Lecture 6: Deadlock - UC San Diego

Lecture6:DeadlockCSE120:PrinciplesofOpera=ngSystems

UCSanDiego:SummerSessionI,2009FrankUyeda

Announcements

•  Homework2isdue.– Nonewhomeworkthisweek.

•  Project1:– Deadlinetonightatmidnight.

•  Project2isnowonline.

•  MidtermExam–  Friday7/17–  Coverseverythingthroughtoday’slecture

2

Review

•  SchedulingMetrics– Averagewait=me,CPUu=liza=on,etc

•  SchedulingAlgorithms– Firstcome,firstserve– ShortestJobFirst/ShortestTimeRemainingFirst– RoundRobin– Priority

3

MarsPathfinder

•  July4,1997landingonMar=alsurface•  Seriesofso[wareglitchesstartedafewdaysa[er

landing–  EventuallydebuggedandpatchedremotelyfromEarth

Note:informa=onandimagesadaptedfrom[mdwelsh,harvard] 4

VxWorksOpera=ngSystem

•  Real‐=meOS(WindRiverSystems)–  OSusesapriorityscheduler

•  Importantforreal‐=metasks

–  Concurrentthreadscommunicateoninforma=onbus•  Sharedmemoryareaguardedbyamonitor

Informa=onBus(Monitor)

MutexWeatherDataThread

Communica=onThread

Informa=onBusThread

5







Communica=onThread

Informa=onBusThread

Obtainmutex;writedata

Waitformutextoreaddata

6







Communica=onThread

Informa=onBusThread

Freemutex;


7







Communica=onThread

Informa=onBusThreadObtainmutex;readdata

8


•  Introducepriori=estothesethreads



Communica=onThread

Informa=onBusThread

Lowpriority

Mediumpriority

Highpriority

9


•  Introducepriori=estothesethreads



Communica=onThread

Informa=onBusThread

Lowpriority

Mediumpriority

Highpriority



10


•  Introducepriori=estothesethreads–  OS=merinterrupt

–  Schedulerrunshigherprioritythread



Communica=onThread

Informa=onBusThread

Lowpriority

Mediumpriority

Highpriority



Interrupt!

11

VxWorksOpera=ngSystem•  Introducepriori=estothesethreads

–  OS=merinterrupt–  Schedulerrunshigherprioritythread

•  Communica=onthreadhashigherprioritythanweatherdatathread•  Butthehighpriorityinforma=onbusthreadisstuckwai=ng

–  Thisscenarioiscalledpriorityinversion



Communica=onThread

Informa=onBusThread

Lowpriority

Mediumpriority

Highpriority



12

PriorityInversion

•  Problemdescrip=on– Ahighprioritythreadisstuckwai=ngforalowprioritythreadtofinish

–  Inthiscase,themediumprioritythreadwasholdingupthelow‐prioritythread

•  Howdowefixthis?

13

PriorityInheritance

•  Lowprioritythreadwinstheraceforthelock.



Communica=onThread

Informa=onBusThread

Lowpriority

Mediumpriority

Highpriority



14

PriorityInheritance

•  Lowprioritythreadwinstheraceforthelock.•  Lowprioritythreadtemporarilyupgradedtohighpriority.



Communica=onThread

Informa=onBusThread

Highpriority

Mediumpriority

Highpriority



15

PriorityInheritance

•  Lowprioritythreadwinstheraceforthelock.•  Lowprioritythreadtemporarilyupgradedtohighpriority.



Communica=onThread

Informa=onBusThread

Highpriority

Mediumpriority

Highpriority



16

Interrupt!



Communica=onThread

Informa=onBusThread

HighpriorityLowpriority

Mediumpriority

Highpriority

Releasemutex


17

•  Lowprioritythreadwinstheraceforthelock.•  Lowprioritythreadtemporarilyupgradedtohighpriority.•  A[erreleasingmutex,downgradedtolowpriority.

PriorityInheritance

PriorityInheritance

•  Solu=onapproach:PriorityInheritance–  Allowathreadtoinheritthepriorityofanythreadthatis

wai=ngforit–  Blocked,higherprioritythreads“donate”theirprioritytolower

prioritythreads•  Dona=ononlyoccursaslongasthehigherprioritythreadisblocked–oncethisisnolongertrue,threadsretaintheiroriginalpriority

•  Forourexample–  Mediumthreadwillnolongerblocklowprioritythread–  FortheMarspathfinder,theyuploadedthefixremotelyand

everythingwasfinea[erthat•  Abletoreplaybugsinlab;resetrealmachinetoknown“goodstate”anduploadfix–goodlessonsfordevelopinglarge,real‐=mesystems

18

Starva=onandDeadlock•  Twotypesofresources:

–  Preemp=ble:cantakeawayfromathread(e.g.,CPU..)–  Non‐preemp=ble:can’ttakeawayfromathread(e.g.,mutex,lock..)

•  Starva=on:–  Athreadnevermakesprogressbecauseotherthreadsareusingaresourceitneeds

•  Deadlock:–  Acircularresourcedependencybetweenmul=plethreads(note:threadsisinterchangeablewithprocesses,jobs,tasksinthiscontext)

•  Starva=onisnotthesameasdeadlock 19

DeadlockExampleMonitor account { double balance;

double withdraw(amount) { balance = balance - amount; return balance; }

double deposit(amount) { balance = balance + amount; return balance; } }

// transfer from Alice to Bob Alice.mutex.acquire(); Alice.withdraw(amount); Bob.mutex.acquire(); Bob.deposit(amount); …Alice.mutex.release();

// two bank accounts protected by a // monitor, with an associated mutex

Monitor Alice, Bob, Eve;

Thread1

// transfer from Bob to Alice Bob.mutex.acquire(); Bob.withdraw(amount); Alice.mutex.acquire(); Alice.deposit(amount); …Bob.mutex.release();

Thread2

Thread1andThread2areDeadlocked!

20

Deadlock

•  Processesthatacquiremul=pleresourcesaredependentonthoseresources–  E.g.,lockssemaphores,monitors,etc.

•  Whatifoneprocesstriestoallocatearesourcethatasecondprocessholds,andvice‐versa?– Neithercanmakeprogress!

•  Wecallthissitua=ondeadlockandwe’lllookat:– Defini=onandcondi=onsnecessaryfordeadlock–  Representa=onofdeadlockcondi=ons– Approachestodealingwithdeadlock

21

DeadlockDefini=on

•  Defini=on:– Deadlockexistsamongasetofprocessesifeveryprocessiswai=ngforaneventthatcanbecausedonlybyanotherprocessintheset.

•  Deadlockisaproblemthatcanarise:– Whenprocessescompeteforaccesstolimitedresources

– Whenprocessesareincorrectlysynchronized

22

Condi=onsforDeadlock

Deadlockcanexistifandonlyiffourcondi=onshold:•  Mutualexclusion–Atleastoneresourcemustbeheld

inanon‐sharablemode(i.e.,onlyoneinstance)•  Holdandwait–Theremustbeoneprocessholding

oneresourceandwai=ngforanotherresource•  Nopreemp=on–resourcescannotbepreempted

(i.e.,cri=calsec=onscannotbeabortedexternally)•  Circularwait–Theremustexistasetofprocesses{P1,

P2,P3,…,Pn}suchthatP1iswai=ngforaresourceheldbyP2,P2iswai=ngforP3,…,andPnforP1

23

ResourceAlloca=onGraph

•  Deadlockcanbedescribedusingaresourcealloca=ongraph(RAG)

•  TheRAGconsistsofsetsofver=cesP={P1,P2,P3,…,Pn}ofprocessesandR={R1,R2,R3,…,Rm}resources–  AdirectededgefromaprocesstoaresourcePiRj,impliesthatPihasrequestedRj

–  AdirectededgefromaresourcetoaprocessRiPj,impliesthatRihasbeenacquiredbyPj

–  Eachresourcehasafixednumberofunits

•  Ifthegraphhasnocycles,deadlockcannotexist•  Ifthegraphhasacycle,deadlockmayexist

24

RAGExample

Acycle…deadlock? Acycle…deadlock?

25

DealingwithDeadlock

Therearefourwaystodealwithdeadlock:•  Ignoreit

–  Doyoufeellucky?

•  Preven=on–  Makeitimpossiblefordeadlocktohappen

•  Avoidance–  Controlalloca=onofresources

•  Detec=onandrecovery–  Lookforacycleindependencies

26

DeadlockPreven=on

Preventatleastoneofnecessarycondi=ons:•  Mutualexclusion

–  Makeresourcessharable

•  Holdandwait–  Processcan’tholdoneresourcewhenreques=nganother–  Processrequests,releasesallneededresourcesatonce

•  Preemp=on–  OScanpreemptresource(costly)

•  Circularwait–  Imposeanordering(numbering)ontheresourcesand

requesttheminorder(populartechnique)27

DeadlockAvoidance

•  Howtoavoiddeadlock– Requireprocessestoprovideinforma=oninadvanceaboutwhatresourceswillbeneeded

– Systemgrantsresourcestoavoidcirculari=es(waitdependencies)

•  Toughtodo– Hardtodetermineallresourceneedsinadvance

– Goodtheore=calproblem,notasprac=cal

28

Banker’sAlgorithm

TheBanker’sAlgorithmistheclassicapproachtodeadlockavoidanceforresourceswithmul=pleunits

•  Assignacreditlimittoeachcustomer(task)–  Maximumcreditclaimmustbestatedinadvance

•  Rejectanyrequestthatleadstoadangerousstate–  Adangerousstateisonewhereasuddenrequestbyany

customerforthefullcreditlimitcouldleadtodeadlock–  Arecursivereduc=onprocedurerecognizesdangerous

states•  Inprac=ce,thesystemmustkeepresourceusagewell

belowcapacitytomaintainaresourcesurplus–  Rarelyusedinprac=ceduetolowresourceu=liza=on

29

Banker’sAlgorithmExample

A B C

2 2 1

30

Availablesystemresources

A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1

Currentlyallocatedresources

perProcess

A B C

P1 1 1 0

P2 2 0 1

P3 0 1 1

Processes(maximumresources)


A B C

2 2 1

31


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 1 1 0

P2 2 0 1

P3 0 1 1


Astateissafeifitispossibleforallprocessestoexecuteandterminate


A B C

2 1 0

32


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 1 1 0

P2 2 1 2

P3 0 1 1



P2canterminate


A B C

4 2 2

33


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 1 1 0

P2 0 0 0

P3 0 1 1




A B C

2 1 0

34


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 3 2 2

P2 0 0 0

P3 0 1 1



P1canterminate


A B C

5 3 2

35


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 0 0 0

P2 0 0 0

P3 0 1 1




A B C

3 0 2

36


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 0 0 0

P2 0 0 0

P3 2 4 1



P3canterminate


A B C

2 2 1

37


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 1 1 0

P2 2 0 1

P3 0 1 1


P1’srequestA B C

2 1 1


A B C

0 1 0

38


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 3 2 1

P2 2 0 1

P3 0 1 1


P1’srequestA B C

2 1 1

P1cannotterminate


A B C

0 1 0

39


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 3 2 1

P2 2 0 1

P3 0 1 1


P1’srequestA B C

2 1 1

P2cannotterminate


A B C

0 1 0

40


A B C

P1 3 2 2

P2 2 1 2

P3 2 4 1


perProcess

A B C

P1 3 2 1

P2 2 0 1

P3 0 1 1


P1’srequestA B C

2 1 1

P3cannotterminate

Resourcerequestisdeniedordelayedbecauseitdoesnotleadtoasafestate

DeadlockDetec=onandRecovery

•  Detec=onandRecovery–  Ifwedon’thavedeadlockpreven=onoravoidance,thendeadlockmayoccur

–  Inthiscase,weneedtodetectdeadlockandrecover•  Todothis,weneedtwoalgorithms

– Onetodeterminewhetheradeadlockhasoccurred– Anothertorecoverfromthedeadlock

•  Possible,butexpensive(=meconsuming)–  ImplementedinVMS–  Rundetec=onalgorithmifresourcerequest=mesout

41

DeadlockDetec=on

•  Detec=on–  Traversetheresourcegraphlookingforcycles–  Ifacycleisfound,needtotryanddeterminewhichresourceorprocesstopreempt

•  Expensive– Manyprocessesandresourcestotraverse

•  Onlyinvokedetec=onalgorithmdependingon–  Howo[enorlikelydeadlockis–  Howmanyprocessesarelikelytobeaffectedwhenitoccurs

•  Thereisatradeoffbetweenoverheadfromrunningalgorithmfrequently,andbeingabletoiden=fyoffendingprocess

42

DeadlockRecovery

Onceadeadlockisdetected,wehavetwoop=ons…•  Abortprocesses

– Abortalldeadlockedprocesses•  Processesneedtostartoveragain

– Abortoneprocessata=meun=lcycleiseliminated•  Systemneedstorerundetec=ona[ereachabort

•  Preemptresources(forcetheirrelease)– Needtoselectprocessandresourcetopreempt– Needtorollbackprocesstopreviousstate– Needtopreventstarva=on

43

DeadlockSummary

•  Deadlockoccurswhenprocessesarewai=ngoneachotherandcannotmakeprogress–  CyclesinResourceAlloca=onGraph(RAG)

•  Deadlockrequiresfourcondi=ons:– Mutualexclusion,Holdandwait,Noresourcepreemp=on,circularwait

•  Fourapproachestodealingwithdeadlock:–  Ignoreit:risky–  Preven=on:makeoneofthefourcondi=onsimpossible–  Avoidance:banker’salgorithm(controlalloca=on)–  Detec=onandRecovery:lookforacycle,preemptorabort

44

NextTime

•  MidtermReview–  Bringques=ons

•  Peerwiseques=onsduetomorrowatmidnight.

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

45

Documents

Lecture 6: Deadlock - UC San Diego