Chapter Concurrent Programming As we learned in Chapt - PDF document

Chapter12ConcurrentProgrammingAswelearnedinChapter8,logicalcontrolowsareconcurrentiftheyoverlapintime.Thisgeneralphenomenon,knownasconcurrency,showsupatmanydifferentlevelsofacomputersystem.Hardwareexceptionhandlers,processes,andUnixsignalhandlersareallfamiliarexamples.Thusfar,wehavetreatedconcurrencymainlyasamechanismthattheoperatingsystemkernelusestorunmultipleapplicationprograms.Butconcurrencyisnotjustlimitedtothekernel.Itcanplayanimportantroleinapplicationprogramsaswell.Forexample,wehaveseenhowUnixsignalhandlersallowapplicationstorespondtoasynchronouseventssuchastheusertypingctrl-cortheprogramaccessinganundenedareaofvirtualmemory.Application-levelconcurrencyisusefulinotherwaysaswell:AccessingslowI/Odevices.WhenanapplicationiswaitingfordatatoarrivefromaslowI/Odevicesuchasadisk,thekernelkeepstheCPUbusybyrunningotherprocesses.IndividualapplicationscanexploitconcurrencyinasimilarwaybyoverlappingusefulworkwithI/Orequests.Interactingwithhumans.Peoplewhointeractwithcomputersdemandtheabilitytoperformmultipletasksatthesametime.Forexample,theymightwanttoresizeawindowwhiletheyareprintingadocument.Modernwindowingsystemsuseconcurrencytoprovidethiscapability.Eachtimetheuserrequestssomeaction(say,byclickingthemouse),aseparateconcurrentlogicalowiscreatedtoperformtheaction.Reducinglatencybydeferringwork.Sometimes,applicationscanuseconcurrencytoreducethelatencyofcertainoperationsbydeferringotheroperationsandperformingthemconcurrently.Forexample,adynamicstorageallocatormightreducethelatencyofindividualfreeoperationsbydeferringcoalescingtoaconcurrent“coalescing”owthatrunsatalowerpriority,soakingupspareCPUcyclesastheybecomeavailable.Servicingmultiplenetworkclients.TheiterativenetworkserversthatwestudiedinChapter11areunrealisticbecausetheycanonlyserviceoneclientatatime.Thus,asingleslowclientcandenyservicetoeveryotherclient.Forarealserverthatmightbeexpectedtoservicehundredsorthousandsofclientspersecond,itisnotacceptabletoallowoneslowclienttodenyservicetotheothers.Abetterapproachistobuildaconcurrentserverthatcreatesaseparatelogicalowforeachclient.Thisallows895 896CHAPTER12.CONCURRENTPROGRAMMINGtheservertoservicemultipleclientsconcurrently,andprecludesslowclientsfrommonopolizingtheserver.Computinginparallelonmulti-coremachines.Manymodernsystemsareequippedwithmulti-coreprocessorsthatcontainmultipleCPUs.Applicationsthatarepartitionedintoconcurrentowsoftenrunfasteronmulti-coremachinesthanonuniprocessormachinesbecausetheowsexecuteinparallelratherthanbeinginterleaved.Applicationsthatuseapplication-levelconcurrencyareknownasconcurrentprograms.Modernoperatingsystemsprovidethreebasicapproachesforbuildingconcurrentprograms:Processes.Withthisapproach,eachlogicalcontrolowisaprocessthatisscheduledandmaintainedbythekernel.Sinceprocesseshaveseparatevirtualaddressspaces,owsthatwanttocommunicatewitheachothermustusesomekindofexplicitinterprocesscommunication(IPC)mechanism.I/Omultiplexing.Thisisaformofconcurrentprogrammingwhereapplicationsexplicitlyscheduletheirownlogicalowsinthecontextofasingleprocess.Logicalowsaremodeledasstatemachinesthatthemainprogramexplicitlytransitionsfromstatetostateasaresultofdataarrivingonledescriptors.Sincetheprogramisasingleprocess,allowssharethesameaddressspace.Threads.Threadsarelogicalowsthatruninthecontextofasingleprocessandarescheduledbythekernel.Youcanthinkofthreadsasahybridoftheothertwoapproaches,scheduledbythekernellikeprocessows,andsharingthesamevirtualaddressspacelikeI/Omultiplexingows.Thischapterinvestigatesthesethreedifferentconcurrentprogrammingtechniques.Tokeepourdiscus-sionconcrete,wewillworkwiththesamemotivatingapplicationthroughout–aconcurrentversionoftheiterativeechoserverfromSection11.4.9.12.1ConcurrentProgrammingWithProcessesThesimplestwaytobuildaconcurrentprogramiswithprocesses,usingfamiliarfunctionssuchasfork,exec,andwaitpid.Forexample,anaturalapproachforbuildingaconcurrentserveristoacceptclientconnectionrequestsintheparent,andthencreateanewchildprocesstoserviceeachnewclient.Toseehowthismightwork,supposewehavetwoclientsandaserverthatislisteningforconnectionrequestsonalisteningdescriptor(say,3).Nowsupposethattheserveracceptsaconnectionrequestfromclient1andreturnsaconnecteddescriptor(say,4),asshowninFigure12.1.Afteracceptingtheconnectionrequest,theserverforksachild,whichgetsacompletecopyoftheserver'sdescriptortable.Thechildclosesitscopyoflisteningdescriptor3,andtheparentclosesitscopyofcon-necteddescriptor4,sincetheyarenolongerneeded.ThisgivesusthesituationinFigure12.2,wherethechildprocessisbusyservicingtheclient.Sincetheconnecteddescriptorsintheparentandchildeachpointtothesameletableentry,itiscrucialfortheparenttocloseitscopyoftheconnecteddescriptor.Other-wise,theletableentryforconnecteddescriptor4willneverbereleased,andtheresultingmemoryleakwilleventuallyconsumetheavailablememoryandcrashthesystem. 12.1.CONCURRENTPROGRAMMINGWITHPROCESSES897 Client 2 clientfd Client 1 clientfd Server connfd (4) listenfd (3) Connectionrequest Figure12.1:Step1:Serveracceptsconnectionrequestfromclient. Client 2 clientfd Client 1 clientfd Server listenfd (3) Child 1 connfd (4) Data transfers Figure12.2:Step2:Serverforksachildprocesstoservicetheclient.Nowsupposethataftertheparentcreatesthechildforclient1,itacceptsanewconnectionrequestfromclient2andreturnsanewconnecteddescriptor(say,5),asshowninFigure12.3. Client 2 clientfd Client 1 clientfd Server listenfd (3) Child 1 connfd (4) Connectionrequest connfd (5) Data transfers Figure12.3:Step3:Serveracceptsanotherconnectionrequest.Theparentthenforksanotherchild,whichbeginsservicingitsclientusingconnecteddescriptor5,asshowninFigure12.4.Atthispoint,theparentiswaitingforthenextconnectionrequestandthetwochildrenareservicingtheirrespectiveclientsconcurrently.12.1.1AConcurrentServerBasedonProcessesFigure12.5showsthecodeforaconcurrentechoserverbasedonprocesses.Theechofunctioncalledinline29comesfromFigure11.21.Thereareseveralimportantpointstomakeaboutthisserver:First,serverstypicallyrunforlongperiodsoftime,sowemustincludeaSIGCHLDhandlerthat 898CHAPTER12.CONCURRENTPROGRAMMING Client 2 clientfd Client 1 clientfd Server listenfd (3) Child 1 connfd (4) Data transfers Child 2 connfd (5) Data transfers Figure12.4:Step4:Serverforksanotherchildtoservicethenewclient.reapszombiechildren(lines4–9).SinceSIGCHLDsignalsareblockedwhiletheSIGCHLDhandlerisexecuting,andsinceUnixsignalsarenotqueued,theSIGCHLDhandlermustbepreparedtoreapmultiplezombiechildren.Second,theparentandthechildmustclosetheirrespectivecopiesofconnfd(lines33and30,respectively).Aswehavementioned,thisisespeciallyimportantfortheparent,whichmustcloseitscopyoftheconnecteddescriptortoavoidamemoryleak.Finally,becauseofthereferencecountinthesocket'sletableentry,theconnectiontotheclientwillnotbeterminateduntilboththeparent'sandchild'scopiesofconnfdareclosed.12.1.2ProsandConsofProcessesProcesseshaveacleanmodelforsharingstateinformationbetweenparentsandchildren:letablesaresharedanduseraddressspacesarenot.Havingseparateaddressspacesforprocessesisbothanadvantageandadisadvantage.Itisimpossibleforoneprocesstoaccidentallyoverwritethevirtualmemoryofanotherprocess,whicheliminatesalotofconfusingfailures–anobviousadvantage.Ontheotherhand,separateaddressspacesmakeitmoredifcultforprocessestosharestateinformation.Toshareinformation,theymustuseexplicitIPC(interprocesscommunications)mechanisms.(SeeAside.)Anotherdisadvantageofprocess-baseddesignsisthattheytendtobeslowerbecausetheoverheadforprocesscontrolandIPCishigh.Aside:UnixIPCYouhavealreadyencounteredseveralexamplesofIPCinthistext.ThewaitpidfunctionandUnixsignalsfromChapter8areprimitiveIPCmechanismsthatallowprocessestosendtinymessagestoprocessesrunningonthesamehost.ThesocketsinterfacefromChapter11isanimportantformofIPCthatallowsprocessesondifferenthoststoexchangearbitrarybytestreams.However,thetermUnixIPCistypicallyreservedforahodge-podgeoftechniquesthatallowprocessestocommunicatewithotherprocessesthatarerunningonthesamehost.Examplesincludepipes,FIFOs,SystemVsharedmemory,andSystemVsemaphores.Thesemechanismsarebeyondourscope.ThebookbyStevens[108]isagoodreference.EndAside.PracticeProblem12.1: 12.1.CONCURRENTPROGRAMMINGWITHPROCESSES899 code/conc/echoserverp.c 1 #include"csapp.h" 2 voidecho(intconnfd); 34 voidsigchld_handler(intsig) 5 { 6 while(waitpid(-1,0,WNOHANG)�0) 7 ; 8 return; 9 } 1011 intmain(intargc,char**argv) 12 { 13 intlistenfd,connfd,port; 14 socklen_tclientlen=sizeof(structsockaddr_in); 15 structsockaddr_inclientaddr; 1617 if(argc!=2){ 18 fprintf(stderr,"usage:%sport&#x-2.2;䥢\n",argv[0]); 19 exit(0); 20 } 21 port=atoi(argv[1]); 2223 Signal(SIGCHLD,sigchld_handler); 24 listenfd=Open_listenfd(port); 25 while(1){ 26 connfd=Accept(listenfd,(SA*)&clientaddr,&clientlen); 27 if(Fork()==0){ 28 Close(listenfd);/*Childclosesitslisteningsocket*/ 29 echo(connfd);/*Childservicesclient*/ 30 Close(connfd);/*Childclosesconnectionwithclient*/ 31 exit(0);/*Childexits*/ 32 } 33 Close(connfd);/*Parentclosesconnectedsocket(important!)*/ 34 } 35 } code/conc/echoserverp.cFigure12.5:Concurrentechoserverbasedonprocesses.Theparentforksachildtohandleeachnewconnectionrequest. 900CHAPTER12.CONCURRENTPROGRAMMINGAftertheparentclosestheconnecteddescriptorinline33oftheconcurrentserverinFigure12.5,thechildisstillabletocommunicatewiththeclientusingitscopyofthedescriptor.Why?PracticeProblem12.2:Ifweweretodeleteline30ofFigure12.5,whichclosestheconnecteddescriptor,thecodewouldstillbecorrect,inthesensethattherewouldbenomemoryleak.Why?12.2ConcurrentProgrammingWithI/OMultiplexingSupposeyouareaskedtowriteanechoserverthatcanalsorespondtointeractivecommandsthattheusertypestostandardinput.Inthiscase,theservermustrespondtotwoindependentI/Oevents:(1)anetworkclientmakingaconnectionrequest,and(2)ausertypingacommandlineatthekeyboard.Whicheventdowewaitforrst?Neitheroptionisideal.Ifwearewaitingforaconnectionrequestinaccept,thenwecannotrespondtoinputcommands.Similarly,ifwearewaitingforaninputcommandinread,thenwecannotrespondtoanyconnectionrequests.OnesolutiontothisdilemmaisatechniquecalledI/Omultiplexing.Thebasicideaistousetheselectfunctiontoaskthekerneltosuspendtheprocess,returningcontroltotheapplicationonlyafteroneormoreI/Oeventshaveoccurred,asinthefollowingexamples:Returnwhenanydescriptorinthesetf0;4gisreadyforreading.Returnwhenanydescriptorinthesetf1;2;7gisreadyforwriting.Timeoutif152:13secondshaveelapsedwaitingforanI/Oeventtooccur.Selectisacomplicatedfunctionwithmanydifferentusagescenarios.Wewillonlydiscusstherstscenario:waitingforasetofdescriptorstobereadyforreading.See[109,110]foracompletediscussion. #includeunistd.h&#x-2.2;䥢#includesys/types.h&#x-2.2;䥢intselect(intn,fd set*fdset,NULL,NULL,NULL);Returnsnonzerocountofreadydescriptors,1onerrorFD ZERO(fd set*fdset);/*Clearallbitsinfdset*/FD CLR(intfd,fd set*fdset);/*Clearbitfdinfdset*/FD SET(intfd,fd set*fdset);/*Turnonbitfdinfdset*/FD ISSET(intfd,fd set*fdset);/*Isbitfdinfdsetturnedon?*/Macrosformanipulatingdescriptorsets Theselectfunctionmanipulatessetsoftypefd set,whichareknownasdescriptorsets.Logically,wethinkofadescriptorsetasabitvector(introducedinSection2.1)ofsizen:bn1;:::;b1;b0: 12.2.CONCURRENTPROGRAMMINGWITHI/OMULTIPLEXING901Eachbitbkcorrespondstodescriptork.Descriptorkisamemberofthedescriptorsetifandonlyifbk=1.Youareonlyallowedtodothreethingswithdescriptorsets:(1)allocatethem,(2)assignonevariableofthistypetoanother,and(3)modifyandinspectthemusingtheFD ZERO,FD SET,FD CLR,andFD ISSETmacros.Forourpurposes,theselectfunctiontakestwoinputs:adescriptorset(fdset)calledthereadset,andthecardinality(n)ofthereadset(actuallythemaximumcardinalityofanydescriptorset).Theselectfunctionblocksuntilatleastonedescriptorinthereadsetisreadyforreading.Adescriptorkisreadyforreadingifandonlyifarequesttoread1bytefromthatdescriptorwouldnotblock.Asasideeffect,selectmodiesthefd setpointedtobyargumentfdsettoindicateasubsetofthereadsetcalledthereadyset,consistingofthedescriptorsinthereadsetthatarereadyforreading.Thevaluereturnedbythefunctionindicatesthecardinalityofthereadyset.Notethatbecauseofthesideeffect,wemustupdatethereadseteverytimeselectiscalled.Thebestwaytounderstandselectistostudyaconcreteexample.Figure12.6showshowwemightuseselecttoimplementaniterativeechoserverthatalsoacceptsusercommandsonthestandardinput.Webeginbyusingtheopen listenfdfunctionfromFigure11.17toopenalisteningdescriptor(line17),andthenusingFD ZEROtocreateanemptyreadset(line19):listenfdstdin3210 read set(;): 0 0 0 0 Next,inlines20and21,wedenethereadsettoconsistofdescriptor0(standardinput)anddescriptor3(thelisteningdescriptor),respectively:listenfdstdin3210 read set(f0;3g): 1 0 0 1 Atthispoint,webeginthetypicalserverloop.Butinsteadofwaitingforaconnectionrequestbycallingtheacceptfunction,wecalltheselectfunction,whichblocksuntileitherthelisteningdescriptororstandardinputisreadyforreading(line25).Forexample,hereisthevalueofready setthatselectwouldreturniftheuserhittheenterkey,thuscausingthestandardinputdescriptortobecomereadyforreading:listenfdstdin3210 ready set(f0g): 0 0 0 1 Onceselectreturns,weusetheFD ISSETmacrotodeterminewhichdescriptorsarereadyforreading.Ifstandardinputisready(line26),wecallthecommandfunction,whichreads,parses,andrespondstothecommandbeforereturningtothemainroutine.Ifthelisteningdescriptorisready(line28),wecallaccepttogetaconnecteddescriptor,andthencalltheechofunctionfromFigure11.21,whichechoeseachlinefromtheclientuntiltheclientclosesitsendoftheconnection.Whilethisprogramisagoodexampleofusingselect,itstillleavessomethingtobedesired.Theproblemisthatonceitconnectstoaclient,itcontinuesechoinginputlinesuntiltheclientclosesitsendof 902CHAPTER12.CONCURRENTPROGRAMMING code/conc/select.c 1 #include"csapp.h" 2 voidecho(intconnfd); 3 voidcommand(void); 45 intmain(intargc,char**argv) 6 { 7 intlistenfd,connfd,port; 8 socklen_tclientlen=sizeof(structsockaddr_in); 9 structsockaddr_inclientaddr; 10 fd_setread_set,ready_set; 1112 if(argc!=2){ 13 fprintf(stderr,"usage:%sport&#x-2.2;䥢\n",argv[0]); 14 exit(0); 15 } 16 port=atoi(argv[1]); 17 listenfd=Open_listenfd(port); 1819 FD_ZERO(&read_set);/*Clearreadset*/ 20 FD_SET(STDIN_FILENO,&read_set);/*Addstdintoreadset*/ 21 FD_SET(listenfd,&read_set);/*Addlistenfdtoreadset*/ 2223 while(1){ 24 ready_set=read_set; 25 Select(listenfd+1,&ready_set,NULL,NULL,NULL); 26 if(FD_ISSET(STDIN_FILENO,&ready_set)) 27 command();/*Readcommandlinefromstdin*/ 28 if(FD_ISSET(listenfd,&ready_set)){ 29 connfd=Accept(listenfd,(SA*)&clientaddr,&clientlen); 30 echo(connfd);/*EchoclientinputuntilEOF*/ 31 Close(connfd); 32 } 33 } 34 } 3536 voidcommand(void){ 37 charbuf[MAXLINE]; 38 if(!Fgets(buf,MAXLINE,stdin)) 39 exit(0);/*EOF*/ 40 printf("%s",buf);/*Processtheinputcommand*/ 41 } code/conc/select.cFigure12.6:AniterativeechoserverthatusesI/Omultiplexing.Theserverusesselecttowaitforconnectionrequestsonalisteningdescriptorandcommandsonstandardinput. 12.2.CONCURRENTPROGRAMMINGWITHI/OMULTIPLEXING903theconnection.Thus,ifyoutypeacommandtostandardinput,youwillnotgetaresponseuntiltheserverisnishedwiththeclient.Abetterapproachwouldbetomultiplexatanergranularity,echoing(atmost)onetextlineeachtimethroughtheserverloop.PracticeProblem12.3:InmostUnixsystems,typingctrl-dindicatesEOFonstandardinput.Whathappensifyoutypectrl-dtotheprograminFigure12.6whileitisblockedinthecalltoselect?12.2.1AConcurrentEvent-DrivenServerBasedonI/OMultiplexingI/Omultiplexingcanbeusedasthebasisforconcurrentevent-drivenprograms,whereowsmakeprogressasaresultofcertainevents.Thegeneralideaistomodellogicalowsasstatemachines.Informally,astatemachineisacollectionofstates,inputevents,andtransitionsthatmapstatesandinputeventstostates.Eachtransitionmapsan(inputstate,inputevent)pairtoanoutputstate.Aself-loopisatransitionbetweenthesameinputandoutputstate.Statemachinesaretypicallydrawnasdirectedgraphs,wherenodesrepresentstates,directedarcsrepresenttransitions,andarclabelsrepresentinputevents.Astatemachinebeginsexecutioninsomeinitialstate.Eachinputeventtriggersatransitionfromthecurrentstatetothenextstate.Foreachnewclientk,aconcurrentserverbasedonI/Omultiplexingcreatesanewstatemachineskandassociatesitwithconnecteddescriptordk.AsshowninFigure12.7,eachstatemachineskhasonestate(“waitingfordescriptordktobereadyforreading”),oneinputevent(“descriptordkisreadyforreading”),andonetransition(“readatextlinefromdescriptordk”). Transition: ”read a text line from descriptor dk " State: ”waiting for descriptor dk to be ready for reading” State: ”waiting for descriptor dk to be ready for reading”Input event: ”descriptor dk is ready for reading" Figure12.7:Statemachineforalogicalowinaconcurrentevent-drivenechoserver.TheserverusestheI/Omultiplexing,courtesyoftheselectfunction,todetecttheoccurrenceofinputevents.Aseachconnecteddescriptorbecomesreadyforreading,theserverexecutesthetransitionforthecorrespondingstatemachine,inthiscasereadingandechoingatextlinefromthedescriptor.Figure12.8showsthecompleteexamplecodeforaconcurrentevent-drivenserverbasedonI/Omultiplex-ing.Thesetofactiveclientsismaintainedinapoolstructure(lines3–11).Afterinitializingthepoolbycallinginit pool(line29),theserverentersaninniteloop.Duringeachiterationofthisloop,theservercallstheselectfunctiontodetecttwodifferentkindsofinputevents:(a)aconnectionrequestarrivingfromanewclient,and(b)aconnecteddescriptorforanexistingclientbeingreadyforreading.Whenaconnectionrequestarrives(line36),theserveropenstheconnection(line37)andcallstheadd clientfunctiontoaddtheclienttothepool(line38).Finally,theservercallsthecheck clientsfunctiontoechoasingletextlinefromeachreadyconnecteddescriptor(line42). 904CHAPTER12.CONCURRENTPROGRAMMING code/conc/echoservers.c 1 #include"csapp.h" 23 typedefstruct{/*representsapoolofconnecteddescriptors*/ 4 intmaxfd;/*largestdescriptorinread_set*/ 5 fd_setread_set;/*setofallactivedescriptors*/ 6 fd_setready_set;/*subsetofdescriptorsreadyforreading*/ 7 intnready;/*numberofreadydescriptorsfromselect*/ 8 intmaxi;/*highwaterindexintoclientarray*/ 9 intclientfd[FD_SETSIZE];/*setofactivedescriptors*/ 10 rio_tclientrio[FD_SETSIZE];/*setofactivereadbuffers*/ 11 }pool; 1213 intbyte_cnt=0;/*countstotalbytesreceivedbyserver*/ 1415 intmain(intargc,char**argv) 16 { 17 intlistenfd,connfd,port; 18 socklen_tclientlen=sizeof(structsockaddr_in); 19 structsockaddr_inclientaddr; 20 staticpoolpool; 2122 if(argc!=2){ 23 fprintf(stderr,"usage:%sport&#x-2.2;䥢\n",argv[0]); 24 exit(0); 25 } 26 port=atoi(argv[1]); 2728 listenfd=Open_listenfd(port); 29 init_pool(listenfd,&pool); 30 while(1){ 31 /*Waitforlistening/connecteddescriptor(s)tobecomeready*/ 32 pool.ready_set=pool.read_set; 33 pool.nready=Select(pool.maxfd+1,&pool.ready_set,NULL,NULL,NULL); 3435 /*Iflisteningdescriptorready,addnewclienttopool*/ 36 if(FD_ISSET(listenfd,&pool.ready_set)){ 37 connfd=Accept(listenfd,(SA*)&clientaddr,&clientlen); 38 add_client(connfd,&pool); 39 } 4041 /*Echoatextlinefromeachreadyconnecteddescriptor*/ 42 check_clients(&pool); 43 } 44 } code/conc/echoservers.cFigure12.8:ConcurrentechoserverbasedonI/Omultiplexing.Eachserveriterationechoesatextlinefromeachreadydescriptor. 12.2.CONCURRENTPROGRAMMINGWITHI/OMULTIPLEXING905Theinit poolfunction(Figure12.9)initializestheclientpool.Theclientfdarrayrepresentsasetofconnecteddescriptors,withtheinteger1denotinganavailableslot.Initially,thesetofconnecteddescriptorsisempty(lines5–7),andthelisteningdescriptoristheonlydescriptorintheselectreadset(lines10–12). code/conc/echoservers.c 1 voidinit_pool(intlistenfd,pool*p) 2 { 3 /*Initially,therearenoconnecteddescriptors*/ 4 inti; 5 p-�maxi=-1; 6 for(i=0;iFD_SETSIZE;i++) 7 p-�clientfd[i]=-1; 89 /*Initially,listenfdisonlymemberofselectreadset*/ 10 p-�maxfd=listenfd; 11 FD_ZERO(&p-�read_set); 12 FD_SET(listenfd,&p-�read_set); 13 } code/conc/echoservers.cFigure12.9:init pool:Initializesthepoolofactiveclients.Theadd clientfunction(Figure12.10)addsanewclienttothepoolofactiveclients.Afterndinganemptyslotintheclientfdarray,theserveraddstheconnecteddescriptortothearrayandinitializesacorrespondingRIOreadbuffersothatwecancallrio readlinebonthedescriptor(lines8–9).Wethenaddtheconnecteddescriptortotheselectreadset(line12),andweupdatesomeglobalpropertiesofthepool.Themaxfdvariable(lines15–16)keepstrackofthelargestledescriptorforselect.Themaxivariable(lines17–18)keepstrackofthelargestindexintotheclientfdarraysothatthecheck clientsfunctionsdoesnothavetosearchtheentirearray.Thecheck clientsfunctionechoesatextlinefromeachreadyconnecteddescriptor.Ifwearesuc-cessfulinreadingatextlinefromthedescriptor,thenweechothatlinebacktotheclient(lines15–18).Noticethatinline15wearemaintainingacumulativecountoftotalbytesreceivedfromallclients.IfwedetectEOFbecausetheclienthascloseditsendoftheconnection,thenwecloseourendoftheconnection(line23)andremovethedescriptorfromthepool(lines24–25).IntermsofthenitestatemodelinFigure12.7,theselectfunctiondetectsinputevents,andtheadd clientfunctioncreatesanewlogicalow(statemachine).Thecheck clientsfunctionper-formsstatetransitionsbyechoinginputlines,anditalsodeletesthestatemachinewhentheclienthasnishedsendingtextlines.12.2.2ProsandConsofI/OMultiplexingTheserverinFigure12.8providesaniceexampleoftheadvantagesanddisadvantagesofevent-drivenprogrammingbasedonI/Omultiplexing.Oneadvantageisthatevent-drivendesignsgiveprogrammers 906CHAPTER12.CONCURRENTPROGRAMMING code/conc/echoservers.c 1 voidadd_client(intconnfd,pool*p) 2 { 3 inti; 4 p-�nready--; 5 for(i=0;iFD_SETSIZE;i++)/*Findanavailableslot*/ 6 if(p-�clientfd[i]0){ 7 /*Addconnecteddescriptortothepool*/ 8 p-�clientfd[i]=connfd; 9 Rio_readinitb(&p-�clientrio[i],connfd); 1011 /*Addthedescriptortodescriptorset*/ 12 FD_SET(connfd,&p-�read_set); 1314 /*Updatemaxdescriptorandpoolhighwatermark*/ 15 if(connfd�p-�maxfd) 16 p-�maxfd=connfd; 17 if(i�p-�maxi) 18 p-�maxi=i; 19 break; 20 } 21 if(i==FD_SETSIZE)/*Couldn'tfindanemptyslot*/ 22 app_error("add_clienterror:Toomanyclients"); 23 } code/conc/echoservers.cFigure12.10:add client:Addsanewclientconnectiontothepool. 12.2.CONCURRENTPROGRAMMINGWITHI/OMULTIPLEXING907 code/conc/echoservers.c 1 voidcheck_clients(pool*p) 2 { 3 inti,connfd,n; 4 charbuf[MAXLINE]; 5 rio_trio; 67 for(i=0;(i=p-&#x-2.2;䥢maxi)&&(p-&#x-2.2;䥢nready&#x-2.2;䥢0);i++){ 8 connfd=p-�clientfd[i]; 9 rio=p-�clientrio[i]; 1011 /*Ifthedescriptorisready,echoatextlinefromit*/ 12 if((connfd�0)&&(FD_ISSET(connfd,&p-�ready_set))){ 13 p-�nready--; 14 if((n=Rio_readlineb(&rio,buf,MAXLINE))!=0){ 15 byte_cnt+=n; 16 printf("Serverreceived%d(%dtotal)bytesonfd%d\n", 17 n,byte_cnt,connfd); 18 Rio_writen(connfd,buf,n); 19 } 2021 /*EOFdetected,removedescriptorfrompool*/ 22 else{ 23 Close(connfd); 24 FD_CLR(connfd,&p-�read_set); 25 p-�clientfd[i]=-1; 26 } 27 } 28 } 29 } code/conc/echoservers.cFigure12.11:check clients:Servicesreadyclientconnections. 908CHAPTER12.CONCURRENTPROGRAMMINGmorecontroloverthebehavioroftheirprogramsthanprocess-baseddesigns.Forexample,wecanimaginewritinganevent-drivenconcurrentserverthatgivespreferredservicetosomeclients,whichwouldbedifcultforaconcurrentserverbasedonprocesses.Anotheradvantageisthatanevent-drivenserverbasedonI/Omultiplexingrunsinthecontextofasingleprocess,andthuseverylogicalowhasaccesstotheentireaddressspaceoftheprocess.Thismakesiteasytosharedatabetweenows.Arelatedadvantageofrunningasasingleprocessisthatyoucandebugyourconcurrentserverasyouwouldanysequentialprogram,usingafamiliardebuggingtoolsuchasGDB.Finally,event-drivendesignsareoftensignicantlymoreefcientthanprocess-baseddesignsbecausetheydonotrequireaprocesscontextswitchtoscheduleanewow.Asignicantdisadvantageofevent-drivendesignsiscodingcomplexity.Ourevent-drivenconcurrentechoserverrequiresthreetimesmorecodethantheprocess-basedserver.Unfortunately,thecomplexityincreasesasthegranularityoftheconcurrencydecreases.Bygranularity,wemeanthenumberofinstructionsthateachlogicalowexecutespertimeslice.Forinstance,inourexampleconcurrentserver,thegranularityofconcurrencyisthenumberofinstructionsrequiredtoreadanentiretextline.Aslongassomelogicalowisbusyreadingatextline,nootherlogicalowcanmakeprogress.Thisisneforourexample,butitmakesourevent-driverservervulnerabletoamaliciousclientthatsendsonlyapartialtextlineandthenhalts.Modifyinganevent-drivenservertohandlepartialtextlinesisanontrivialtask,butitishandledcleanlyandautomaticallybyaprocess-baseddesign.Anothersignicantdisadvantageofevent-baseddesignsisthattheycannotfullyutilizemulti-coreprocessors.PracticeProblem12.4:IntheserverinFigure12.8,wearecarefultoreinitializethepool.ready setvariableimmediatelybeforeeverycalltoselect.Why?12.3ConcurrentProgrammingWithThreadsTothispoint,wehavelookedattwoapproachesforcreatingconcurrentlogicalows.Withtherstap-proach,weuseaseparateprocessforeachow.Thekernelscheduleseachprocessautomatically.Eachprocesshasitsownprivateaddressspace,whichmakesitdifcultforowstosharedata.Withthesec-ondapproach,wecreateourownlogicalowsanduseI/Omultiplexingtoexplicitlyscheduletheows.Becausethereisonlyoneprocess,owssharetheentireaddressspace.Thissectionintroducesathirdapproach—basedonthreads—thatisahybridofthesetwo.Athreadisalogicalowthatrunsinthecontextofaprocess.Thusfarinthisbook,ourprogramshaveconsistedofasinglethreadperprocess.Butmodernsystemsalsoallowustowriteprogramsthathavemultiplethreadsrunningconcurrentlyinasingleprocess.Thethreadsarescheduledautomaticallybythekernel.Eachthreadhasitsownthreadcontext,includingauniqueintegerthreadID(TID),stack,stackpointer,programcounter,general-purposeregisters,andconditioncodes.Allthreadsrunninginaprocesssharetheentirevirtualaddressspaceofthatprocess.LogicalowsbasedonthreadscombinequalitiesofowsbasedonprocessesandI/Omultiplexing.Likeprocesses,threadsarescheduledautomaticallybythekernelandareknowntothekernelbyanintegerID.LikeowsbasedonI/Omultiplexing,multiplethreadsruninthecontextofasingleprocess,andthusshare 12.3.CONCURRENTPROGRAMMINGWITHTHREADS909theentirecontentsoftheprocessvirtualaddressspace,includingitscode,data,heap,sharedlibraries,andopenles.12.3.1ThreadExecutionModelTheexecutionmodelformultiplethreadsissimilarinsomewaystotheexecutionmodelformultipleprocesses.ConsidertheexampleinFigure12.12.Eachprocessbeginslifeasasinglethreadcalledthemainthread.Atsomepoint,themainthreadcreatesapeerthread,andfromthispointintimethetwothreadsrunconcurrently.Eventually,controlpassestothepeerthreadviaacontextswitch,becausethemainthreadexecutesaslowsystemcallsuchasreadorsleep,orbecauseitisinterruptedbythesystem'sintervaltimer.Thepeerthreadexecutesforawhilebeforecontrolpassesbacktothemainthread,andsoon. Time Thread 1(main thread)Thread 2(peer thread) Thread context switch Thread context switch Thread context switch Figure12.12:Concurrentthreadexecution.Threadexecutiondiffersfromprocessesinsomeimportantways.Becauseathreadcontextismuchsmallerthanaprocesscontext,athreadcontextswitchisfasterthanaprocesscontextswitch.Anotherdifferenceisthatthreads,unlikeprocesses,arenotorganizedinarigidparent-childhierarchy.Thethreadsassociatedwithaprocessformapoolofpeers,independentofwhichthreadswerecreatedbywhichotherthreads.Themainthreadisdistinguishedfromotherthreadsonlyinthesensethatitisalwaystherstthreadtorunintheprocess.Themainimpactofthisnotionofapoolofpeersisthatathreadcankillanyofitspeers,orwaitforanyofitspeerstoterminate.Further,eachpeercanreadandwritethesameshareddata.12.3.2PosixThreadsPosixthreads(Pthreads)isastandardinterfaceformanipulatingthreadsfromCprograms.Itwasadoptedin1995andisavailableonmostUnixsystems.Pthreadsdenesabout60functionsthatallowprogramstocreate,kill,andreapthreads,tosharedatasafelywithpeerthreads,andtonotifypeersaboutchangesinthesystemstate.Figure12.13showsasimplePthreadsprogram.Themainthreadcreatesapeerthreadandthenwaitsforittoterminate.Thepeerthreadprints“Hello,world!\n”andterminates.Whenthemainthreaddetectsthatthepeerthreadhasterminated,itterminatestheprocessbycallingexit.