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Processes11/3/16

Recall:thekernel’sjob

Ensurethatallrunningprocesseshavereasonableaccesstosystemresources:

• CPU• Memory• IOdevices

Providealoneview abstraction:eachprocessshouldrunasifitweretheonlyoneonthesystem.

ManagingCPUcycles:timesharing

• MultipleprocessesperCPU• Eachprocessmakesprogressovertime• IllusionofparallelprogressbyrapidlyswitchingCPU• Whenscheduled,aprocessrunsforafewmilliseconds(1ms=10-3 seconds) beforegettingpreempted.

P1 P2

P3

P1 P2 P3quantum

P1P2P3

time

• Statetransitions• Dispatch:allocatetheCPUtoaprocess• Preempt:takeawayCPUfromprocess• Sleep:processgivesupCPUtowaitforevent• Wakeup:eventoccurred,makeprocessready

ProcessStatenewprocess

Exit

wakeup

sleep

Ready Blocked

ExitedRunning

(onCPU)

dispatch preempt

Whymightaprocessbeblocked(unabletomakeprogress/useCPU)?

A. It’swaitingforanotherprocesstodosomething.

B. It’swaitingformemorytofindandreturnavalue.

C. It’swaitingforanI/Odevicetodosomething.

D. Morethanoneoftheabove.(Whichones?)

E. Someotherreason(s).

HowisTimesharingImplemented?

• Kerneltracksthestateofeachprocess:• Running:actuallymakingprogress,usingCPU• Ready:abletomakeprogress,butnotusingCPU• Blocked:notabletomakeprogress,can’tuseCPU

• Aprocessrunsuntilstopped:• Canstopitselfbymakingasystemcall(TRAP)• Canbestoppedbythekernel(interrupt)

• Kernelrunstoperformacontextswitch:• Savethestateofthecurrentprocess• Selectsanotherreadyprocessandloaditsstate

KernelMaintainsProcessTable

• Listofprocessesandtheirstates• Alsosometimescalled“processcontrolblock(PCB)”

• Otherstateinfoincludes• CPUcontext(registervalues)• areasofmemorybeingused• otherinformation

Process ID(PID) State Otherinfo1534 Ready Savedcontext,…

34 Running Memoryareasused,…

487 Ready Saved context,…

9 Blocked Conditiontounblock,…

HowaContextSwitchOccurs

• Processmakessystemcall(TRAP)orisinterrupted• Thesearetheonlywaysofenteringthekernel

• Inhardware• Switchfromusertokernelmode:amplifiespower• Gotofixedkernellocation:interrupt/traphandler

• Insoftware(inthekernelcode)• Savecontextoflast-runningprocess• Selectnewprocessfromthosethatareready• Restorecontextofselectedprocess• OSreturnscontroltoaprocessfrominterrupt/trap

Whyshouldthekernel(noteachprocess)controlcontextswitching?

A. Itwouldcausetoomuchoverhead.

B. TheycouldrefusetogiveuptheCPU.

C. Theydon’thaveenoughinformationaboutotherprocesses.

D. Someotherreason(s).

Policyquestion:howshouldthekernelpickthenextprocesstorun?

Therearelotsofreasonableoptions:• Keeprunningthesameprocesswhenpossible.• maximizethroughput

• Runtheprocessthathasbeenreadylongest.• ensurefairness

• Prioritizesomeprocesses.• Shoulduser(s)setthepriorities?• Shouldprioritybedeterminedautomatically?

Linux’sPolicy(You’renotresponsibleforthis.)

• Special“realtime”processclasses(highpriority)

• Otherprocesses:• Keepred-blackBSTofprocess,organizedbyhowmuchCPUtimethey’vereceived.• Pickthereadywithprocessthathasrunfortheshortesttimethusfar.• Runit,updateit’sCPUusagetime,addtotree.

• Interactiveprocesses:Usuallyblocked,lowtotalruntime,highpriority.

Wheredoprocessescomefrom?

Thefork() systemcallcreatesanewprocess.

Onboot,thekernelspawnsthe“init”process.

Init callsfork() tocreatechildprocesses.

Thoseprocessescanalsocreatechildrenwithfork.

fork()

• Systemcall(functionprovidedbyOSkernel).

• Createsaduplicateoftherequestingprocess.• Processiscloningitself:

• CPUcontext• Memory“addressspace”

OS

Stack

TextDataHeap

OS

Stack

TextDataHeap

OS

Stack

TextDataHeap

(Almost)identicalclones

fork() returnvalue• Thetwoprocessesareidenticalineveryway,exceptforthereturnvalueoffork().• Thechildgetsareturnvalueof0.• Theparentgetsareturnvalueofchild’sPID.

pid_t pid = fork(); // both continue after callif (pid == 0) {

printf("hello from child\n");} else {

printf("hello from parent\n");}

Whichprocessexecutesnext?Child?Parent?Someotherprocess?

UptoOStodecide.Noguarantees.Don’trelyonparticularbehavior!

Howmanyhello’swillbeprinted?

fork();

printf(“hello”);

if (fork()) {


}

fork();


A.6B.8C.12D.16E.18

Afterfork(),callexec()

• Familyoffunctions(execl,execlp,execv,…).

• Replacethecurrentprocesswithanewone.

• Loadsprogramfromdisk:• Oldprocessisoverwritteninmemory.• Doesnotreturnunlesserror.

Commonfork()usage:Shell

• A“shell”istheprogramcontrollingyourterminal(e.g.,bash).

• Itcallsfork() tocreatenewprocesses,butwedon’twantaclone(anothershell).

• Wewantthechildtoexecutesomeotherprogram:callexec().


1. fork()childprocess.

2. exec()desiredprogramtoreplacechild’saddressspace.

2. wait()forchildprocesstoterminate.

3. repeat…

Theparentandchildeachdosomethingdifferentnext.


1. fork()childprocess.

Shell

fork()

Shell(p)

Shell(c)


2. parent:wait()forchildtofinish

Shell

fork()

Shell(p)

Shell(c)

wait()


2. child:exec()user-requestedprogram

Shell

fork()

Shell(p)

Shell(c)

wait() exec()


2. child:exec()user-requestedprogram

Shell

fork()

Shell(p)

Shellnewprog

wait() exec()

Runstocompletion


3. childprogramterminates,cyclerepeats

Shell

fork()

Shell(p)

Shellnewprog

wait() exec()

Runstocompletion

Childterminates


3. childprogramterminates,cyclerepeats

Shell

fork()

Shell(p)

Shellnewprog

wait() exec()

Runstocompletion

ChildterminatesShell(p)

Parentprocess(shell)resumes

ProcessTermination

Aprocessterminateswhen:• main returns• Itcallsexit()• Itreceivesaterminationsignal(fromtheOSoranotherprocess)

• Inallcases,itprocessproducesstatusinformation.• Theparentprocessreceivesthisinformation.

WhatHappenswhenaprocessexits?Itbecomesazombieprocessuntilitsparentreapsit.

Zombieprocess:• exited,mostlydead,notrunnableanymore• waitingforparenttocompletelyremoveallofitsstatefromthesystem

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Theparentprocessisresponsibleforcleaningupchildprocessstate.Twooptions:

1. Parentexplicitlywaitsforchildtoexitbycallingawaitfunction:• wait:waitforanychildtoexit(pid returned).• waitpid:waitforaspecificchildtoexit.

2. ParentreceivesaSIGCHILDsignal,andthesignalhandlercodecallswait toreapthechild.

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Reapingzombies

Summary:systemcallsforprocesses• fork:spawnsnewprocess.• Calledonce,Returnstwice(inparentandchildprocess).

• exit:terminatesownprocess.• Calledonce,neverreturns.• Putsitinto“zombie”status.

• wait orwaitpid:reapterminatedchildren.• exec family:runsnewprograminexistingprocess.• Calledonce,(normally)neverreturns.

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SignalsSignal:asoftwareinterrupt:asmallmessagetotell

aprocessthatsomeeventhashappened.• OSsends asignaltoaprocess

• Onbehalfofanotherprocessthatcalledthekill syscall• Astheresultofsomeevent (NULLpointerdereference)

• Aprocessreceives asignalAsynchronous:signalee doesn’tknowwhenitwillgetoneAsignalispending beforeaprocessreceivesit

• Asignalinterrupts thereceivingprocess,whichthenrunssignalhandlercode• defaulthandlersforeachsignaltypeinOS• programmercanalsoaddsignalhandlercode

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SignalsOSidentifiesspecificsignalbyitsnumber,examples:

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ID Name DefaultAction CorrespondingEvent2 SIGINT Terminate Interrupt(e.g.,ctl-cfromkeyboard)9 SIGKILL Terminate Killprogram(cannotoverrideorignore)

11 SIGSEGV Terminate Invalidmemoryreference(e.g.NULLptr)14 SIGALRM Terminate Timersignal17 SIGCHLD Ignore Childstoppedorterminated

SendingSignals:Unixcommand:$ kill -9 1234 #sendSIGKILL signaltoprocess1234Systemcall:kill(1234, SIGKILL); //sendSIGKILL toprocess1234

Implicitlysent: side-effectofprogramdoingsomething(NULLptr dereferencecausesSIGSEGV)

ReceivingaSignal• Adestinationprocessreceives asignalwhenitisforcedbythekerneltoreactinsomewaytothedeliveryofthesignal.

• Threepossiblewaystoreact:• Ignore thesignal(donothing)notallsignalscanbeignored(e.g.SIGKILL)• Terminate theprocessonreceiptofsignal• Catch thesignalbyexecutingauser-levelfunctioncalledsignalhandler

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InstallingSignalHandlerssignal(int signum, handler_t *handler);

• Modifiesthedefaultactionassociatedwiththereceiptofaparticularsignal.

• handler isasignalhandlerfunction• Whenprogramreceivessignal,itjumpstostartexecutingthehandler function.• Whenthehandler doneexecuting,controlpassesbacktoinstructioninthecontrolflowoftheprocessthatwasinterruptedbyreceiptofthesignal.

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Summary

• ProcessesarecycledoffandonCPUrapidly.• Mechanism:contextswitch• Policy:CPUscheduling

• Processesarecreatedbyfork().

• Otherfunctionstomanageprocesses:• exec():replaceaddressspacewithnewprogram• exit():terminateprocess• wait():reapchildprocess,getstatusinfo

• Signalscommunicateeventstoprocesses.

Processes - cs.swarthmore.edu€¦ · • Other processes: • Keep red-black BST of process,...

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