Synchroniza+on

CS4410,Opera+ngSystems

Fall2016CornellUniversity

RachitAgarwalAnneBracy

See:Ch5&6inOSPPtextbook

TheslidesaretheproductofmanyroundsofteachingCS4410byProfessorsSirer,Bracy,Agarwal,George,andVanRenesse.

Synchroniza+on:Topic1

2

Synchroniza+onMo+va+on&Basics• RaceCondi+ons• Cri+calSec+ons• Example:TooMuchMilk• BasicHardwarePrimi+ves• BuildingaSpinLock

ThreadsShareMemory

3

Threadshave:•Privateregisters

Contextswitchingsavesandrestoresregisterswhenswitchingfromthreadtothread

•Shared“global”memoryGlobalmeansnotstackmemory

•UsuallyprivatestackPointersintostacksacrossthreadsfrownedupon

2threadsupda+ngasinglesharedvariableamount• Onethreadwantstodecrementamountby$10K• Otherthreadwantstodecrementamountby50%

Whathappenswhentwothreadsexecuteconcurrently?

Memory

TwoTheads,OneVariable

4

...amount-=10,000;

...

...amount*=0.5;

...

100,000amount

T1 T2

Itwon’tactuallyexecutelikethis:

Memory

TwoTheads,OneVariable

5

...r1=loadfromamountr1=r1–10,000storer1toamount...

...r2=loadfromamountr2=0.5*r2storer2toamount...

100,000amount

T1 T2

Itmightexecutelikethis:

Memory

TwoTheads,OneVariable

6

...r1=loadfromamountr1=r1–10,000storer1toamount...

...r2=loadfromamountr2=0.5*r2storer2toamount...

100,000amount

Everythingworks:althoughdifferent,eitherorderiscorrect

T1

T2

Oritmightexecutelikethis:

Memory

TwoTheads,OneVariable

7

...r1=loadfromamountr1=r1–10,000storer1toamount...

...r2=loadfromamount

...

r2=0.5*r2storer2toamount...

100,000amount

LostUpdate:Wrong..andverydifficulttodebug

T1

T2

RaceCondiLons

8

Allpossibleinterleavingsshouldbesafe!

=+mingdependenterrorinvolvingsharedstate•OncethreadAstarts,itneedsto“race”tofinish•Whetherracecondi+onhappensdependsonthreadschedule•Different“schedules”or“interleavings”(totalorderonmachineinstruc+ons)

RaceCondiLonsareHardtoDebug

9

•Numberofpossibleinterleavingsishuge•Someinterleavingsaregood•Someinterleavingsarebad:

•Butbadinterleavingsmayrarelyhappen!•Works100x≠noracecondi+on

•Timingdependent=smallchangescanhidebug

CaseStudy:Therac-25

Example:RaceswithQueues

10

2concurrentenqueue()opera+ons?2concurrentdequeue()opera+ons?

Whatcouldpossiblygowrong?

tail head

CriLcalSecLon

11

Codethatcanbeexecutedbyonlyonethreadata+me

GoalsSafety:1threadinacri+calsec+onat+meLiveness:allthreadseventuallymakeitintoCSifdesiredFairness:allhaveequalchancesofgemngintoCS…inprac+ce,fairnessrarelyguaranteed

...CSEnter();CriticalsectionCSExit();...

...CSEnter();CriticalsectionCSExit();...

T1 T2

TooMuchMilkProblem

12

2roommatesfridgealwaysstockedwithmilk•fridgeisempty→needtorestockit•don’twanttobuytoomuchmilk

Caveats•Onlycommunicatebyanotepadonthefridge•Notepadhascellswithnames,likevariables:outtobuymilk

TASK:Writethepseudo-codetoensurethatatmostoneroommategoestobuymilk

0

SoluLon#1:NoProtecLon

13

iffridge_empty(): buy_milk()

iffridge_empty(): buy_milk()

T1 T2

Safety:Onlyoneperson(atmost)buysmilkLiveness:Ifmilkisneeded,someoneeventuallybuysit.Fairness:Roommatesequallylikelytogotobuymilk.

Safe?Live?Fair?

SoluLon#2:addabooleanflag

14

while(outtobuymilk):do_nothing();iffridge_empty():outtobuymilk=1buy_milk() outtobuymilk=0

while(outtobuymilk):do_nothing();iffridge_empty():outtobuymilk=1buy_milk() outtobuymilk=0

T1 T2

Safety:Onlyoneperson(atmost)buysmilkLiveness:Ifmilkisneeded,someoneeventuallybuysit.Fairness:Roommatesequallylikelytogotobuymilk.Safe?Live?Fair?

outtobuymilkini+allyfalse

SoluLon#3:addtwobooleanflags!

15

blueonit=1ifnotgreenonitandfridge_empty(): buy_milk()blueonit=0

greenonit=1ifnotblueonitandfridge_empty(): buy_milk()greenonit=0

T1 T2

oneforeachroommate(ini+allyfalse)blueonit,greenonit

Safety:Onlyoneperson(atmost)buysmilkLiveness:Ifmilkisneeded,someoneeventuallybuysit.Fairness:Roommatesequallylikelytogotobuymilk.Safe?Live?Fair?

SoluLon#4:asymmetricflags!

16

blueonit=1whilegreenonit:do_nothing()iffridge_empty(): buy_milk()blueonit=0

greenonit=1ifnotblueonitandfridge_empty(): buy_milk()greenonit=0

T1 T2

‒ complicated (and this is a simple example!) ‒ hard to ascertain that it is correct ‒ asymmetric code is hard to generalize & unfair

oneforeachroommate(ini+allyfalse)blueonit,greenonit

Safe?Live?Fair?

LastSoluLon:Peterson’sSoluLon

17

blueonit=1turn=greenwhile(greenonitandturn==green):do_nothing()iffridge_empty(): buy_milk()blueonit=0

greenonit=1turn=bluewhile(blueonitandturn==blue):do_nothing()iffridge_empty(): buy_milk()greenonit=0

T1 T2

‒ complicated (and this is a simple example!) ‒ hard to ascertain that it is correct ‒ hard to generalize, inefficient

anotherflagturn{blue,green}

Safe?Live?Fair?

HardwareSoluLon

18

•Hardwareprimi+vestoprovidemutualexclusion•Typicallyreliesonamul+-cyclebusopera+onthatatomicallyreadsandupdatesamemoryloca+on

•ExampleSpecofTest-And-Set:

setsthevalueto1,returnsformervalue

ATOMICintTestAndSet(int*var){ intoldVal=*var; *var=1; returnoldVal;}

Spinlocks

19

SL_acquire(int*lock){while(test_and_set(lock)) /*donothing*/;}

SL_release(int*lock){ *lock=0;}

BuyingMilkwithSpinlock

20

Sharedspinlock:intbuyingmilk,ini+ally0

SL_acquire(&buyingmilk)iffridge_empty(): buy_milk()SL_release(&buyingmilk)

SL_acquire(&buyingmilk)iffridge_empty(): buy_milk()SL_release(&buyingmilk)

T1 T2

SpinLockIssues

21

Par+cipantsnotincri+calsec+onmustspin→was<ngCPUcycles

• Replacethe“donothing”loopwitha“yield()”?Processeswoulds+llbescheduledanddescheduled

Needbeqerprimi+ve:• allowsoneprocesstopassthrough• allotherstosleepun+ltheycanbeexecutedagain

Synchroniza+on:Topic2

22

Semaphores• Defini+on• BinarySemaphores• Coun+ngSemaphores• Implemen+ngSemaphores• ClassicSynchroniza+onProblems(w/Semaphores)

• Producer-Consumer(w/aboundedbuffer)• Readers/WritersProblems

• ClassicSemaphoreMistakes• SemaphoresConsideredHarmful

Non-nega+veintegerw/atomicincrement,decrementS=newSemaphore(init)//mustinitialize!Canonlybemodifiedby:

• P(S):decrementorblockifalready0• V(S):increment&wakeupanywai+ngthreads• Nointerfacetoreadthevalue

WhatisaSemaphore?

23

Opera+onshavethefollowingseman+cs:

[Dijkstra 1962]

P(S){while(S==0);S-=1;}

V(S){S+=1;}

Dutch 4410: P = Probeer (‘Try') and V = Verhoog ('Increment', 'Increase by one')

BinarySemaphore

24

Semaphorevalueiseither0or1• Usedformutualexclusion

(semaphoreasamoreefficientlock)• Ini+ally1inthatcase

P(S)CriticalSection()V(S)

P(S)CriticalSection()V(S)

T1 T2

SemaphoreSS.init(1)

Semacountcanbeanyinteger• Usedforsignalingorcoun+ngresources• Typically:

• onethreadperformsP()toawaitevent• anotherthreadperformsV()toalertwai+ngthreadthateventhasoccurred

CounLngSemaphores

25

PacketProcessor():x=get_packet_from_card()enqueue(packetq,x);V(packetarrived);

NetworkingThread():P(packetarrived);x=dequeue(packetq);print_contents(x);

T1 T2

Semaphorepacketarrivedpacketarrived.init(0)

StarterSemaphoreimplementaLon

26

P(Sema*s){while(TRUE){ ifs.countbigenoughbreak; } decrements.count}

V(Sema*s){increments.count

}

structSemaphore{intcount;}

Dowelikethis?

2.Busy-wai<ngisbad!Countnotbigenough?→makenoteofthread’sinterestandstopit.(haveVstartitagainwhencountbigenough)

Assumeathread“stuck”inP()willbeeventuallyinterrupted

1.SharedstateisnotprotectedThreadscanbeinterruptedatanypoint?→needalock

Producer-ConsumerProblem

27

2+threadscommunicate:somethreadsproducedatathatothersconsume

Boundedbuffer:sizeNProducerprocesswritesdatatobuffer• Writestoinandmovesrightwards• Don’twritemorethanN!Consumerprocessreadsdatafrombuffer• Readsfromoutandmovesrightwards• Don’tconsumeifthereisnodata!

Example:“pipe”(|)inUnix>catfile|sort|uniq|more

0 N-1

in out

SoluLon#1:NoProtecLon

28

//additemtobuffervoidproduce(intitem){buf[in]=item;in=(in+1)%N;}

//removeitemintconsume(){intitem=buf[out];out=(out+1)%N;returnitem;}

Problems:1.Unprotectedsharedstate(mul+pleproducers/consumers)2.Inventory:•Consumercouldconsumewhennothingisthere!•Producercouldoverwritenot-yet-consumeddata!

Shared:intbuf[N];intin,out;

SoluLon#2:AddMutexSemaphores

29

//additemtobuffervoidproduce(intitem){P(mutex_prod);

buf[in]=item;in=(in+1)%N;V(mutex_prod);

}

//removeitemintconsume(){P(mutex_cons);intitem=buf[out];out=(out+1)%N;V(mutex_cons);returnitem;}

Shared:intbuf[N];intin,out;Semaphoremutex_prod(1),mutex_cons(1);

now atomic

SoluLon#3:AddCommunicaLonSemaphores

30

voidproduce(intitem){P(nRoom);//need spaceP(mutex_prod);

buf[in]=item;in=(in+1)%N;V(mutex_prod);V(nData);//new item!

}

intconsume(){P(nData);//need itemP(mutex_cons);intitem=buf[out];out=(out+1)%N;V(mutex_cons);V(nRoom);//more space!

returnitem;}

Shared:intbuf[N];intin,out;Semaphoremutex_prod(1),mutex_cons(1);SemaphorenRoom(N),NData(0);

Readers-WritersProblem

31

Modelsaccesstoadatabase:shareddatathatsomethreadsreadandotherthreadswriteWanttoallow:•mul+pleconcurrentreaders—OR—(exclusive)•onlyasinglewriterata+me

Example:makinganairlinereserva+on• Whenyoubrowsetolookatflightschedulesthewebsiteactsasareaderonyourbehalf

• Whenyoureserveaseat,websitehastowriteintodatabasetomakethereserva+on

[Courtois+ 1971]

Readers-WritersConstraints

32

Nthreadsshare1objectinmemory• Somewrite:1writerac+veata+me• Someread:nreadersac+vesimultaneously

Insight:generalizesthecri+calsec+onconcept

Ques+ons:1.Writerac+ve&comboofreaders/writersarrive.Whoshouldgetinnext?

2.Writerwai+ng&endlessof#ofreaderscome.FairforthemtobecomeacLve?

Fornow:back-and-forthturn-taking:• Ifareaderiswai+ng,readersgetinnext• Ifawriteriswai+ng,onewritergetsinnext

Readers-WritersSoluLon

33

voidwrite(){

P(rw_lock);.../*performwrite*/...V(rw_lock);

}

intread(){P(count_mutex);rcount++;if(rcount==1)P(rw_lock);V(count_mutex);.../*performread*/...P(count_mutex);rcount--;if(rcount==0)V(rw_lock);V(count_mutex);}

Shared:intrcount;Semaphorecount_mutex(1);Semaphorerw_lock(1);

Readers-Writers:UnderstandingtheSoluLon

34

Ifthereisawriter:• Firstreaderblocksonrw_lock• Otherreadersblockonmutex

Onceareaderisac+ve,allreadersgettogothrough• Whichreadergetsinfirst?

Thelastreadertoexitsignalsawriter• Ifnowriter,thenreaderscancon+nue

Ifreadersandwriterswai+ngonrw_lock&writerexits• Whogetstogoinfirst?

Readers-Writers:AssessingtheSoluLon

35

Whenreadersac+venowritercanenter✔• Writerswait@P(rw_lock)

Whenwriterisac+venobodycanenter✔• Anyotherreaderorwriterwillwait(where?)

Back-and-forthisn’tsofair:• Anynumberofreaderscanenterinarow• Readerscan“starve”writers

Fairback-and-forthsemaphoresolu+onistricky!• Tryit!(don’tspendtoomuch+me…)

ClassicSemaphoreMistakes

36

P(S)CSP(S)

I

V(S)CSV(S)

P(S)CS

J

K

P(S)if(x)return;CSV(S)

L

Istuckon2ndP().Subsequentprocessesfreezeupon1stP().

Underminesmutex:•Jdoesn’tgetpermissionviaP()•“extra”V()sallowotherprocessesintotheCSinappropriately

NextcalltoP()willfreezeup.Confusingbecausetheotherprocesscouldbecorrectbuthangswhenyouuseadebuggertolookatitsstate!

ConditionalcodecanchangecodeflowintheCS.Causedbycode

updates(bugfixes,etc.)bysomeoneotherthanoriginalauthorofcode.

⬅typo

⬅typo

⬅omission

SemaphoresConsideredHarmful

37

“Duringsystemconcep+onittranspiredthatweusedthesemaphoresintwocompletelydifferentways.Thedifferenceissomarkedthat,lookingback,onewonderswhetheritwasreallyfairtopresentthetwowaysasusesoftheverysameprimi+ves.Ontheonehand,wehavethesemaphoresusedformutualexclusion,ontheotherhand,theprivatesemaphores.”

—Dijkstra“Thestructureofthe’THE’-Mul+programmingSystem”Communica+onsoftheACMv.11n.5May1968.

SemaphoresNOTtotherescue!

38

Semaphoresare“low-level”primi+ves.Smallerrors:• Easilybringsystemtogrindinghalt• Verydifficulttodebug

Twousagemodels:• Mutualexclusion:“real”abstrac+onisacri+calsec+on• Communica<on:threadsusesemaphorestocommunicate(e.g.,boundedbufferexample)

Simplifica<on:Provideconcurrencysupportincompiler➙EnterMonitors

Synchroniza+on:Topic3

39

Monitors&Condi<onVariables• Defini+on• Seman+cs• SimpleMonitorExample• vs.Semaphores• ClassicSynchroniza+onProblems(w/Monitors)

• BoundedBufferProducer-Consumer• Readers/WritersProblems

• ClassicSynchroniza+onMistakes(w/Monitors)

WhatisaMonitor?

40

AbstractDataTypeforhandlingsharedresources,comprising:1.SharedPrivateData

• theresource• cannotbeaccessedfromoutside

2.Proceduresthatoperateonthedata• gatewaytotheresource• canonlyactondatalocaltothemonitor

3.Synchroniza+onprimi+ves• amongthreadsthataccesstheprocedures

[Hoare 1974]

MonitorSemanLcsguaranteemutualexclusion

41

Onlyonethreadcanexecutemonitorprocedureatany+me(aka“inthemonitor”)

Monitormonitor_name{// shared variable declarationsprocedureP1(){}

procedureP2(){}..procedurePN(){}

initialization_code(){}}

Monitorstack{inttop;

voidpush(any_t*){}

any_t*pop(){}

initialization_code(){}}

intheabstract forexample:

only one operation can execute at a time

MonitorscandefineCondi+onVariables

42

Amechanismtowaitforevents3opera+onsonCondi+onVariableConditionx;•x.wait():atomically release monitor lock and relinquish

processor, sleep until woken up (or you wake up on your own)• x.signal():wake at least one process waiting on condition (if there is one). No history associated with signal. • x.broadcast():wakeallprocesseswai+ngoncondition (useful for resource manager)

You must hold the monitor lock to call these operations.

!! NOT the same thing as UNIX wait and UNIX signal !!

UsingCondiLonVariables

43

Towaitforsomecondi+on: whilenotsome_predicate(): CV.wait()

•Atomicallyreleasesthemonitorlockandyieldsprocessor

•asCV.wait()returns,lockisautoma+callyreacquired

Whenthecondi+onbecomessa+sfied: CV.broadcast(): wakesupallthreads CV.signal():wakesupatleastonethread

TypesofWaitQueues

44

Monitorshavetwokindsof“wait”queues• Entrytothemonitor:hasaqueueofthreadswai+ngtoobtainmutualexclusion&enter

• Condi<onvariables:eachcondi+onvariablehasaqueueofthreadswai+ngontheassociatedcondi+on

KidandCookThreads

45

kid_main(){

dig_in_mud();BK.kid_eat();bathe();draw_on_walls();BK.kid_eat();

facetime_Karthik();facetime_oma();BK.kid_eat();}

cook_main(){wake();shower();drive_to_work();while(not_5pm)BK.makeburger();drive_to_home();watch_got();sleep();

}

MonitorBurgerKing{Lockmlock;

intnumburgers=0;conditionhungrykid;

voidkid_eat(){mlock.acquire() while(numburgers==0) hungrykid.wait() numburgers-=1mlock.release()}

voidmakeburger(){mlock.acquire() ++numburger; hungrykid.signal();mlock.release()}}

0

Ready Running

notalways

thisexplicit

SynchronizaLon:TheBigPicture

46

Shared Objects

Synchronization Variables

Atomic Instructions

Hardware

Interrupt Disable

Bounded Buffer

Multiple Processors

Semaphores Locks

Test-and-Set

Barrier

Hardware Interrupts

Condition Variables

Concurrent Applications

LanguageSupport

47

Canbeembeddedinprogramminglanguage:• Compileraddssynchroniza+oncode,enforcedatrun+me

• Mesa/CedarfromXeroxPARC• Java:synchronized,wait,no+fy,no+fyall• C#:lock,wait(with+meouts),pulse,pulseall• Python:acquire,release,wait,no+fy,no+fyAll

Producer-Consumer

48

MonitorProducer_Consumer{charbuf[SIZE];intn=0,tail=0,head=0;conditionnot_empty,not_full;voidproduce(charch){ while(n==SIZE) wait(not_full); buf[head]=ch; head=(head+1)%SIZE; n++;notify(not_empty);}

charconsume(){ while(n==0) wait(not_empty); ch=buf[tail]; tail=(tail+1)%SIZE; n--; notify(not_full); returnch;}}

Whatifnothreadiswai+ngwhenno+fy()called?

Thensignalisanop.VerydifferentfromcallingV()onasemaphore–semaphoresrememberhowmany+mesV()wascalled!

CondiLonVariablesvs.Semaphores

49

Accesstomonitoriscontrolledbyalock.Tocallwaitorsignal,threadmustbeinmonitor(=havelock).Waitvs.P:

• SemaphoreP()blocksthreadonlyifvalue<1• waitalwaysblocks&givesupthemonitorlock

Signalvs.V:causeswai+ngthreadtowakeup• V()increments➙futurethreadsdon'twaitonP()• Nowai+ngthread➙signal=nop• Condi+onvariableshavenohistory!

Monitorseasierandsaferthansemaphores• Lockacquire/releaseareimplicit,cannotbeforgoqen• Condi+onforwhichthreadsarewai+ngexplicitlyincode

ReadersandWriters

50

MonitorReadersNWritersintwaitingWriters=0,waitingReaders=0,nReaders=0,nWriters=0;ConditioncanRead,canWrite;

voidBeginWrite()assert(nReaders==0ornWriters==0)++waitingWriterswhile(nWriters>0ornReaders>0) canWrite.wait();--waitingWritersnWriters=1;

voidEndWrite()assert(nWriters==1andnReaders==0)nWriters=0ifWaitingWriters>0 canWrite.signal();elseifwaitingReaders>0canRead.broadcast();

voidBeginRead() assert(nReaders==0ornWriters==0) ++waitingReaders while(nWriters>0orwaitingWriters>0) canRead.wait(); --waitingReaders ++nReaders

voidEndRead() assert(nReaders>0andnWriters==0) --nReaders; if(nReaders==0andwaitingWriters>0)canWrite.signal();

[slide31]

UnderstandingtheSoluLon

51

Awritercanenterif:• nootherac+vewriter

and• nowai+ngreaders

Whenawriterfinishes:checkstoseeifreaderswai+ng:Y➙letsallenterN➙ifwriterwai+ng,lets1enter

Areadercanenterif:• noac+vewriterand• nowai+ngwriters

Whenlastreaderfinishes:• itlets1writerin(ifany)

Fair?

52

Wantstobefair:• Ifawriteriswai+ng,readersqueueup• Ifareader(oranotherwriter)isac+veorwai+ng,writersqueueup

…mostlyfair,althoughonceitletsareaderin,itletsALLwai+ngreadersinallatonce,evenifsomeshowedup“a�er”otherwai+ngwriters

Hoarevs.MesaSemanLcsforMonitors

53

HoareSeman<cs:monitorlockistransferreddirectlyfromthesignalingthreadtothenewlywokenupthread

- notusuallydesirabletoforcesignalingthreadtorelinquishthemonitorlockimmediatelytoawokenupthread

- confoundsschedulingwithsynchroniza+on,penalizesthreads

MesaSeman<cs:putsawokenupthreadonthemonitorentryqueue,butdoesnotimmediatelyrunthatthread,ortransferthemonitorlock

WhichisMesa?WhichisHoare?

54wikipedia.org

ClassicMistakeswithMonitors

55

#1:NakedWaits

whilenotsome_predicate(): CV.wait()

Whatiswrongwiththis? random_fn1() CV.wait() random_fn2()

Howaboutthis?withself.lock:a=Falsewhilenota:self.cv.wait()a=True

ClassicMistakeswithMonitors

56

#2:Ifvs.While

Whatiswrongwiththis? ifnotsome_predicate(): CV.wait()

ClassicMistakeswithMonitors

57

#3:SplitPredicatesWhatiswrongwiththis?withlock:whilenotcondA:condA_cv.wait()whilenotcondB:condB_cv.wait()

Beqer:withlock:whilenotcondAornotcondB:ifnotcondA:condA_cv.wait()ifnotcondB:condB_cv.wait()

Synchroniza+on:FinalTopics

58

MappingtoRealLanguages

BarrierSynchroniza<on

AnotherClassic:BarbershopProblem

MonitorsinPython

59

classRWlock:def__init__(self):

self.lock=Lock()self.canRead=Condition(self.lock)self.canWrite=Condition(self.lock)self.nReaders=0self.nWriters=0self.nWaitingReaders=0self.nWaitingWriters=0

defend_read(self):withself.lock:self.nReaders-=1ifself.nReaders==0andself.nWaitingWriters>0:self.canWrite.notify()

signal()➙notif

y()

broadcast)➙

notifyAll()

defbegin_read(self):withself.lock: self.nWaitingReaders+=1whileself.nWriters>0orself.nWaitingWriters>0:self.canRead.wait()self.nWaitingReaders-=1self.nActiveReaders+=1

Remember that wait

• releases the lock when called

• re-acquires the lock when it returns

Wheredoestheactualreadingtakeplace?

Monitorsin“4410Python”:__init__

60

fromrvrimportMP,MPthread

classMonitorExample(MP):def__init__(self):MP.__init__(self,None)

self.lock=Lock(“monitorlock”)self.canRead=self.Lock.Condition(“canread”)self.canWrite=self.Lock.Condition(“canwrite”)self.nReaders=self.Shared(“numreaders”,0)self.nWriters=self.Shared(“numwriters”,0)self.nWaitingReaders=self.Shared(“nwaitingreaders”,0)self.nWaitingWriters=self.Shared(“nwaitingwriters”,0)

classRWlock:def__init__(self):

self.lock=Lock()self.canRead=Condition(self.lock)self.canWrite=Condition(self.lock)self.nReaders=0self.nWriters=0self.nWaitingReaders=0self.nWaitingWriters=0

Monitorsin“4410Python”:begin_read

61

defbegin_read(self):withself.lock:

self.nWaitingReaders.inc()whileself.nWriters.read()>0orself.nWaitingWriters.read()>0:

self.canRead.wait()self.nWaitingReaders.dec()self.nActiveReaders.write(self.nActiveReaders.read()+1)

defbegin_read(self):withself.lock: self.nWaitingReaders+=1whileself.nWriters>0orself.nWaitingWriters>0:self.canRead.wait()self.nWaitingReaders-=1self.nActiveReaders+=1

Whydowedothis?• helpfulfeedbackfromauto-grader• helpfulfeedbackfromdebugger

Lookinthe10-P/P2/docdirectoryfordetailsandexamplecode.

BarrierSynchronizaLon

62

• Importantsynchroniza+onprimi+veinhigh-performanceparallelprograms

• nThreadsthreadsdivvyupwork,runroundsofcomputa+onsseparatedbybarriers.

• couldfork&waitbut- threadstartupcosts- wasteofawarmcache

Createnthreads&abarrier.

Eachthreaddoesround1()barrier.checkin()

Eachthreaddoesround2()barrier.checkin()

Checkinwith1condiLonvariable

63

What’swrongwiththis?

self.allCheckedIn=Condition(self.lock)

defcheckin():withself.lock: nArrived++ ifnArrived<nThreads: whilenArrived<nThreads: allCheckedIn.wait() else: allCheckedIn.broadcast()

Checkinwith2condiLonvariables

64

• Implemen+ngbarriersisnoteasy.• Solu+onhereusesa“double-turns+le”

self.allCheckedIn=Condition(self.lock)self.allLeaving=Condition(self.lock)

defcheckin(): nArrived++ ifnArrived<nThreads://noteveryonehascheckedin

whilenArrived<nThreads: allCheckedIn.wait()//waitforeveryonetocheckinelse: nLeaving=0//thisthreadisthelasttoarrive allCheckedIn.broadcast()//telleveryonewe’reallhere!

nLeaving++ ifnLeaving<nThreads://noteveryonehasleftyet whilenLeaving<nThreads: allLeaving.wait()//waitforeveryonetoleaveelse: nArrived=0//thisthreadisthelasttoleave allLeaving.broadcast()//telleveryonewe’reouttahere!

BarbershopProblem

65

Onepossibleversion:• Abarbershopholdsuptokclients• Nbarbersworkonclients• Mclientstotalwanttheirhaircut• Eachclientwillhavetheirhaircutbythefirstbarberavailable

ImplemenLngtheBarbershop

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(1)Iden+fythewaitsCustomers?Barbers?

(2)Createcondi+onvariablesforeach(3)Createcounterstotriggerthewai+ng(4)Createsignalsforthewaits

RulestoLiveBy

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• Useconsistentstructure• Whenpossible,uselocksandcondi+onvariables• Alwaysacquirelockatbeginningofprocedure,releaseatend

• Alwaysholdlockwhenusingacondi+onvariable• Alwayswaitinwhileloop• Neverspininsleep()

Goreadthe12CommandmentsofSynchronizaLon

Conclusion:RaceCondiLonsareabigpain!

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Severalwaystohandlethem• Eachhasitsownprosandcons

Programminglanguagesupportsimplifieswri+ngmul+threadedapplica+ons

• Pythoncondi+onvariables• JavaandC#supportatmostonecondi+onvariableperobject,soareslightlymorelimited

Someprogramanalysistoolsautomatechecking•makesurecodeisusingsynchroniza+oncorrectly• Hardpartistodefining“correct”

deal