanddistributeddatabasesystems,however,pose
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uniqueproblemsoftheirown.Theconcernisnotonlywithavoidingdeadlocksituationsbutalsowithprotectingagainstdatabaseinconsistencies.
Theschemesforhandlingdeadlockinoperatingsystems,discussedabove,formthebasis
fordatabasecontrol,butthenatureofdatabaseaccessissuchthatprevention
andavoidancemethodsarelessfeasible.Basically,toomanydatabase
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applicationsaredata-driven|thatis,thenextactionisbasedonthevalueofthecurrentitemretrieved.Thusdetectionmethodsandassociatedrecoveryschemes
whichselectresources
9
forpreemptiononabasisof
theirrecoverycostbecomeimportant.Theserecoverytechniquesaredesignedtoavoidpreemption
ofresourceswithhighinherentlosses.
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PPRRR11223e esseP3e ePPRRR11223
e esseP3e ePPRRjr
isResource-process active edge.Process-resource wait edge.
e: Exclusive access.s: Shared access.Figure5.Generalresourcegraphfor
ExampleA.
Databasesystems.Indatabasesystems,the
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resourceswhichprocessesmaywishtolockforexclusiveuseincludepiecesofshareddata.Veryoftentheprocessesmayissuelockrequests,such
as\LockthepersonnelrecordsofallemployeesintheSystemsProgrammingDept.,
forwhichthelockingcriteriondependsondatavalues.Thedi cultiesintroducedby
thismethodarenormallyabsentinanoperatingsystemenvironment.
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Inaddition,aparticulardataresourcemaybedescribedinmorethanoneway,orthenatureofadataresourcemaybealtered
byaprocessoperatingonit.Locksmaybeinterdependentinthesense
thatfurtherlocksmayhavetoberequesteddependingonthe
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rstlock.Becauseofthesedi cultiesinanticipatingfuturerequirements,manyconventionalapproachestoresourcemanagementinoperatingsystemsaredi cultorimpossiblein
thedatabaseenvironment.Inthecaseofpreemptionofadataresource,the
necessityofmaintaininguniformlycorrectdatainthesystemdictatestheabortion,rollback,
andrestartingofoneormoreprocesses.Thisisgenerally
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11
information.Adirectededgeexistsbetweentwonodeswheneveronenodecontainstheaddressoftheother.Suchadatabaseis
assumedtobeaccessedbyasetofroutinescalledprimitives.Underconcurrent
operation,twoormoreprimitivesmayconictwitheachotherwhileaccessinga
node.Toovercometheseconictsaprocedurecalledthe"walking
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algorithmwasformulated.Itrequiresaprimitiveto(1)setthelockofthenextnodeitwishestoaccessbeforeunlockingthenode
itiscurrentlyaccessingand(2)keepanodelockedonlyforthe
durationofitsaccess.Thewalkingalgorithmavoidsthepossibilityoftwoprimit
iveswritingthesamenodesimultaneously.Also,thealgorithmrequires
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thatanodebeunlockedandallitsdatabasepointerserasedbeforeitcanbedeletedandreturnedtothefreelist.Thisovercomes
aproblemthatcouldariseifaprimitiveerasedapointerfroma
nodeimmediatelyafteritwasreadbysomeotherprimitive.Ifsucherasure
wereallowed,subsequentactionsbythereadingprimitivewhichdepend
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onthatpointermightbeinvalid.
TheCodasyl16approachtodatamanagementusesaLOCK-UNLOCKmechanismtoenableincremental
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allocationofdataresourcestoprocesses.Thestatusofallaccessestothedatabaseismaintainedinanaccessstategraph;12adirected
graphwhosenodescorrespondtotheunionofthesetofactiveprocesses
andthesetofallocatabledataelements.Thesetofedgesisreferred
toasthelocklist,andeachelementrepresentsan
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edgeemanatingfromtheactiveprocessnodeandterminatingatthedataelementallocatedtothatprocess.Thebasicmechanismoftheschemeis
tomodeleachoperation,LOCK,UNLOCK,ALLOCATE,andDEALLOCATE.Forinstance,alock
actioncanberepresentedbymappingtheaccessstategraphbeforeaprocess
wasallowedtolockadataelementontotheaccess
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cannotwaitformorethanoneresourceatatime.
Oneproposedtechnique17modi
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esandcombinesthefollowingsteps:
(1)trytopreclaimneededresources;(2)preemptdataresourceswhenpreclaimingleadstoadeadlock;
and(3)imposeapresequencingschemeforprocessesbytimestamping,toavoid
deadlockduetoinde
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nitedelay.Themethodrequireseachprocesstolockallofitsdataresourcesduringaseizephasebeforestartingtheexecutionphase.Competition
forresourcesthereforeoccursonlyduringseizephases.Backingupaprocessto
thestartofthisphaseiseasy,sincetherearenorepercussionsfrom
preemptingaresource,eventhoughithasbeententativelyallocated
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nitedelay,anageindicatorisattachedtoblockedprocesses,foruseasaprioritybythescheduler.Thusthemethodisdeadlock-free.
One-levelandtwo-levellockoutmechanisms14havealsobeenconsideredforsynchronizingdatabase
access.Intheone-levellockoutscheme,sharedaccess(byreaders)tothe
databaseisallowedatanytime,regardlessoftheallocation
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state.Conversely,writersarerequiredtolockthedataresourcesbeforealteringthem.Asinotherschemes,thepresenceofacircuitinthe
stategraphisanecessaryandsu cientconditionfordeadlock.Schlageterprovidesan
algorithmfordetectingdeadlockbytraversingthegraphfromablockedprocessnode,
inanattempttoreturntothatblockednode.In
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thetwo-levellockoutscheme,readersaresplitintotwoclasses:thosewhichareinsensitivetoconcurrentupdatesandthosewhichpreventwritersfromconcurrently
accessingthedata.ThisisimplementedbyusingprimitivesLOCKR(read)andLOCKW
(write).DatalockedbyLOCKRcanbeaccessedbyanyreader.Datalocked
byLOCKWcanbeaccessedonlybyreaderswhichdo
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notneedtobeprotectedagainstchangesofdata.Underthetwo-levellockoutmechanism,startingatablockedprocessnodeandtraversingpathsto
detectdeadlocksisnolongersimple,becausearesourcemaybeheldby
severalprocessessimultaneouslyandeachofthosemayberegardedasblockingany
waitingprocess.
Theschemeproposedfordeadlockavoidance
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indatabasesystems15requirestheprocessestopre-declaretheiranticipatedresourcerequirements,withthesystemgrantingonlysaferequests.Thealgorithmistailoredto
theneedsofdatabasesystems,unlikeotherapproaches.5;8Aseriesoftime-varying
graphrepresentationsarede
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nedfordatabaseinteractions.A\holds-claimsgraphrepresentsonlythoseprocessesthatarecurrentlymakingaclaimonsomecommonsystemresources.A\claims-claims
graphrepresentstheprocesseswhicharepotentiallycapableofdenyingresourcestoone
another,theirclaimsbeingincontention.A\holds-holdsgraphrepresentstheallocationstatus
ofthesystem.Adeadlockexistsifandonlyif
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thereisacircuitintheholds-holdsgraph,whereasadeadlockcanbeavoidedifandonlyiftheholds-claims
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nitedelayisnotentirelyeliminated,itisreducedbythestrategyofgrantingrequestsinanincrementalway.
Distributeddatabases.There
isagrowingbodyofworkondistributeddatabases,butearlyattemptsto
handledeadlockshadpracticaldrawbacks.Thesalientfeaturesofsomemethodswillbe
reviewedhere,whileanon-lineapproach17willbedealtwith
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inthenextsection.
Themaincharacteristicofdistributedcomputingisthatoperatingsystemsmustcontendwithmoreautonomousbehavior,thusaggravating
thecontrolproblem.Presenceofappreciabletime-lagsrenderscoordinationofthevariouscontrollers
inthesystemmuchmoredi cult.Moreover,ingeographicallydistributeddatabasesallinformation
neededtodetectdeadlocksisnotnecessarilyavailableatany
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singleinstallation,sothedeadlockproblemissomewhatdi erent.ConsideravariationoftheexampleinFigure2,butwithtwoseparatecomputersC1
andC2,asshowninFigure6.AssumethatprocessPandresource
entityMresideatC1andthatprocessQandresourceentityN
resideatC2.ProcessesPandQ,afterlockingand
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deadlockmaynotbedetected,oradeadlockmaybeindicatedwhereonenolongerexists.Synchronizingtheupdatesof
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leswhicharereplicatedatdi erentsitesisalsonontrivial,andabortion,rollback,andrecoverybecomeveryinvolvedandrequirecomplexinterprocesscommunication.
Someworkonthepreventionofdeadlocksindistributeddatabaseshasbeen
reported.Atypicalapproachrequiresthatalldataresourcesbeallocatedtothe
processbeforeinitiation.Consequently,processinitiationisneedlesslydelayed.An
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shareddataresourceslist(conceptuallysimilartoaprocessset)toallotherprocesses
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beforeitcanproceed.Thisshareddataresourceslistisdeterminedbyusingwhatiscalledaprocesspro
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theglobalnetworkstatusbyusing
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schemedegrades.Inthedistributedcontrolapproach,supportforNcomputersrequiresthetransmissionof(N-1)identicalmessagescontainingstatusandqueuesof
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les.Alsoeachinstallationreceives(N-1)di erentmessagesfromotherinstallations.
Goldman'sdeadlockdetectionschemes22arebasedonthecreationand
expansionofanorderedblockedprocesslist.EachprocessinanOBPLis
waitingforaresourceheldbythenextprocessinthelist.Whenever
anOBPListransmittedbetweeninstallations,adataresourcename
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isinsertedintotheidenti
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cationpartoftheOBPL.Thelastprocessinthelisthasaccessto,oriswaitingfor,thatresource.Intheformercase
thestate(blockedoractive)ofthelastprocessintheOBPLmust
bedetermined,whileinthelattercaseoneneedstoknowthestate
oftheprocesswhichholdsthedataresource.Todetermine
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theschemerequiresthecreationandexpansionofonedi erentcopyoftheOBPLforeachreadersince,ifoneofthereadersis
deadlocked,anyprocesswhichrequestsaccesstothatresourceisblockedforever.It
ispossiblethatOBPLs,whileundergoingexpansion,couldbetransferred(sequentially)amongseveral
installationsorseveraltimesbetweenthesametwoinstallationsbefore
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adeadlockisdetected.Also,OBPLscouldbecomelarge,leadingtosubstantialoverhead,especiallywhenrecordsorentities,insteadof
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les,areconsideredasdataresources.
20;21
OthercontributionsfromGoldman'sworkareexamplesofdeadlocksnotdetectedbytheearlier
schemes.Thebasicawintheotherschemesistheassumptionthatallowing
alocalprocessatonesitetowaitforalocalresourcewill
notcauseanetwork-widedeadlock.
Inanycase,
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Table2.Summaryofdeadlock-handlingtechniquesindistributeddatabases.
PRINCIPLEADVANTAGESMAJORCOSTSRELEVANTREFERENCESDETECTION
Facilitateson-lineapproach.
Suitsdistributeddatabaseenvironmentfairlywell.
Run-timecost.
Highrollbackand
restartcosts.
Communicationcostsinmaintaininginformationfordeadlockdetection.
Isloor
andMarsland23
MarslandandIsloor17
Goldman22
Mahmoud
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andRiordon21
Chandra,Howe,andKarp20PREVENTION
Rollbackandrestartisnotnecessary.
Intheabsenceofsoftwareerrors,consistencyof
databaseisguaranteed.
Costsofwaitingforhighlydata-drivenprocesses.
Communication
costsintransmittingtheadvanceinformationonfuturerequests.
Maryanski19
Chu
andOhlmacher18AVOIDANCE
Minimaluseofrollbackandrestart.
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Dataresourceallocationpolicyismidwaybetweenthatofhighlyconservativepreventionandveryliberaldetectionprinciples.
Costsofwaitingprocessesand
run-timecosts.
Moderaterollbackcost.
Communicationcostsoftransmittingfuturedata
resourceallocationrequests.
ChuandOhlmacher18
On-linedetectionindistributed
databases
17;23
Theconceptofon-linedetection
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ofdeadlocksinadistributeddatabasehasbeenintroduced.Itisde
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nedtobetherecognitionofdeadlockasrequestsaremadeorgranted,bybothlocalandremoteresourceallocators,withoutthenecessityof
furthercommunicationamonginstallations.Existingalgorithms20..22fordeadlockdetectionindistributeddatabases,because
theywerenotdesignedforon-linedetection,haveseveral-problemsforon-lineuse:
(1)Foreveryrequest,grantedornot,existingalgorithms
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needtoobtaintheglobalnetworkstatusbysimultaneoustransmissionofeachinstallation'sstatus,leadingtoheavycommunicationtra c.(2)Sincethistra cresults
incommunicationdelays,anewdeadlockmaygoundetectedoranapparentdeadlock
maynolongerexist.(3)Afterobtainingthecompletenetworkstatus,thealgorithms
stillhavetoperformdeadlockdetectioncomputations.8
The
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useofareachablesetmayovercometheseproblemstosomeextent.Thus,inasystemgraphrepresentingprocessinteractions,thesetofall
nodestraversedbyadirectedpathfromagivennodeconstitutesitsreachable
set,andaprocessisdeadlockedifandonlyifthecorrespondingprocess
nodebelongstoitsownreachableset.23Maintainingreachable
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setsbyincrementalupdates,asresourcesareallocatedandfreed
15
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RRRRPP PP2 21 13 34 4eeseeC1 C2sRRRRPP PP
2 21 13 34 4eeseeC1 C2sRPi j
Resource-process access edge.RP s rProcess-resource wait edge.
e: Exclusive access.s: Shared access.Figure7.On-linedetection.
ateachinstallation,enableslocalrecognitionofdeadlocks,andallows
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theallocationdecisiontobemadewithoutfurthermessages.
Theon-lineapproachtodeadlockhandlingeliminatestheneedforperiodictransferof
lengthymessagesamongthecomputersinthenetworktoascertainthedeadlockstatus
ofthesystem.Becausecommunicationdelaysarenegligible,themethodreducesthetime
intervalduringwhichdeadlocksgoundetected.
Consider,for
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instance,Figure7representingcomputersystemsC1andC2.LetprocessesfP1;P2}andresourcesfR1;R2}resideonC1andsimilarlyprocessesfP3;P4}andresources
fR3;R4}resideonC2.ForsimplicityassumethateachresourceRiisheld
byprocessPiforexclusiveaccess(foralli),thatP4iswaiting
foraccesstoR1,andthatP2iswaitingfor
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R3.Atanyinstant(say,t0)bothC1andC2possessupdatedreachablesetsforR2andR4,namelyfP2;R3;P3}andfP4;R1;P1g,respectively.Subsequently,
letP1andP3requestaccesstoR2andR4,respectively.Deadlockis
nowinevitableandwillberecognizedupontheintroductionoftheprocess-resourcewait
edgesfromP3toR4andP1toR2.At
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theinstantt1,neithercomputerdetectstrouble,butwhenC1andC2receivetheinformationontheadditionoftheedgesateachother's
installation,thedeadlockisdetectedatbothsitesasthereachablesetsare
updated.TheessenceofthisexampleisthatrequestsbyP1forR2
andP3forR4occurwithinthesametimeframe.
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Thus,nomatterwhatdetectionisused,theallocationwilloccurbecauseneithercomputercanbeawareoftheactivitiesoftheother.Under
thesecircumstances,apreemptionmechanismisnecessaryinordertorecovercontrolof
thesystem.ItwasjustsuchasituationasthiswhichallowedGoldman
todeducethatothermethods20;21fordeadlockdetectionindistributed
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systemswereawed.
Themotivationforon-linedetectionisaconsequenceofseveralfactors.Aslargeintegrateddatabasesrespondtomoreusers,
thepossibilityofdeadlocksincreasesrapidly.Furthermore,thearrivalofdistributedprocessingon
thesceneenhancestheneedforimmediatedeadlockdetection.Inadistributedenvironment,
anydelayinthedetectionofdeadlockshasadversee ects
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ondatabaseconsistency.
Severalaspectsofon-linedetectionmaybeconsideredforthefollowingcompletesetofprocess-resourceinteractions:
(1)
anewprocessentersthesystem;(2)anewdataresourceisaccessed;
16
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21PRRP R PP R2 4 41 3 3s ee e ee es21PRRP R PP R2 4 41 3 3s ee e ee ese: Exclusive access.
s: Shared access.Figure8.SystemgraphforExampleB.
(3)aprocess
runstocompletionandreleasesallthedataresourcesheld;(4)aprocess
inthesystemrequestsaccesstoadataresourceheld
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byanotherprocess;and(5)adataresourceheldbyaprocessispreemptedfromit.Incases(1)and(2),adeadlock-freesystem
remainsdeadlock-free.Forcase(3),allocationdecisionsforreleaseddataresourceswithoutany
waiting-accessrequests,orwithasinglewaiting-accessrequest,donotleadtodeadlock.
However,anallocationdecisionforareleaseddataresourcewith
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multiplewaiting-accessrequestscanleadtopotentialdeadlocksasshowninExampleB.Evenso,atleastonesafeallocationexists.17
ExampleB.ConsiderprocessesP1,P2,P3,andP4whichholddataresources
R1,R2,R3,andR4,respectively,forexclusiveuse.Letusassumethat
P1iswaitingforsharedaccesstoR3,P2for
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R1,andP4forexclusiveuseofR2andR3.SupposethatprocessP3runstocompletionandreleasesresourceR3.R3hastwo
waiting-accessrequestsfromP4andP1.AssumethatP4issueditsrequestbefore
P1did.IftheallocationofR3toP4isdoneina
FIFOmanner,thenprocessesP1,P2,andP4willbe
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deadlocked.ItisobviouslymoreadvantageoustomaketheallocationofR3toP1,andletP1runtocompletion,thantomakethe
allocationofR3toP4andbedeadlocked.Anecessaryandsu cientcondition,
basedonreachablesets,hasbeenderived23torecognizesituationsinwhichprocesses
havemorethanoneoutstandingrequest,andtoavoiddeadlock
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accordingly.InFigure8itisprocessP4whichhaswaiting-accessrequestsforbothR2andR3.
Marsland17showsthat,inthe
caseofadeadlock-freesystemwithmultipleprocesseswaitingforaccesstoa
releaseddataresource,thereexistsatleastoneprocesssuchthatanallocation
madetothisprocessmaintainsthesystemdeadlock-free.Furtherimprovement
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maybepossiblebyallocatingtheresourcetoasafeprocesswhichhasminimumwaiting-accessrequestsonotherdataresources.Inbatchoperatingsystems,
aprocessistypicallyallowedtohavemorethanoneoutstandingrequestat
atime;however,ifthesystemisunabletosatisfyalltheoutstanding
requestsatonce,theprocessisrequiredtodropthem.
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8ThisinturnrulesouttheoccurrenceofasituationanalogoustothatdescribedinExampleB.Consequently,earlyresearchersofthedeadlock
problemindatabasesalsodisallowedmultipleoutstandingrequests.Sincedatabaserequestsaredata-driven
andcontent-based,thepossibilityofmultipleoutstandingrequestsishigh,andsothe
on-lineapproachisuseful.
Incase(4),honoring
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therequestmayleadtoadeadlock,whileincase(5)anattemptisbeingmadetobreakdeadlockthroughpreemptionofaresource.
Forallthesecasesalgorithmsareavailableforupdating
17;23
thereachablesetsforeveryedgeaddedordeletedinthesystemgraph.
Besidesitslowlevelofcommunicationactivity,theon-lineapproach
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hasthefollowingmajoradvantages:
17
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Aprocessisallowedtohaveanynumberofoutstandingrequests.
Whenanumberofreadersshareadataresource,no
specialtreatmentisneeded,unlikeschemes22
whichrequirethatadi erent
copyoftheOBPLbeformedforeachsuchreader.
Everyrequestisdealtwithinauniformway,since
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thedetectiontechniquedoesnotclassifyrequestsaccordingtotherelationshipbetweentheoriginoftheprocessandtheaddresseddataresource.Inall
otherapproaches20..22thealgorithmsdealwitheachaccessrequestaccordingtosomeclassi
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cation.
Combinedapproachtodeadlockhandling
Howard24hypothesizesthatnoneofthethreebasicapproachesalone|detection,avoidance,orpreventionis
appropriatefortheentirespectrumofresourceallocationproblemsencounteredinoperatingsystems.
Instead,individualtechniquescanbetailoredtooptimallyhandlesubproblemsofresourceallocation
andstilloperategloballytopreventdeadlocks.Thebasisfor
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amixedtechniqueliesinthestructureofoperatingsystems.Resourceorderinginahierarchicalstructureprovidestheframework.Insomeoperatingsystems
thishierarchyisintheformofenvelopinglayersofcapability.Eachlayer
modi
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mechanisms.Inthecaseofresourceordering,thisimpliesthatanouterprocesscannotholdanyresourcesrequiredbytheinnerprocessitcreates.
Sucharestrictionautomaticallyenforcesa\naturalorderingofresources,sinceresourcesrequired
byinner-levelprocesseswouldautomaticallyappearlaterintheordering.
By
wayofillustration,considerthefollowingexampleinvolvingatypical
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edasfollows24:
(1)spaceintheswappingstore;(2)assignabledevicessuchastapedrives,andjobresourcessuchasaccess
to
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les;(3)centralmemoryforuserjobs;and(4)internalresourcessuchasmemoryfortransientsystemoverlays,andaccesstochannelsandcont
rollers.InHoward'sidealmixed-methodsolutiontoresourcemanagement,theresourcesareorganized
intothefourorderedclassesabove,andadi erentschemeisusedfor
eachclass.Thus,preallocationmaybeusedfortheswapping
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space,sincethemaximumstoragerequirementsareusuallyknown.Sinceajobcanalwaysbeswappedout,centralmemorycanbepreempted,sodeadlock
preventionisfeasible.Forjobresources,muchinformationabouttheintendedrequirementscan
beobtainedfromthejobcontrolcards,sodeadlockavoidancecanbeused.
Inprinciple,deadlockdetectionisalsopossible,sinceresourceordering
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canbeused,butproblemsmayariseif
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lepreemptionisattemptedduringanupdate.Finally,forinternalsystemresources,preventionthroughresourceorderingispossible,sincerun-timechoicesbetweenpendingrequests
areunnecessary.AsHowardconcludes,\Thepracticaladvantagesofthecombinedapproachin
operatingsystemswilloutweighthetheoreticaldesirabilityofusingasinglemethodthroughout:"24
Acomprehensivecombinedapproachtodeadlockhandlingin
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databasesystemsordistributeddatabaseshasnotbeendevisedsofar.However,theideahasbeenappliedforon-linedetectionindistributeddatabases,where
theremaybemultipleoutstandingrequestsonadataresourcereleasedbya
completingprocess.In
18
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DURODESTROY1111UPDATE1110READ1100OPEN100
0
Figure9.Blockingmatrixfor
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leoperations.
thecaseofadeadlock-freesystem,anunwiseresourceallocationdecisioncanpotentiallyleadtodeadlockasdepictedinExample
B.But,asshown,atleastoneprocessexists(inthesetof
waitingprocesses)towhichtheresourcecanbeallocatedthatmaintainsthesystem
deadlock-free.Inotherwords,apotentialdeadlockisdetectedand
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avoidedaccordingly.Theapproachusesbothdetectionandavoidanceprinciples.Thus,inthecaseofmultipleprocesseswaitingforaccesstoareleaseddata
resource,theresourceisallocatedtothe
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rstprocesswhichmaintainsthesystemdeadlock-freeaftertheallocation.Furtherimprovementmaybepossiblebyallocatingtheresourcetothe
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rstprocesswhichnotonlymaintainsthesystemdeadlock-freebutalsohasminimumwaiting-accessrequestsonotherdataresources.
Somecurrentdeadlock-handling
implementations
Inmoderntimesharingcomputersystems,thepotentialfordeadlockduring
accesstothe
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c
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leoperationisperformed,the
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les
26
arelockedinoneofthreeinclusivelevels(read,update,ordestroy).Inordertoensurethattherules
ofconcurrentusagearenotviolatedbeforelocking,MTSmaintainsatableindicating,
atanyinstant,
allthe
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lescurrentlyopenand/orlocked,
howtheyarelockedandbywhattask(process),
whattasksarecurrentlywaitingtolocka
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leandwhytheyarewaiting.Fromthistable,onecandeterminewhetherornotaparticulartypeofopeningand/orlockingofa
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lecan
26
beallowed,accordingtothefollowingrulesofconcurrentusage.
(1)Ifa
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leisnotlockedforupdatingordestroying,anynumberoftaskscanhaveshared(read)accesstothis
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le.(2)Ifa
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lelockedforupdating(writing,emptying,ortruncating).(3)Ifa
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leisnotbeingused,thenonlyonetaskcanhavethis
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lecannotbelockedasrequested,thetaskisqueuedtowaitforthe
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le,butwaitingcanleadtodeadlockduetomutualblockingoftasks.Asde
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nedbythematrixinFigure9,aprocesswishingtoupdatea
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le.Similarly,openinga
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leisonlyblockedbyaprocessintheactofdestroyingthe
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le.
MTSmaintainstheinformationabout
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leusageinanMMmatrix,B.EachelementBij=1ifandonlyifTASKihasa
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leopenorlockedinsuchawaythatitblocksTASKj.NotethatMisthenumberof
19
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P1P2P3P4P10100001000011000P20
0100001100001
00P30001100001
000010P4100
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0010000100001B.=B+BB+BBB+BBBB
Figure10.ComputationoftransitiveclosureforexampleshowninFigure7.
tasksblockingorbeingblockedbyanothertask.Usuallythisisfar
fewerthanthenumberofactivetasksorthenumber
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ofopen
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les.ThedeadlocksituationisdeterminedfromthestatusofmatrixB ,thetransitiveclosure10ofB.InessenceB.isobtainedbyORing
therowsofBwitheachother.Thusfora33
matrix,B.=B+BB+BBB.Consequently,ifB.=1
thenTASKiisinvolvedinadeadlock.Fortheexample
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ii
showninFigure7,afterP1andP3havebeenmadetowait,theexistenceofdeadlockisprovided
bythecomputationofB.fromB,asshowninFigure10.
Datamanagementsystems,ontheotherhand,varyinthewaythey
detectandresolveadeadlock.As
27
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othershavepointedout,manyoftheearlysystemsimplementedtechniqueswhichwerequiterudimentary.InproductssuchasIDMS|IntegratedDatabaseManagementSystem|Total,and
Model204,deadlockisnotpossiblebecauseofrestrictionsonprocesses.Inthese
systems,sinceaprocesscanlockonlyonerecordatatime,heavy
restrictionsareplacedonuserstoensurethatdeadlocksnever
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occur.27Consequently,theyareneithergeneralnorrealisticandexhibitlowlevelsofconcurrentdataresourceusage.InAdabas|AdaptableDataBaseSystem27|onthe
otherhand,ifaprocessrequestsalockedrecord
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vetimes,therecordisunlockedandcontroltransferredtotherequester.Thusitispossibleforaprocesstolosecontrolofa
recordafterithaslockedit,necessitatingareacquisitionbeforetheupdateis
repeated.Thismethodleadstounnecessarypreemptionsandisalsosubjecttocyclic
restart,asillustratedinFigure3.
Advancedmodels
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fordeadlockanalysis
Althoughthebasicmodelsforanalyzingthedeadlocksituationincomputersystemshavebeenreviewed,manyotheraspectshave
notbeencovered.Inparticular,wehavenotmentionedprobabilisticorgame-playing
modelswhichattempttoavoiddeadlockbyensuring,withhighlikelihood,thatevery
stateinthesystemissecure.Abrieftreatmentof
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theresourcemanager,RM,iscompetingagainstindependentprocesses
28
demandingservice.Inthiscase,themanagermaybethoughtof
aswinningifalltheprocessescompletesuccessfully;theoppositionwinsifit
createsadeadlock.Devillers28informallyde
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nesdeadlockavoidance:\Fromsomeaprioriinformationabouttheprocesses,theresources,theoperatingsystem,etc.,determinewhatsituationsmayberealizedwithout
endangeringthesmoothrunningofthesystem.
Healsopresentsa
owchartmodelwhichinsomesensegeneralizesthemaximum-claims7andtask-step9methods.Unfortunately,
withthismodelitisnoteasytode
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nethesafestates,notonlybecauseaprocess
20
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mayhavemorethanonepossibleseriesofstepsthatitmaytraverse(futurehistory),butalsobecausetheremaynotexista
simpleworstpossiblefuturehistoryforeachprocess.
Toovercomethis
problemDevillersproposesaglobalapproach,inwhichastateisde
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nedsafeifandonlyifastrategyexistsfortheRMwhichensuresitssuccesswhateveroperationtheprocessesinthatstatechoose.
Astatewillbelosingifanoperationexistsfortheprocessessuch
thattheRMwilllosethegamewhateverstrategyitchooses.Thisapproach
throwsnewlightonthedeadlockproblembyprovidinga
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waytoconstructthesetofunsafestates,andhenceprovidingabasisforasystematicstudyofthepropertiesofthesafestates.
29
Probabilisticmodel.Aprobabilisticapproachtothedeadlockproblem
hasalsobeentried,andaninvestigationmadeofthelikelihoodofdeadlock
incertainclassesofsystems.Anystatediagramusedto
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representprocess-resourceinteractionscanalsobeviewedasa
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nitestateautomaton.Aprobabilitymeasurecanbeattachedtoanoccurrenceofeachpossibletransition.Arandomresourceallocationmodelisassumed,and
thesumoftheprobabilitiesassociatedwithtransitionsoutofagivenstate
isrequiredtobeunity.Byaddingauxiliarystoragetotheautomaton,
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rst-come-
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rst-serve(FCFS)andlast-come-
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rst-serve(LCFS)schedulerscanbemodeledtoform,respectively,aprobabilisticqueueautomatonandaprobabilistic
29
push-downautomaton.Thelikelihood
ofdeadlockismeasuredintermsoftheexpectedvalueof(1)the
numberofsystemactionstodeadlockor(2)thenumberofresourceallocations
todeadlock.
Calculationsarecarriedoutforsystems
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containingsmallnumbersofprocessesandresources.ForasystemwithtworesourcesandtwoprocessesthemeantimetodeadlockunderFCFSor
LCFSschedulingisshowntobeslightlylessthanthatunderrandomallocation.
Thefactthatonewouldexpecttheprobabilityofdeadlocktodecreaseif
thenumberofunitsoftheresourcesincreases(whilethe
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numberofprocessesremains
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xed)issubstantiated.Conversely,ina
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xed-resourcesystem,increasingthenumberofprocessesincreasestheprobabilityofdeadlocksincemoreprocessescompeteforthesamenumberofresources.However,it
isnotintuitivelyclearwhathappenstothedeadlockprobabilityifthenumber
ofprocessesandresourcesareuniformlyincreased.Forsmallsystemsithasbeen
shownthattheprobabilityofdeadlockactually
29
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increases.However,sincethemodelconsiderednomorethan
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toolittleisunderstoodabouttherelativeprobabilityofinterferenceanddeadlock.Fortransactionprocessingsystems,itisthoughtthatinterferenceisrareand
thatelaborateavoidancealgorithmswouldnotbeeconomical.30
Becauseof
theincreasingcomplexityofdistributeddatabases,thedeadlocksituationmustbehandlede ciently.
However,itisdi culttoestimatetheperformancee ectsof
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thesetechniques,ortheprobabilityofoccurrenceofdeadlocks,sincecommunicationtimeiscritical.Asdistributeddatabasesbecomemorewidelyavailable,experimentaldatacan
begatheredtomeasureperformancesandprobabilities,permittingexplorationofthecommunicationaspects.
Acombinedapproachtohandlingdeadlocksindatabasesystemsisprobably
themostpracticalsolution,buttheremaybefurtherproblems
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inintegrateddatabasesystemswhichallowprocessesaccesstoclassi
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eddata.Additionallevelsoflockingmaybenecessary,whichattheveryleastwillincreasethelockingcomplexity.
21
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networkofcomputers,resultinginthedatabasesbecomingmoreaccessibletolargernumbersofdiverseapplicationjobs,thedatabaseadministrator'sfunctionbecomesincreasinglycomplex.
Theactionsoftheoperatingsystem(whichmanagesapplicationjobs)andthoseof
theDBA(whomaintainsprocessintegrityandtheconsistencyofthedatabase)have
tobecoordinated.BoththeDBAandtheoperatingsystem
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designermustthoroughlyunderstandrelationshipsamongconcurrencycontrols,processors,processes,deadlock-handlingandrecoverytechniques,communicationaspects,andprotocols.Thisareaofresponsibilitybetweenthe
systemdesignerandtheDBAcallsfordeeperstudy,especiallyinanetwork
environment.
Acknowledgments
Thisresearchwassupportedinpartby
agrantfromtheNationalScienceandEngineeringCouncilof
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Canada.
Thee ortsofallCMPUT516studentswhocarefullyreadandcommentedonearlierversionsofthisarticlearemuchappreciated,
butspecialthanksaredueTerryCrocker,MurrayReid,andArnoldRiverafor
theirconstructivecriticismsandinsights.
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ConcurrentProcesses,Proc.1973NCC,Vol.42,AFIPSPress,Montvale,N.J.,June1973,pp.271{275.TheLOCK-UNLOCKmechanismoftheCodasylapproachto
datamanagement,whichenablesincrementalallocations,isdescribed.Anecessaryandsu cient
conditionfortheexistenceofdeadlockintermsofthee ectofthe
ALLOCATEfunctionisderived.Usingthisadeadlockdetectionscheme
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Thetechniqueisaveryshrewdmodi
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cationandcombinationof(a)trytopreclaimneededresources;(b)ifpreclaimingresourcesleadstoadeadlock,preemptresources;and(c)imposea
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nitedelay.
14.G.Schlageter,\AccessSynchronizationandDeadlockAnalysisinDatabaseSystems:AnImplementation-OrientedApproach,InformationSystems,Vol.1,No.2,
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tostartattheblockedprocessnodeandtestif
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-
8/2/2019 10.1.1.72.4304_DeadLock
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developed.Necessaryandsu cientconditionsfortheexistenceofadeadlockandtheavoidanceofdeadlockarederived.Adeadlockavoidanceschemeinthe
eventofaprocessentry/deletionandresourcerequest/releaseisdevised.
16.
CODASYLDataBaseTaskGroupReport,Conf.onDataSystemLanguages,ACM,New
York,Apr.1971.17.T.A.MarslandandS.S.
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Isloor,\DetectionofDeadlocksinDistributedDatabaseSystems,INFOR,Vol.18,No.1,Feb.1980,pp.1{20.Adiscussionofdeadlock-handlingtechniquesindistributed
systemsisprovided.Atutorialapproachtotheon-linedetectionmethodistaken.
Examplesfromrealworldapplicationsaredepictedtoshowtherealityofa
processhavingmultipleoutstandingresourcerequests.Forthiscase,the
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-
8/2/2019 10.1.1.72.4304_DeadLock
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rstapproachrequirestheallocationofallneededresourcesbeforeprocessinitiation.Theconceptofaprocesssetisintroducedandusedinthe
secondapproachproposedfordeadlockprevention.
19.F.J.Maryanski,\A
DeadlockPreventionAlgorithmforDistributedDataBaseManagementSystems,TechnicalReportCS77-02,
ComputerScienceDept.,KansasStateUniversity,Manhattan,Kan.,Feb.1977,
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24pp.Preventionalgorithmrequireseachprocesstocommunicateitsshareddataresourceslisttoallotherprocessesbeforeitcanproceed.Theshared
dataresourceslistisdeterminedbyusingprocesspro
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le.Proofofcorrectnessofthealgorithmissketched.
20.A.N.Chandra,W.G.Howe,andD.P.Karp,\CommunicationProtocol
forDeadlockDetectioninComputerNetworks,IBMTechnicalDisclosureBulletin,Vol.16,No.
10,Mar.1974,pp.3471{3481.Thetechniqueofdeadlockdetectionrequiresthemaintenance
ofresourcetablesateachinstallation,containinginformationonprocesses
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(local/remote)havingaccesstoresources(local/remote).Schemesfortheexpansionofresourcetablesaregiveninthenetworkenvironment.
21.S.A.
MahmoudandJ.S.Riordon,\SoftwareControlledAccesstoDistributedDataBases,INFOR,
Vol.15,No.1,Feb.1977,pp.22{36.24
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Twoapproaches{centralizedcontrolanddistributedcontrol{todeadlockdetectionaresuggested.Thecentralizedapproachdetectsdeadlocksbycreatinganoverallglobalpictureofthe
networkstatus.Inthedistributed-controlapproacheachcomputersendsidenticalmessagestoevery
otherone,andreceivesdi erentmessagesfromeachone,sothatadeadlock
maybedetectedatanyparticularinstallation.
22.
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B.Goldman,\DeadlockDetectioninComputerNetworks,TechnicalReportMIT/LCS/TR-185,LaboratoryforComputerScience,MIT,Cambridge,Mass.,Sept.1977,180pp.Thescheme
isbasedonthecreationandexpansionofwhatiscalledanordered
blockedprocesslist.TechniquestoexpandOBPLsbetweeninstallationsareprovidedtodetect
deadlocksinthenetworkenvironment.Formalproofofcorrectness
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oftheschemeisprovided.Failureoftwopublishedalgorithmsisdemonstrated.
23.S.S.IsloorandT.A.Marsland,\AnE ective
'On-LineDeadlockDetectionTechniqueforDistributedDataBaseManagementSystems,Proc.COMPSAC78,
Chicago,Ill.,Nov.1978,pp.283-288.
Theapplicabilityofdeadlock
detectionprinciplestodistributeddatabasesisinvestigated.Theconcept
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ofon-linedetectionofdeadlocksindistributedsystemsisde
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nedandintroduced,andane ectivealgorithmforon-linedetectionissuggested.Severalhighlightsoftheproposalinadistributedenvironmentareemphasized.Further
researchintheareaisidenti
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ed.
24.J.H.Howard,Jr.,\MixedSolutionsfortheDeadlockProblem,Comm.ACM,Vol.16,No.7,July1973,pp.427{430.A
thoroughdiscussionontheinappropriatenessoftheapplicabilityofdetection,avoidance,orprevention
principlesalonefortheentirespectrumofresourceallocationproblemsinoperatingsystems
isprovided.Amixedsolutioncombiningthebasicprinciplesand
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allowingtheselectionoftheoptimaloneforeachclassofresourcesinasystemissuggested.Withintheframeworkofresourceordering,the
methodcloselycorrespondstothehierarchicalstructureofoperatingsystems.
25.
D.W.BoettnerandM.T.Alexander,\TheMichiganTerminalSystem,Proc.IEEE,
Vol.63,No.6,June1975,pp.912{918.26.G.
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C.Pirkola,\AFileSystemforaGeneralPurposeTimesharingEnvironment,Proc.IEEE,Vol.63,No.6,June1975,pp.918{924.27.D.C.
TsichritzisandF.H.Lochovsky,DataBaseManagementSystems,AcademicPress,NewYork,
1977.28.R.Devillers,\GameInterpretationoftheDeadlockAvoidanceProblem,Comm.ACM,
Vol.20,No.10,Oct.1977,pp.741{745.Thedeadlockavoidanceproblem
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isde
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nedasdeterminingsafesituationswhichmayberealizedwithoutendangeringthesmoothrunningofthesystem,fromsomecausalinformationabouttheprocesses,
resources,andoperatingsystem.Aglobalapproachtothedeadlockphenomenonistaken,
andtheevolutionofthesystemisinterpretedasagamebetweenthe
operatingsystemand\naturerepresentedbytheprocesses.Thenotion
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ofriskandsafetyarepreciselyde
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ned,andcanbeusedtostudythepropertiesofthesafestates.
29.C.A.Ellis,\ProbabilisticModelsofComputerDeadlock,
ReportCU-CS-041-74,Dept.ofComputerScienceUniversityofColorado,Boulder,Colo.,Apr.1974,
25pp.Theprobabilityofincreaseordecreaseindeadlocks,asthenumber
ofprocessesandresourceswithinacomputersystemincreases,is
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investigated.Amodelapplicabletotheinvestigationofthisproblemis
25
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presented.Basicallytheprocess-resourceinteractionsaretreatedaspotentialmembersofthesetofstringsacceptedbyaprobabilisticautomaton.Resultsofcalculations
onactualcomputersystemmodelsarealsodescribed,indicatingthattheprobabilityof
deadlockincreasesforthetypesofsystemsconsidered.
30.R.Peebles
andE.Manning,\SystemArchitectureforDistributedDataManagement,Computer,
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Vol.11,No.1,Jan.1978,pp.40{47.Bibliography
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