Unit 3.2 Process Control.ppt (Process Control and Scheduling)

This is for learning purpose.

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Computer Engineering
(NBA Accredited)
Prof. A. V. Brahmane
Assistant Professor
E-mail : [email protected]
Contact No: 91301 91301 Ext :145, 9922827812
Subject-Operating System and Administration (CO2013)
Unit III-The Structure of Process, Process Control and
Process Scheduling

2. Content
•Process state and transitions,
•Layout of the system memory,
•Context of the process,
•Saving the context of the process,
•Manipulation the process address space,
•Sleep,
•Process creation,
•Signal,
•Process termination, Awaiting the process termination,
•Invoking other program, Process Scheduling
•Case Study -Access Control, Rootly Powers and Controlling Processes
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3. Process Control –Scheduling
•Therelationshipbetweenthesystemcallsandmemorymanagementalgorithms
isshowninthefollowingdiagram:
•Almost all calls usesleepandwakeupso its not shown in the figure.execalso interacts with the
file system algorithms
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4. Process Creation
•TheonlywaytocreateanewprocessinUNIXistousetheforksystemcall.The
processwhichcallsforkiscalledtheparentprocessandthenewlycreated
processiscalledthechildprocess.
•pid=fork();
•Onreturnoftheforksystemcall,thetwoprocesseshaveidenticaluser-level
contextexceptforthevalueofpid.pidfortheparentprocessistheprocessIDof
thechildprocess.Andpidforchildprocessis0.Theprocess0istheonlyprocess
whichisnotcreatedviafork.
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5. •Thestepsfollowedbythekernelforforkare:
•Itcreatesanewentryintheprocesstable.
•ItassignsauniqueIDtothenewlycreatedprocess.
•Itmakesalogicalcopyoftheregionsoftheparentprocess.Ifaregionscanbe
shared,onlyitsreferencecountisincrementedinsteadofmakingaphysicalcopy
oftheregion.
•Thereferencecountsoffiletableentriesandinodesoftheprocessareincreased.
•ItturnedthechildprocessIDtotheparentand0tothechild.
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6. The algorithmfork
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7. Fork creating a New process context
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8. Signals
•Signalsinformprocessesoftheoccurrenceofasynchronousevents.Processes
maysendeachothersignalswiththekillsystemcall,orthekernelmaysend
signalsinternally.
•Thereare19signalsintheSystemV(Release2)UNIXsystemthatcanbe
classifiedasfollows:
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9. Signal Classifications
•Signalshavingtodowiththeterminationofaprocess,sendwhenaprocessexitsorwhenaprocessinvokes
thesignalsystemcallwiththedeathofchildparameter.
•Signalshavingtodowithprocessinducedexceptionssuchaswhenaprocessaccessesanaddressoutsideits
virtualaddressspace,whenitattemptstowritememorythatisread-only(suchasprogramtext),orwhenit
executesaprivilegedinstructionorforvarioushardwareerrors.
•Signalshavingtodowiththeunrecoverableconditionsduringasystemcall,suchasrunningoutofsystem
resourcesduringexecaftertheoriginaladdressspacehasbeenreleased
•Signalscausedbyanunexpectederrorconditionduringasystemcall,suchasmakinganonexistentsystem
call(theprocesspassedasystemcallnumberthatdoesnotcorrespondtoalegalsystemcall),writingapipe
thathasnoreaderprocesses,orusinganillegal"reference"valueforthelseeksystemcall.Itwouldbemore
consistenttoreturnanerroronsuchsystemcallsinsteadofgeneratingasignal,buttheuseofsignalsto
abortmisbehavingprocessesismorepragmatic.
•Signalsoriginatingfromaprocessinusermode,suchaswhenaprocesswishestoreceiveanalarmsignal
afteraperiodoftime,orwhenprocessessendarbitrarysignalstoeachotherwiththekillsystemcall.
•Signalsrelatedtoterminalinteractionsuchaswhenauserhandsupaterminal(orthe"carrier"signaldrops
onsuchalineforanyreason),orwhenauserpressesthe"break"or"delete"keysonaterminalkeyboard.
•Signalsfortracingexecutionofaprocess.
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10. Checking and Handling Signals in the Process State Diagram
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11. The algorithmissig
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12. Handling Signals
•The handling of signal is done through the algorithmpsig,
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13. Types of Signals
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14. Process Groups
•Thesystemhastoidentifyprocessesby"groups"insomecases,forinstance,a
signalmightrelatetoalltheprocesseswhichareancestorsoftheloginshell.The
kernelusestheprocessgroupIDtoidentifygroupsofrelatedprocessesthat
shouldreceiveacommonsignalforcertainevents.ItsavesthegroupIDinthe
processtable.
•Thesetpgrpsystemcallinitializestheprocessgroupnumberofaprocessandsets
itequaltothevalueofitsprocessID.
•grp=setpgrp();
•wheregrpisthenewprocessgroupnumber.Achildretainstheprocessgroup
numberofitsparentduringfork.
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15. Sending Signals from Processes
•Processesusethekillsystemcalltosendsignals.
•kill(pid,signum);
•wherepididentifiesthesetofprocessestoreceivethesignal,andsignumisthesignalnumber
beingsent.Thefollowinglistshowsthecorrespondencebetweenvaluesofpidandsetsof
processes.
•Ifpidisapositiveinteger,thekernelsendsthesignaltotheprocesswithprocessIDpid.
•Ifpidis0,thekernelsendsthesignaltoallprocessesinthesender'sprocessgroup.
•Ifpidis-1,thekernelsendsthesignaltoallprocesseswhoserealuserIDequalstheeffectiveuser
IDofthesender(realandeffectiveuserIDsarestudiedlater).Ifthesendingprocesshaseffective
userIDofsuperuser,thekernelsendsthesignaltoallprocessesexceptprocesses0and1.
•Ifpidisanegativeintegerbutnot-1,thekernelsendsthesignaltoallprocessesintheprocess
groupequaltotheabsolutevalueofpid.
•Inallcases,ifthesendingprocessdoesnothaveeffectiveuserIDofsuperuser,oritsrealor
effectiveuserIDdonotmatchtherealoreffectiveuserIDofthereceivingprocess,killfails.
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16. Process Termination
•ProcessesontheUNIXsystemexitbyexecutingtheexitsystemcall.Whena
processexits,itentersthezombiestate,relinquishesallofitsresources,and
dismantlesitscontextexceptforitsprocesstableentry.
•exit(status);
•wherestatusistheexitcodereturnedtotheparent.Theprocessmay
callexitexplicitly,butthestartuproutineinCcallsexitafterthemainfunction
returns.Thekernelmaycallexitonreceivinganuncaughtsignal.Insuchcases,
thevalueofstatusisthesignalnumber.
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17. The algorithm for exit
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18. Awaiting Process Termination
•Aprocesscansynchronizeitsexecutionwiththeterminationofachildprocessby
executingthewaitsystemcall.
•pid=wait(stat_addr);
•wherepidistheprocessIDofthezombiechild,andstat_addristheaddressin
userspaceofanintegerthatwillcontaintheexitstatuscodeofthechild.
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19. The algorithm forwait
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20. Invoking Other Programs
•Theexecsystemcalloverlaystheaddressspaceofaprocesswiththecontentsof
anexecutablefile.
•execve(filename,argv,envp)
•wherefilenameisnameofthefilebeinginvoked,argvisapointertoarrayof
characterpointerswhichareargumentstotheprogramintheexecutablefile,
andenvpisapointertoarrayofcharacterpointerswhicharetheenvironmentof
theexecutedprogram.Thereareseverallibraryfunctionsthatcallexec,
likeexecl,execv,execle,andsoon.Allcallexecveeventually.Thecharacterstrings
intheenvparrayareoftheform,"name=value".Cprogramscanaccessthe
environmentvariablesbyaccessingtheglobalvariableenviron,initializedbythe
Cstartuproutine.
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21. The algorithm forexec
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22. The User ID of a Process
•TherearetwouserIDassociatedwithaprocess,therealuserIDandtheeffective
userIDorsetuid(setuserID).TherealuserIDidentifiestheuserwhois
responsiblefortherunningprocess.TheeffectiveuserIDisusedtoassign
ownershipofnewlycreatedfiles,tocheckfileaccesspermissions,andtocheck
permissiontosendsignalstoprocessesviathekillsystemcall.Thekernelallowsa
processtochangeitseffectiveuserIDwhenitexecsasetuidprogramorwhenit
invokesthesetuidsystemcallexplicitly.
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23. •Asetuidprogramisanexecutablefilethathasthesetuidbitsetinitspermission
modefield.Whenaprocessexecsasetuidprogram,thekernelsetstheeffective
userIDfieldsintheprocesstableandu-areatotheownerIDofthefile.To
distinguishthetwofields,letuscallthefieldintheprocesstablethesaveduser
ID.
•setuid(uid);
•whereuidisthenewuserID,anditsresultdependsonthecurrentvalueofthe
effectiveuserID.IftheeffectiveuserIDofthecallingprocessissuperuser,the
kernelresetstherealandeffectiveuserIDfieldsintheprocesstableandu-area
touid.Ifitsnotthesuperuser,thekernelresetstheeffectiveuserIDintheu-area
touidifuidhasthevalueoftherealuserIDorifithasthevalueofthesaveduser
ID.Otherwise,thesystemcallreturnsanerror.Generally,aprocessinheritsits
realandeffectiveuserIDsfromitsparentduringtheforksystemcalland
maintainstheirvaluesacrossexecsystemcalls.
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24. Changing the Size of a Process
•Aprocesscanincreaseordecreasethesizeofitsdataregionbyusing
thebrksystemcall.
•brk(endds);
•whereenddsbecomesthevalueofthehighestvirtualaddressofthedataregion
oftheprocess(calleditsbreakvalue).Alternatively,ausercancall
•oldendds=sbrk(increment);
•whereincrementchangesthecurrentbreakvaluebythespecifiednumberof
bytes,andoldenddsisthebreakvaluebeforethecall.sbrkisaClibraryroutine
thatcallsbrk.Thekernelchecksthatthenewprocesssizeislessthanthesystem
maximumandthatthenewdataregiondoesnotoverlappreviouslyassigned
virtualaddressspace.
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25. Algorithm: brk
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26. •Thank you …..
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