Introduction to Computer ScienceCSCI109

AndrewGoodneySpring2018

China– Tianhe-2

Lecture8:OperatingSystemsMarch5th, 2018

ìOperating SystemsWorkingTogether

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ScheduleDate Topic Assigned Due Quizzes/Midterm/Final

8-Jan IntroductionWhatiscomputing,howdidcomputerscometobe?

HW1

15-Jan MLKHoliday

22-Jan ComputerarchitectureHowisamoderncomputerbuilt?Basicarchitectureandassembly

29-Jan DatastructuresWhyorganizedata?Basicstructuresfororganizingdata

HW1Quiz1onmaterialtaughtinclass1/8and1/22

30-Jan

5-Feb DatastructuresTrees,GraphsandTraversals

HW2

12-FebMoreAlgorithms/DataStructures,ComplexityandCombinatorics Recursionandrun-time

Quiz2onmaterialtaughtinclass1/29,2/5

19-Feb PresidentsDay

23-Feb

26-Feb Algorithmsandprogramming(Somewhat)Morecomplicatedalgorithmsandsimpleprogrammingconstructs,compilers

HW2Quiz3onmaterialtaughtinclass2/12

5-Mar Operatingsystems WhatisanOS?Whydoyouneedone? HW3Quiz4onmaterialtaughtinclass2/26

12-Mar SpringBreak

19-Mar Midterm MidtermMidtermonallmaterialtaughtsofar.

26-Mar ComputernetworksHowarenetworksorganized?HowistheInternetorganized?

HW3

2-Apr ArtificialintelligenceWhatisAI?Search,plannningandaquickintroductiontomachinelearning

HW4Quiz5onmaterialtaughtinclass3/26

6-Apr

9-Apr Thelimitsofcomputation Whatcan(andcan't)becomputed?Quiz6onmaterialtaughtinclass4/2

16-Apr RoboticsRobotics:backgroundandmodernsystems(e.g.,self-drivingcars)

HW4Quiz7onmaterialtaughtinclass4/9

23-Apr Summary,recap,review Summary,recap,reviewforfinalQuiz8onmaterialtaughtinclass4/16

4-MayFinalonallmaterialcoveredinthesemester

Finalexam11am-1pminSAL101

LastdaytodropaMonday-onlyclasswithoutamarkof“W”andreceivearefundorchangetoPass/NoPassorAuditforSession001

Lastdaytodropacoursewithoutamarkof“W”onthetranscript

Lastdaytodropaclasswithamarkof“W”forSession001

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Agenda

u Talkaboutoperatingsystemsu Reviewquizzes1-3(midtermstudying)

u Takequiz4

3

Operating Systems

uWhatisanOS?uThekernel,processesandresources

uProtection/Isolation/SecurityuCompetingfortimeuCompetingforspace

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Reading:St.Amant Ch.6

The need for an OS

5

CPU,Memory,Disk,I/OProgram

Problem

Solution:Algorithm+DataStructures

Pseudocode

Low-levelinstructions

Executeson

Compilesto

The OS as a executive manager

6

CPU,Memory,Disk,I/OProgram

Problems

Solution:Algorithms +DataStructures

Pseudocode

Low-levelinstructions

Executionsmanagedby

Compileto

Low-levelinstructionsLow-levelinstructions

ProgramProgramProgram

OperatingSystem

What is an Operating System?

uAnexecutivemanagerforthecomputeruManagesresources

v Space(i.e.memory)vTime(i.e.CPUcomputetime)vPeripherals(i.e.inputandoutput)

uOSisaprogramthatstarts,runs,pauses,restarts,andendsotherprograms

u (somecontentfromthefollowingslidesiscourtesyofMarkRedekopp andCS350)7

8

4

Definition

• A piece of software that manages a computer’s resources

• What resources need managing?– CPU (threads and processes)– Memory (Virtual memory,

protection)– I/O (Abstraction, interrupts,

protection)DISK

Processor

Mem. Management Unit

Network

Graphics

I/O Drivers & Protocols

File Systems

Memory Translation

Processes & Scheduling

System Library

System Library

User App User App

Ker

nel

Mod

eH

ardw

are

Use

r m

ode

Examples of Operating Systems

u MicrosoftFamilyv MSDOS,Windows3.1– 98,WindowsNT ->Windows10v Predominatelyx86(Intel)hardware,somePowerPC,someARMv FreeDOS

u POSIX(UNIX/like)v macOS,FreeBSD,openBSD,netBSD,Solaris,AIX,andothers

u Runonmostprocessorarchitecturesv iOSv Linux

u Littlesideprojectofuniversitystudentu ”UNIXclone”thatwonthewaru 20+populardistributionsu Android:heavilycustomizedLinuxandJavaonphone/tablet

u Othersv PlaystationOS,VxWorks

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10

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Genealogy of Operating Systems

Figure 1.9,OS:PP 2nd Ed.

Important Vocabulary

u Resourcev Somepartofthecomputerthatprogramsuse:

u Memory,CPU,Input/Output devices

u Policyv Rulesenforcedbyalgorithmsthatshareaccesstoresources

u OSDevelopers(humans)writepoliciesthatachievesomesetofgoalsfortheoperatingsystems

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What does an Operating System do?

uAbarecomputerisjusthardwareuProgramsarewrittentousethathardware,butexclusiveuseisinefficient

uInsimpleterms,theOS:v Enablesmorethanoneprogramatatimetousethecomputerhardware

v Presentcomputerresources(CPU,disk,I/O)throughabstractinterfacestoallowsharing

v Enforcepoliciestomanage/regulatethesharingofresources

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Roles

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9

Roles

• Textbook regularly revisits 3 roles an OS plays:

• Referee

– Protection against other applications

– Enforce fair resource sharing

• Why doesn't an infinite loop require a reboot?

• Illusionist (Virtualization)

– Each program thinks it is running separately

– Each program thinks it has full access to computer's resources (or unlimited

resources)

• Glue

– Common services (such as copy/paste)

– Files can be read by any application

– UI routines for look & feel

– Separate applications from hardware

• so you don’t need to know which keyboard, disk drive, etc

OS Design Criteria

u Reliability(andavailability)u Security&Privacy

u Performanceu Portability

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Reliability and Availably

u Reliablesystemsworkproperlyv Correct(orexpected)outputsaregeneratedforasetofinputsv Ifthisisnotthecase,thesystemhasfailed

u Examples?

u Availablesystemsareavailabletodoworku Availabledoesnotimplyreliable

v Systemcanbeavailablebutnotreliable(systemhasbugs,generateswrongresults)

v Systemcanbereliablebutnotavailableu Crashevery5minutes,butsavesresultsandrestarts5minuteslater

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Privacy, Security, Isolation

u ForanOSsecuritymeanstheOSdoesnotrununintendedcodeorgetintoacompromisedstatev Novirus/malware

u OSprivacymeansprogramsshouldnotgetaccesstodatatheyshouldnothavev Passwordkeychains,filesinotherusersdirectories

u SecurityandPrivacyrequiresometradeoffswithperformance,whichiswhyOS’sarenot100%securev Somearebetterthanothers!

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Portability

u Manymachinetypesexist:x86,x86_64,PPC,ARM,MIPSu Manydifferentmotherboardsorhardwareplatformsexist:serverwith8CPUs12PCIe slotstoRaspberryPi,toAppleTV,etc.

u OSwithgoodportabilityabstractsthesedifferencesintoastableAPIsoprogrammersdon’tnotice

u Also,cantheOSitselfbeportedtonewhardwareeasily?

u Goodportabilityleadstowideadoptionv Linux,Windows

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Performance

u Whatdoesperformancemean?v Lotsofcomputation?v FluidGUIforgame?v Lowlatencydiskfordatabase?

u OSbalancesthesewithpoliciesv Majoraxisisthroughputvs.responsetimev DifferentOS’saretunedbasedonusecasev DBserverhasdifferentpoliciesthanWindowsgamingrig

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Examples of Policies

uTasksaregivenpriorities;higherprioritytasksarehandledfirst

uSomekindoftasksareneverinterrupteduAlltasksareequalpriority;round-robinuSometaskscanonlyusepartofadiskuSometaskscanusenetwork

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The kernel

uThekernelisthecoreofanOSuKernelcoordinatesotherprogramsuWhenthecomputerstartsupthekerneliscopiedfromthedisktothememory

uKernelrunsuntilsomeotherprogramneedstousetheCPU

uKernelpausesitselftorunotherprogram21

How memory is used

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FreeMemory

FreeMemory

Kernel

FreeMemory

ExecutingProgram

Kernel

FreeMemory

Kernel

Start Kernelloaded

Userprogramruns

Userprogramdone

Multitasking

uOneprogramusestheCPUatatimeuOSswitchesCPUusage(rapidly)uCreatesanillusionthatalltheprogramsarerunningatthesametime

uChangeoverfromoneprogramtoanotheriscalledacontextswitch

uExamplesofcontextswitching?uCancontextswitchingbegoodforaprogram?uCancontextswitchingbegoodforaCPU?

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Abstractions: Processes and Resources

uResourcesv Space(memory)v Time(CPU)v Peripherals(printersetc.)

uProcess:anexecutingprogramv Programcounterv Contentsofregistersv Allocatedmemory&contents

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uOSdoesn’tworryaboutwhateachprogramdoes

u InsteadOScaresaboutv Whatresourcesdoesaprocessneed?

v Howlongwillitrun?v Howimportantisit?

Protection/Isolation

uOtherprocesseshavetobepreventedfromwritingtothememoryusedbythekernel

uCrashinoneprogramshouldn’tcrashOSorotherprograms

uOShasaccesstoallresources:privilegedmodeuUserprogramshaverestrictedaccess:usermodeuWhenauserprogramneedsaccesstoprotectedresourcesitmakesasystemcall(e.g.,managingfiles,accessingaprinter)

uPrincipleofleastprivilege (kernelhashighestprivilege)

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Competing for time

uThinkofthetimetheCPUspendsinchunksorblocks

uHowcanblocksoftimebeallocatedtodifferentprocessessothatworkcanbedoneefficiently?

uPolicy:rulestoenforceprocessprioritization

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Process Scheduling Policies

uTheprocessqueueuRound-robinuFirst-come,first-serveduPriority-based

v Presetpriorityforeachprocessv Shortest-remaining-time

uAllthesepolicieskeeptheCPUbusyuArethereotherwaystojudgeapolicy?

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Keeping CPU busy

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How to evaluate a policy?

CPUutilization Throughput Latency Fairness

Round-robin Good Variable Potentiallyhigh

YesNostarvation

First-comefirst-served

Good Variable Yes

Shortestremainingtime

Good High Potentiallyhigh

NoCouldhavestarvation

Fixedpriority Good

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u Utilization:howmuchworktheCPUdoes

u Throughput:#ofprocessesthatusetheCPUinacertaintime

u Latency:averageamountoftimethatprocesseshavetowaitbeforerunning

u Fairness:everyprocessgetsachancetousetheCPU

Everyday policies

uPlanestakingoff:firstcomefirstservedv Highefficiencyfortherunwayv Ifseveralsmallerplanesinlinebeforealargeone,notefficientfortheaveragepassenger

uTrafficbeingdirectedataccident:roundrobinv Firsttrafficinonedirection,thenanotherv Ifapolicecararrives,thenswitchtopriority-basedv Unlikelytoeverbeshortestremainingtime

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First-come, first serve (non-pre-emptive)

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Round Robin (pre-emptive)

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Weighted Round Robin

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Shortest-time Remaining

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Competing for space

uUnliketime,spacecanbereused

uWhenaprocessisrunning,itisallocatedasingleregionofmemory

uWhenaprocessfinishesrunning,thememoryallocatedtoitisgivenbacktotheOS

u Fragmentation35

Kernel

A

B

CD

E

Kernel

A

D

Dealing with fragmentation

uMovememoryallocationaroundallthetimeinthebackgroundsowhenanewprocessneedsmemory,thereisalargeenoughcontiguousblocktoallocate

u Indirectionv Physicalmemoryordering:memorydividedintofixedsizeblockscalledframes

v Logicalmemoryordering:inlogicalmemoryeachframecorrespondstoapage(arenumberedframe)

v Whenaprocessusesmemoryassignedtoit,ituseslogicaladdresses;theOStranslateslogicalmemorylocationstophysicalmemorylocations

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Virtual memory

uUseindirectiontopretendRAMisbiggerthanitisuDemandpaging

v Whenaprocessstarts,theOSdoesn’tallocateitallthememoryitneeds

v Insteadonlyallocatememoryneededtodoimmediateworkv Keeprestoftherequestedmemoryinsecondarystorage(disk)v Ifapageinmemoryisn’tusedforawhile,it’smovedtodiskv Whenapageisneededbyaprocess,it’scopiedfromdisktoRAM

uOtherexamplesofvirtualization:virtualenvironments(renderondemand),half-courtbasketball,etc.

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The need for an OS

38

CPU,Memory,Disk,I/OProgram

Problem

Solution:Algorithm+DataStructures

Pseudocode

Low-levelinstructions

Executeson

Compilesto

The OS as a executive manager

39

CPU,Memory,Disk,I/OProgram

Problems

Solution:Algorithms +DataStructures

Pseudocode

Low-levelinstructions

Executionsmanagedby

Compileto

Low-levelinstructionsLow-levelinstructions

ProgramProgramProgram

OperatingSystem

Quiz #4

40

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