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Processes&Threads

CS4410,Opera5ngSystems

Fall2016CornellUniversity

RachitAgarwalAnneBracy

See:Ch3&4inOSPPtextbook

TheslidesaretheproductofmanyroundsofteachingCS4410byProfessorsSirer,Bracy,Agarwal,George,andVanRenesse.SomecontentfromMarkusPüschelatCMU.

WhatisaProcess?

2

•Aninstanceofaprogram•Anabstrac5onofacomputer:

AddressSpace+Execu5onContext+Environment

AgoodabstracOon:•isportableandhidesimplementa5ondetails•hasanintui5veandeasy-to-useinterface•canbeinstan5atedmany5mes•isefficientandreasonablyeasytoimplement

ProcessManagement

3

Canaprogram…• Createaninstanceofanotherprogram?• Waitforittocomplete?• Stoporresumeanotherrunningprogram?• Senditanasynchronousevent?

Whoshouldbeallowedtostartaprocess?

4

Possibility#1:Onlythekernelmaystartaprocess

Possibility#2:User-levelprocessesmaystartprocesses

SystemCallInterface

5

System CallInterface

Portable OperatingSystem Kernel

PortableOS Library

Web ServersCompilers Source Code Control

Web Browsers Email

Databases Word Processing

x86 ARM PowerPC

10Mbps/100Mbps/1Gbps Ethernet

802.11 a/b/g/n SCSI IDE

Graphics Accelerators LCD Screens

Whysoskinny?

Example:Crea%ngaProcess

Windows:CreateProcess(…);

UNIXfork+exec

BeginningaProcessviaCreateProcess

6

Kernelhasto:•Create&ini5alizePCBinthekernel• Createandini5alizeanewaddressspace• Loadtheprogramintotheaddressspace• Copyargumentsintomemoryinaddressspace• Ini5alizehwcontexttostartexecu5onat“start”• Informschedulerthatnewprocessisreadytorun

[Windows]

AbstractLifeofaProcess

7

New

Runnable Running

Zombie

Waiting

admiSed done

I/OOperaOonI/OCompleOon

dispatch

interrupt,descheduling

DetailsonThursday.

CreateProcess(Simplified)

8

SystemCall

if(!CreateProcess(NULL,//Nomodulename(usecommandline)argv[1],//CommandlineNULL,//ProcesshandlenotinheritableNULL,//ThreadhandlenotinheritableFALSE,//SethandleinheritancetoFALSE0,//NocreationflagsNULL,//Useparent'senvironmentblockNULL,//Useparent'sstartingdirectory&si,//PointertoSTARTUPINFOstructure&pi)//PtrtoPROCESS_INFORMATIONstructure)

[Windows]

Fork+Exec

9

pid=fork();if(pid==0)exec(B);elsewait(pid);

PC

pid?

ProgramA

pid=fork();if(pid==0)exec(B);elsewait(pid);

PC

pid0

ProgramA

pid=fork();if(pid==0)exec(B);elsewait(pid);

PC

pid42

ProgramA

main(){...}

PC

pid0

ProgramB

[UNIX]

Process1

ifandelsebothexecuted!Process1

Process42 Process42

BeginningaProcessviaCreateProcessFork

10

Kernelhasto:•Create&ini5alizePCBinthekernel• Createandini5alizeanewaddressspace

• Loadtheprogramintotheaddressspace• Copyargumentsintomemoryinaddressspace• IniOalizetheaddressspacewithacopyoftheenOrecontentsoftheaddressspaceoftheparent

• Ini5alizehwcontexttostartexecu5onat“start”• InheritexecuOoncontextofparent(e.g.openfiles)

• Informschedulerthatnewprocessisreadytorun[UNIX]

Codeexample

11

/* * Corresponds to Figure 3.5 in the textbook * */

#include <stdio.h> #include <unistd.h>

int main() {

int child_pid = fork();

if (child_pid == 0) { // child process printf("I am process #%d\n", getpid()); return 0; } else { // parent process. printf("I am the parent of process #%d\n", child_pid); return 0; } }

Possibleoutputs?

CreaOngandManagingProcesses

12[UNIX]

fork Create a child process as a clone of the current process. Returns to both parent and child.

exec(prog, args)

Run the application prog in the current process.

exitTell the kernel the current process is complete, and its data structures (stack, heap, code) should be garbage collected.

Why not necessarily PCB?

wait(pid) Pause until the child process has exited.

kill(pid, type)

Send an interrupt of a specified type to a process.

QuesOons

13

•CanUNIXfork()returnanerror?Why?

•CanUNIXexec()returnanerror?Why?

•CanUNIXwait()everreturnimmediately?Why?

WhatisaShell?

14

Jobcontrolsystem• runsprogramsonbehalfoftheuser•allowsprogrammertocreate/managesetofprograms

• sh OriginalUnixshell(StephenBourne,AT&TBellLabs,1977)• csh BSDUnixCshell(tcsh:enhancedcsh

atCMUandelsewhere)• bash “Bourne-Again”Shell

Runsatuser-level.Whatsystemcallsdoesituse?

Built-InUNIXShellCommands

15[UNIX]

jobs List all jobs running in the background + all stopped jobs.

bg <job> Run the application prog in the current process.

fg <job> Change a stopped or running background job to a running in the foreground.

kill <job> Terminate a job.

showinacOon,+exec

Signals

16[UNIX]

ID Name Default Action Corresponding Event

2 SIGINT Terminate Interrupt (e.g., ctrl-c from keyboard)

9 SIGKILL Terminate Kill program (cannot override or ignore)

14 SIGALRM Terminate Timer signal

17 SIGCHLD Ignore Child stopped or terminated

20 SIGTSTP Stop until next SIGCONT

Stop signal from terminal (e.g. ctrl-z from keyboard)

Avirtualizedinterrupt.Allowapplica5onstobehavelikeopera5ngsystems.

youtube?

SendingaSignal

17

Kerneldeliversasignaltoades5na5onprocess

Foroneofthefollowingreasons:• Kerneldetectedasystemevent(e.g.div-by-zero(SIGFPE)ortermina5onofachild(SIGCHLD))• Aprocessinvokedthekillsystemcallreques5ngkerneltosendsignaltoanotherprocess

• debugging• suspension• resump5on• 5merexpira5on

ReceivingaSignal

18

Ades5na5onprocessreceivesasignalwhenitisforcedbythekerneltoreactinsomewaytothedeliveryofthesignal

Threepossiblewaystoreact:1.Ignorethesignal(donothing)2.Terminateprocess(+op5onalcoredump)3.Catchthesignalbyexecu5ngauser-levelfunc5oncalledsignalhandler• Likeahardwareexcep5onhandlerbeingcalledinresponsetoanasynchronousinterrupt

showhandler.c

SignalExample

19

void int_handler(int sig) { printf("Process %d received signal %d\n", getpid(), sig); exit(0); }

int main() { pid_t pid[N]; int i, child_status; signal(SIGINT, int_handler); for (i = 0; i < N; i++) // N forks if ((pid[i] = fork()) == 0) { while(1); //child infinite loop } for (i = 0; i < N; i++) { // parent continues executing printf("Killing proc. %d\n", pid[i]); kill(pid[i], SIGINT); } for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) // parent checks for each child’s exit printf("Child %d terminated w exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminated abnormally\n", wpid); } exit(0); }

BlockedSignals

20

Aprocesscanblockthereceiptofcertainsignals•Blockedsignalscanbedelivered,butwillnotbereceivedun5lthesignalisunblocked

Kernelmaintainspendingandblockedbitvectorsinthecontextofeachprocess•blocked:representsthesetofblockedsignals

Canbesetandclearedbyusingthesigprocmaskfunc5on

ProcessGroupsEveryprocessbelongstoexactlyoneprocessgroupShell

Foreground job

Background job #1

Background job #2

Child Child

getpgrp()Returnprocessgroupofcurrentprocesssetpgid()Changeprocessgroupofaprocess/bin/kill–921SendSIGKILLtoprocess24818/bin/kill–9–20SendSIGKILLtoeveryprocessinprocessgroup20

pid=20pgid=20

pid=21pgid=20

pid=22pgid=20

Foregroundprocessgroup20

Backgroundprocessgroup32

Backgroundprocessgroup40

pid=32pgid=32

pid=40pgid=40

pid=10pgid=10

ImplemenOnga(really,reallysimple)Shell

22

void eval(char *cmdline) { char *argv[MAXARGS]; /* argv for execve() */ int bg; /* should the job run in bg or fg? */ pid_t pid; /* process id */

bg = parseline(cmdline, argv); if (!builtin_command(argv)) { if ((pid = Fork()) == 0) { /* child runs user job */ if (execve(argv[0], argv, environ) < 0) { printf("%s: Command not found.\n", argv[0]); exit(0); } }

if (!bg) { /* parent waits for fg job to terminate */ int status; if (waitpid(pid, &status, 0) < 0)

unix_error("waitfg: waitpid error"); } else /* otherwise, don’t wait for bg job */

printf("%d %s", pid, cmdline); } }

AndNow…Threads!

23

WhyThreads?

24

• Programstructure:expressinglogicallyconcurrenttasks• Responsiveness:shioingworktoruninthebackground• Performance:managingI/Odevices

DoesmulO-threadingonlymakesenseonmulOcore?

• Performance:exploi5ngmul5pleprocessors

Stack

Whathappenswhen…

25

Mail

Kernel

PCBs

0x00000000

0xFFFFFFFF

Apachewantstorunmul5pleconcurrentcomputa5ons?

Apache

Emacs

ApacheTwoheavyweightaddressspacesfortwoconcurrentcomputa5ons?

Whatisdis5nctabouttheseaddressspaces?

Heap

DataInsns

StackHeap

DataInsns

Stack

Idea!

26

Mail

Kernel

PCBs

0x00000000

0xFFFFFFFF

Apache

Emacs

Heap

DataInsns

Stack

Eliminateduplicateaddressspacesandplaceconcurrentcomputa5onsinthesameaddressspace.

Processvs.Thread

27

Process:•AddressSpace•SharedI/Oresources•OneormoreThreads:

Othertermsforthreads:LightweightProcess,ThreadofControl,Task

NotShared:• Registers,PC,SP• Stack

Shared:• Code• Data• Privileges

Stack 2

ThreadMemoryLayout

28

Data

Insns

Stack 1

Stack 3

PC

Thread1

Thread2PC

Thread3PC

SP

SP

SP

ProcessA

(HeapSharedbutsubdivided.)

SimpleThreadAPI

29

voidthread_create

(thread,func,arg)

Create a new thread, storing information about it in thread. Concurrently with the calling thread, thread executes the function func with the argument arg.

voidthread_yield

()

Calling thread voluntarily gives up processor to let other thread(s) run. Scheduler can resume running

the calling thread whenever it chooses to do so.

intthread_join(thread)

Wait for thread to finish if it has not already done so; then return the value passed to thread_exit by that

thread. Note that thread_join may be called only once for each thread.

voidthread_exit

(ret)

Finish the current thread. Store the value ret in the current thread’s data structure. If another thread is already waiting in a call to thread_join, resume it.

CodingExample

30

process_share.cthread_share.c

Threads

31

Lighter-weightthanprocesses• processisanabstractcomputer(CPU,mem,devices,…)• threadisanabstractcore

Threadsneedtobemutuallytrus5ng(Why?)

Idealforconcurrentprogramswherelotsofcodeanddataareshared• Servers,GUIcode,…

OpOon#1:KernelThreads

32

•Threadsshareasingleloca5oninmemory•SeparatePCBs(TCBs)foreachthread•PCBshave:• same:base&boundregistervalues• different:PC,SP,registers

Mail

Kernel

PCBs

0x00000000

0xFFFFFFFF

Apache

Emacs

Stack 2Heap

DataInsns

Stack 1

21

MulO-threadedkernelwiththreekernelthreadsandtwosingle-threadeduser-levelprocesses.

33

EachkernelthreadhasitsownTCBanditsownstack.Eachuserprocesshasastackatuser-levelforexecu5ngusercodeandakernelinterruptstackforexecu5nginterruptsandsystemcalls.

AmulO-threadedkernelwith3kernelthreadsand2user-levelprocesses,eachwith2threads.

34

Eachuser-levelthreadhasauser-levelstackandaninterruptstackinthekernelforexecu5nginterruptsandsystemcalls.

OpOon#2:UserThreads

35

•Buildamini-OSinuserspace• RealOSdoesn’tknowaboutmul5plethreads• SinglePCB

•Generallymoreefficientthankernelthreads(Why?)

•Butkernelthreadssimplifysystemcallhandlingandscheduling

(Why?) Mail

Kernel

PCBs

0x00000000

0xFFFFFFFF

Apache

Emacs

stack 2Heap

DataInsns

stack 1“os” stack

ImplemenOngUserThreads(4411-P1!)

36

Userprocesssupports:• ThreadControlBlock(TCB)table

• oneentryperthread• “contextswitch”opera5ons

• save/restorethreadstateinTCB• muchlikekernel-levelcontextswitches

• yield()opera5on:• threadreleasescore,allowsanotherthreadtouseit• Automa5cpre-emp5onnotalwayssupported

• Threadscheduler

UserThreads&SystemCalls

37

•Withuserthreads,aprocessmayhavemul5plesystemscallsoutstandingsimultaneously(oneperthread)•KernelPCBmustsupportthis(Why?)

CooperaOvevs.PreempOveMulOthreading

38

Coopera5ve:threadrunsun5lityieldscontroltoanotherthread—yieldisinthecodeitself+besercontrolofscheduling+simplerreasoningaboutsharedresources–starva5on(slowinterfaces)–mul5-core→reasoningnotsimpleraferall!

[Notcommonthesedays]

Mul5-threading==preemp5vemul5-threadingexceptin11-P1“Non-PrempOveMulOtasking”

MulOthreadingProgrammingModel

39

Rule#1:don’tpresumetoknowtheschedule

SharedResources→Synchroniza5onMasers!thread_share.crevisited(NextWeek!)