lecture 2: processes - university of california, san...

Lecture2:ProcessesCSE120:PrinciplesofOpera9ngSystems

UCSanDiego:SummerSessionI,2009FrankUyeda

Announcements

•  PeerWiseaccountsarenowlive.– FirstPeerWiseques9ons/reviewsduetomorrow(Thursday)at11:59pm

•  Homework1isonline(dueonMonday7/6)

•  Project1isonline(duenextFriday7/10)– PleaseemailmeprojectgroupsASAP.

– Milestonetes9ngonMondayatMidnight.

Announcements(cont.)

•  NolectureonFriday!– Happy4thofJuly

•  Discussionsec9ontomorrow@1pminCSE2154.

Review

•  Opera9ngSystemsprovide:– Resourcemanagement

– Abstrac9onsforapplica9ondevelopment

•  Protec9on– UsermodevsKernelmode

•  HardwareSupport–  Interrupts&Excep9ons

GoalsforToday

•  Whatisconcurrencyandwhendowewantit?

•  Whatarethebasicunitsofexecu9oninanOS?•  HowaretheseunitsstoredandmanagedinanOS?•  Whatarethepossiblestatesofthese?

•  Howdotheytransi9onfromstatetostate?•  Howaretheycreatedanddestroyed?•  Mechanismsforinter‐processcommunica9on

Concurrency

•  Uniprogramming– OSrunsasinglejobun9litiscompleted.

•  Mul9programming– OShasaqueueofjobs–  Selectsonejobtorun–  IfthejobhastowaitforI/Oorhasrunforawhile,theOSsavesitsstateandputsitbackintothequeue.

–  Repeatlast2stepsun9lalljobscomplete.

Whichtouse?

•  Whendoesitmakesensetouseauniprogrammingmodel?– Don’twantperformanceoverheadofswitchingjobs– Don’tneed/wantaddi9onalOScomplexity.

•  Whendoesitmakesensetouseamul9programmingmodel?– Wanttoseeprogressonmanyjobsatthesame9me.

–  Beeeru9lizeresourceswhenjobswaitforI/O

Concurrency(cont)

•  Uniprogramming:onejob/taskata9me–  Batchschedulers,suchasinmanysupercompu9ngclusters,jobsrun

sequen9allytocomple9on;MS/DOS,executeonecommandata9me

•  Mul9programming:manyjobs/tasksata9me–  Time‐sharing,wherejobscoexistandresourcesare9me‐sliced

–  Usedinpersonalcomputersandsupportedbynearly*allmodernOpera9ngSystems

•  Whatdoyouneedtosupportconcurrency?–  Don’twanttoforceapplica9onstodealwithit–  We’dliketohavesomewayforOStorepresent(perhapsthrougha

programmingabstrac9on)ajobortask

Process

•  AprocessisthechosenOSabstrac9onforajob/task–  Unitofexecu9on–  Unitofscheduling–  Unitofaccoun9ng

•  Aprocessistheinstan9a9onofaprogram–  Allprocessesareassociatedwithaprogram–  Aprogramorapplica9oncanhavemul9pleprocesses

What’sinaProcess?

Stack

Heap

DataSegment

TextSegment

SP

PC

0x00…….(Star9ngAddress)

0xFFF…..(EndingAddress)

AddressSpaceProgramstate•  Addressspace•  Code

•  Data

•  Stack

•  Registers(SP,PC,FP,..)

ProgramIden9fier(PID)

WhatisinaProcess(2)?

•  ProgramInforma9on

•  Opera9ngSystemresources–  Openfiles,opennetworksockets

•  Execu9onstate–  Running:currentlyusingtheCPU–  Ready:readytousetheCPU(wai9ngforit)

•  AnotherprocesshasCPU– Wai9ng:wai9ngforanevent(nottheCPU)suchasI/O

•  Cannotmakeprogressun9lsignaled

UnixProcesses:pscommand

ProcessID Processexecu9onstate

Whatstatedoyouthinkprocessesareinmostofthe9me?

ProcessRepresenta9on

•  TheOSrepresentsaprocessinthekernelusingaProcessControlBlock(PCB)–  Programinforma9on–  OSresources–  Execu9onstate–  Memorymanagement

–  Schedulinginforma9on

–  Accoun9nginforma9on

•  ProcessControlBlocksarelarge–  referredtoasheavyweightabstrac9ons–  Containseverythingneededtorestorethehardwaretothestatethat

itwasinwhentheprocesswaslastrunning.

ProcessTransi9ons

Running

Wai9ng

Ready

scheduleprocess

unscheduleprocess

blockingonevent(I/O)

eventoccurred(e.g.,I/Oreceived)

When/howdoesaprocesschangeexecu9onstate?

ProcessTransi9ons

•  Arunningprocesshashardwarestate–  Registers,memorymanagement–  Whathappenstothisstatewhenittransi9onstootherstate?

•  IftheOSmovesaprocessoutofrunningstate–  ItshardwarestatemustbesavedinthePCB

•  IftheOSmovesaprocessintotherunningstate–  ItshardwarestateisloadedintothePCB

•  Theprocessofchangingthehardwarestatefromoneprocesstoanotherisacontextswitch–  Contextswitchescanoccurasmanyas100‐10009mes/sec

ContextSwitchTwoprocesses:P0andP1

TheOSconsumesafairamountofprocessing9me

This9meisreferredtoascontextswitchingoverhead

Toomuchcontextswitchingoverheadisabadthing(why?)

Note:imagecourtesyofSilberschatz,2005

SchedulingQueuesTheOSkeepstrackofnon‐runningprocessesinschedulingqueues

head

tail

ReadyqueueFirefoxPCB XServerPCB IdlePCB

head

tail

DiskI/OPCBEmacsPCB BashPCB

TerminalPCBTheremaybemanywaitqueues

• seeNachos.threads.ThreadedQueue

PCB’sandStateQueues

•  PCB’saredatastructuresdynamicallyallocatedinOSmemory

•  Whenaprocessiscreated,theOSallocatesaPCBforit,ini9alizesit,andplacesitintheReadyqueue.

•  Astheprocessexecutes,doesI/O,etc.,itsPCBismovedfromonequeuetoanother.

•  Whentheprocessterminates,it’sPCBisdeallocated.

ProcessCrea9on

•  Aprocessiscreatedbyanotherprocess– Createdprocessiscalledchild– Crea9ngprocessiscalledparent–  InUnix,parentprocessIDisstoredasPPID

•  parent‐childrela9onships:sshd‐>shell‐>ps

ProcessCrea9on(2)

•  Howareparentandchildprocessesrelated?–  IsanyofthestatefromthePCBinherited?

– Unix:ProcessUIDisinherited,andchildreninheritparent’sprivileges

•  Whatelsecanaparentdowithachild?– Parentcaninvokesystemcalltowaitforcomple9onofchild

– Parentcanalsoexecuteinparallelwithchild

ProcessCrea9oninUnix

•  fork()systemcallisusedtocreateanewprocessint fork() –  Createsandini9alizesanewPCB–  Createsanewaddressspace–  Fillschild’saddressspacewithacopyoftheparent’sen9re

addressspace–  Ini9alizeskernelresourcestopointtoparent’s–  PutsPCBonthereadyqueue

•  So…..howdoweknowwhichoneiswhich?–  EachprocesshasadifferentPID–  fork()returnstwice:

•  child’sPIDtotheparent•  “0”tothechild

Unixfork()exampleint main(int argc, char *argv[]) { char *name = argv[0]; int child_pid = fork(); if (child_pid == 0) { printf(“Child of %s is %d\n”, name, getpid()); return 0; } else { printf(“My child is %d\n”, child_pid); return 0; { }

Whatdoesthisprogramprint?

Note:ThisexamplecourtesyofAlexSnoeren

Unixfork()example(2)

Istheorderoftheprintedstatementsalwaysgoingtobethesame?

int main(int argc, char *argv[]) {

char *name = argv[0]; int child_pid = fork(); if (child_pid == 0) {

printf(“Child of %s is %d\n”, name, getpid());

return 0; }

else { printf(“My child is %d\n”, child_pid); return 0;

{ }




return 0; }


{ }

Unixfork()example(3)Parent Child

PC PC




return 0; }


{ }




return 0; }


{ }

Unixfork()example(4)Parent Child

PC PC

Pid=27766 Pid=0




return 0; }


{ }




return 0; }


{ }

Unixfork()example(5)Parent ChildPid=27766 Pid=0

Mychildis27766 Childofforktestis27766

fork()Abstrac9on

while (1) {

int sock = accept();

if (child_pid == 0) { // handle client request

…

}

else {

// close socket

…

{

}


Veryusefulif•  Childiscoopera9ngwithparent•  Childsharesstatewithparent

Webserverexample

Processsynchroniza9oninUnix

•  wait()suspendsthecurrentprocessun9lachildprocessendsintwait(int*status)

–  wait()returns‐1iferrororpidofchild–  statusischild’sexitstatus

•  Whatifparentprocessexitsbeforechild?–  PCBofchildprocessremains

–  Childprocessbecomesan“orphan”or“zombie”process

Unixfork()andwait()exampleint main(int argc, char *argv[]) {

char *name = argv[0];

int child_pid = fork();

int rv, status;

if (child_pid == 0) {


return 0;

}

else {

rv = wait(&status);

printf(“My child is %d, rv=%d, status=%d\n”, child_pid, rv, status);

return 0;

{

} Whatdoesthisprogramprint?


Processcrea9oninUnix(2)

•  exec()systemcallusedtostopand“restart”processint exec(char *prog, char *argv[]) –  Stopsthecurrentprocess–  Loadsprogramprogintoexis9ngaddressspace–  Reini9alizeshardwarestate–  PutsPCBonthereadyqueue

•  Whatshouldexec()return?– Whatabout“execcsh”?–  “execls”?

UnixShells

while(1){

char*cmd=read_command();

intchild_pid=fork();if(child_pid==0){

ManipulateSTDIN/OUT/ERRfiledescriptorsforpipes,redirec=on,etc.

exec(cmd);

panic(“execfailed”);

}else{

waitpid(child_pid);

}

}

ProcessTermina9on

•  exit(intstatus)systemcallterminatesprocess–  exit()freesallresourcesconsumedbyprocess

•  Close/releaseallOSresources•  Freeallocatedmemory

•  Terminateanythreads(nextlecture)

•  RemoveanddeletePCB

•  NotethattheOSdoesnotassumethataprocesswillfreeeverythingabove.– Why?

ProcessTransi9ons

Running

Wai9ng

Ready

scheduleprocess

unscheduleprocess

blockingonevent(I/O)

eventoccurred(e.g.,I/Oreceived)

processexit

New

Terminated

createprocess

InterprocessCommunica9on

•  Wecancreateprocesswithfork(),buthowdowegetthemtoworktogether?– Wanttopassinforma9onbetweenprocesses.

•  Childgetsacopyofparent’saddressspace.– Whatiftheyneedtocommunicateaterfork()?

•  Whathappensifsiblingprocesseswanttocommunicate?

InterprocessCommunica9on(IPC)

•  Opera9ngSystemneedstoprovidesupport– Why?

•  OSmechanisms:–  SharedMemory– MessagePassing

SharedMemoryProcessesreadandwritetoaregionofsharedmemory

ProcessA

ProcessB

PhysicalMemory

Kernel

•  Aprocessneedstocreateasharedmemorysegment•  Unix:shmget()syscall•  Kernelcreatessharedmemory

•  Eachprocessneedstoaeachtothissegment•  Unix:shmat()syscall

•  Eitherprocesscanreadorwritetothisregion•  A:sprinu(shared_mem,“helloworld”);

•  B:prinu(“%s\n”,shared_mem);

helloworld

helloworld(stdout)

shared_mem

MessagePassing

•  Createacommunica9onlinkbetweenAandB–  OScancreatemailboxfor

indirectcommunica9on–  Useothermeansfordirect

communica9on(sharedmemory,network,bus)

•  send(B,message)–  Doesitwaitun9lreceived?

•  receive(A,message)–  Doesitwaitun9lavailable?

•  Synchroniza9on–  Blocking/Synchronous–  Non‐Blocking/Asynchronous

ProcessA

ProcessB

Kernel

ProcessespassmessagesthroughOS


•  Eachcoopera9ngmustknowthenameoftheother•  SharedMemory

–  Canperformfrequentandlargetransfers– Mustsynchronize

•  MessagePassing–  Noexplicitneedforsynchroniza9on–  Kerneloverheadpermessage

•  Othercommonvaria9onsofIPC–  Unixpipes(e.g.,“ls|wc”)– WindowsLocalProcedureCalls(LPCs)


•  WhousesIPCmechanisms?–  Parallelprograms(e.g.,Webserver,stagedcomputa9ons)– Modularprograms(i.e.,composetogethersubtasks,eachbeingaseparateprocesses)

•  Ingeneral,IPCmechanismsare9me‐consuming–  Involvesystemcalls(kernelinvolved)

–  Processesneedtocontextswitchtosend/receivemessages

Summary

•  Whatarethebasicunitsofexecu9oninanOS?–  Aprocess

•  Howaretheseunitsofexecu9onstoredandmanagedinanOS?–  PCBsandinSchedulingQueues

•  Whatarethepossiblestatesofaprocess?–  Running,ready,wai9ng,new,terminated

•  Howdoprocessestransi9onfromstatetostate?–  Schedulingevents,I/Osignals,processcrea9on,termina9on

•  Howdoprocesscommunicate?–  SharedmemoryorMessagePassing

NextTime

•  ReadChapters4(Threads)•  FirstPeerWiseques9on&reviewsduetomorrow(Thursday).

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

Lecture1:Courseoverview

lecture 2: processes - university of california, san...

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