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首页Principles of Computer System Design - An Introduction (Part I)
Principles of Computer System Design - An Introduction (Part I)
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《计算机系统设计原理》是第一本阐述计算机系统设计中的基本原理和抽象的教材,是麻省理工开放式课程计划(MIT Open Courseware)中“计算机系统工程”课程的主教材。计算机系统的基本原理横跨于操作系统、网络、数据库、分布式系统、程序设计语言、软件工程以及计算机体系结构等方面。通过详细分析每个基本原理的案例,《计算机系统设计原理》演示了如何应用这些原理和抽象来解决实际的计算机系统设计问题。
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1
CHAPTER
Systems
Overview ....................................................................................................................2
1.1 Systems and Complexity ................................................................................................ 3
1.1.1 Common Problems of Systems in Many Fields ...........................................3
1.1.2 Systems, Components, Interfaces, and Environments .................................8
1.1.3 Complexity ..........................................................................................10
1.2 Sources of Complexity .......................................................................................13
1.2.1 Cascading and Interacting Requirements ................................................13
1.2.2 Maintaining High Utilization ..................................................................17
1.3 Coping with Complexity I ...................................................................................19
1.3.1 Modularity ...........................................................................................19
1.3.2 Abstraction ..........................................................................................20
1.3.3 Layering ..............................................................................................24
1.3.4 Hierarchy ............................................................................................25
1.3.5 Putting it Back Together: Names make Connections .................................26
1.4 Computer Systems are the Same but Different .....................................................27
1.4.1 Computer Systems have no Nearby Bounds on Composition ......................28
1.4.2 d(technology)/dt is Unprecedented .........................................................31
1.5 Coping With Complexity II .................................................................................35
1.5.1 Why Modularity, Abstraction, Layering, and Hierarchy aren’t Enough .........36
1.5.2 Iteration ..............................................................................................36
1.5.3 Keep it Simple .....................................................................................39
What The Rest of this Book is About ...........................................................................40
Exercises .................................................................................................................41
CHAPTER CONTENTS
Principles of Computer System Design: An Introduction
Copyright © 2009 by Jerome H. Saltzer and M. Frans Kaashoek. All rights of reproduction in any form reserved.
DOI: 10.1016/B978-0-12-374957-4.00009-8
2 CHAPTER 1 Systems
OVERVIEW
4HIS BOOK IS ABOUT COMPUTER SYSTEMS AND THIS CHAPTER INTRODUCES SOME OF THE
VOCABULARYANDCONCEPTSUSEDINDESIGNINGCOMPUTERSYSTEMS)TALSOINTRODUCEShSYS-
TEMSPERSPECTIVEvAWAYOFTHINKINGABOUTSYSTEMSTHATISGLOBALANDENCOMPASSING
RATHERTHANFOCUSEDONPARTICULARISSUES!FULLAPPRECIATIONOFTHISWAYOFTHINKING
CANTREALLYBECAPTUREDINASHORTSUMMARYSOTHISCHAPTERISACTUALLYJUSTAPREVIEW
OFIDEASTHATWILLBEDEVELOPEDINDEPTHINSUCCEEDINGCHAPTERS
4HEUSUALCOURSEOFSTUDYOFCOMPUTERSCIENCEANDENGINEERINGBEGINSWITHLIN-
GUISTICCONSTRUCTSFORDESCRIBINGCOMPUTATIONSSOFTWAREANDPHYSICALCONSTRUCTSFOR
REALIZINGCOMPUTATIONS HARDWARE )TTHEN BRANCHES FOCUSING FOREXAMPLE ON THE
THEORYOFCOMPUTATIONARTIlCIALINTELLIGENCEORTHEDESIGNOFSYSTEMSWHICHITSELFIS
USUALLYDIVIDEDINTOSPECIALITIESOPERATINGSYSTEMSTRANSACTIONANDDATABASESYSTEMS
COMPUTERARCHITECTURESOFTWAREENGINEERINGCOMPILERSCOMPUTERNETWORKSSECURITY
ANDRELIABILITY2ATHERTHANIMMEDIATELYTACKLINGONEOFTHOSESPECIALTIESWEASSUME
THATTHEREADERHASCOMPLETEDTHEINTRODUCTORYCOURSESONSOFTWAREANDHARDWARE
ANDWEBEGINABROADSTUDYOFCOMPUTERSYSTEMSTHATSUPPORTSTHEENTIRERANGEOF
SYSTEMSSPECIALTIES
-ANYINTERESTINGAPPLICATIONSOFCOMPUTERSREQUIRE
FAULTTOLERANCE N
COORDINATIONOFCONCURRENTACTIVITIES N
GEOGRAPHICALLYSEPARATEDBUTLINKEDDATA N
VASTQUANTITIESOFSTOREDINFORMATION N
PROTECTIONFROMMISTAKESANDINTENTIONALATTACKS N
INTERACTIONSWITHMANYPEOPLE N
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IMPOSSIBILITYBUTRATHERFROMLIMITATIONSOFHUMANUNDERSTANDING
3OMEOFTHESESAMEPROBLEMSHAVECOUNTERPARTSORATLEASTANALOGSINOTHERSYSTEMS
THATHAVEATMOSTONLYINCIDENTALINVOLVEMENTOFCOMPUTERS4HESTUDYOFSYSTEMSIS
ONEPLACEWHERECOMPUTERENGINEERINGCANTAKEADVANTAGEOFKNOWLEDGEFROMOTHER
ENGINEERING AREASCIVILENGINEERINGBRIDGESAND SKYSCRAPERS URBAN PLANNINGTHE
Much wisdom about systems that has accumulated over the centuries is passed along
in the form of folklore, maxims, aphorisms and quotations. Some of that wisdom is
captured in the boxes at the bottom of these pages.
Everything should be made as simple as possible, but no simpler.
— commonly attributed to Albert Einstein; it is actually a paraphrase of a comment
he made in a 1933 lecture at Oxford.
3
1.1 Systems and Complexity
DESIGNOFCITIESMECHANICALENGINEERINGAUTOMOBILESANDAIRCONDITIONINGAVIATION
ANDSPACEmIGHTELECTRICALENGINEERINGANDEVENECOLOGYANDPOLITICALSCIENCE7E
START BY LOOKING AT SOME OF THOSE COMMON PROBLEMS 4HEN WE WILL EXAMINE TWO
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ANDSIMILARITIESACROSSDIFFERENTKINDSOFSYSTEMS
!SWEPROCEEDINTHISCHAPTERANDTHROUGHOUTTHEBOOKWESHALLPOINTOUTASERIES
of system design principlesWHICHARERULESOFTHUMBTHATUSUALLYAPPLYTOADIVERSE
RANGEOFSITUATIONS$ESIGNPRINCIPLESARENOTIMMUTABLELAWSBUTRATHERGUIDELINES
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ATIONCAREFULLY
!TTHElRSTENCOUNTEROFADESIGNPRINCIPLETHETEXTDISPLAYSITPROMINENTLY(ERE
ISANEXAMPLEFOUNDONPAGE
Avoid excessive generality
If it’s good for everything, it’s good for nothing.
%ACHDESIGNPRINCIPLETHUSHASAFORMALTITLEh!VOIDEXCESSIVEGENERALITYvANDABRIEF
INFORMALDESCRIPTIONh)FITSGOODFORvWHICHAREINTENDEDTOHELPRECALLTHEPRIN-
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1.1 SYSTEMS AND COMPLEXITY
1.1.1 Common Problems of Systems in Many Fields
4HEPROBLEMSONEENCOUNTERSINTHESEMANYKINDSOFSYSTEMSCANUSEFULLYBEDIVIDED
INTOFOURCATEGORIESemergent properties, propagation of effects, incommensurate
scalingANDtrade-offs
Seek simplicity and distrust it.
— Alfred North Whitehead, The Concept of Nature (1920)
4 CHAPTER 1 Systems
1.1.1.1 Emergent Properties
Emergent propertiesAREPROPERTIESTHATARENOTEVIDENTINTHEINDIVIDUALCOMPONENTSOFA
SYSTEMBUTSHOWUPWHENCOMBININGTHOSECOMPONENTSSOTHEYMIGHTALSOBECALLEDSUR-
PRISES%MERGENTPROPERTIESABOUNDINMOSTSYSTEMSALTHOUGHTHERECANALWAYSBEAFRUIT-
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FOCUSONANUNALTERABLEFACTOFLIFESOMETHINGSTURNUPONLYWHENASYSTEMISBUILT
3OMEEXAMPLESOFEMERGENTPROPERTIESAREWELLKNOWN4HEBEHAVIOROFACOMMITTEE
ORAJURYOFTENSURPRISESOUTSIDEOBSERVERS4HEGROUPDEVELOPSAWAYOFTHINKINGTHAT
COULDNOTHAVEBEENPREDICTEDFROMKNOWLEDGEABOUTTHEINDIVIDUALS4HECONCEPTOF
ANDTHELABELFOREMERGENTPROPERTIESORIGINATEDINSOCIOLOGY7HENTHE-ILLENNIUM
"RIDGEFORPEDESTRIANSOVERTHE2IVER4HAMESIN,ONDONOPENEDITSDESIGNERSHADTOCLOSE
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THEIRFOOTSTEPSWHENTHEBRIDGESWAYSCAUSINGITTOSWAYEVENMORE)NTERCONNECTIONOF
SEVERALELECTRICPOWERCOMPANIESTOALLOWLOADSHARINGHELPSREDUCETHEFREQUENCYOF
POWERFAILURESBUTWHENAFAILURElNALLYOCCURSITMAYTAKEDOWNTHEENTIREINTERCON-
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LARGEENOUGHTOATTRACTTHEUNWANTEDATTENTIONOFGOVERNMENTREGULATORS
1.1.1.2 Propagation of Effects
4HEELECTRICPOWERINTERTIEALSOILLUSTRATESTHESECONDCATEGORYOFSYSTEMPROBLEMS
propagation of effectsWHENATREEFALLINGONAPOWERLINEIN/REGONLEADSTOTHE
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DECISIONTOCHANGETHETIRESIZEONAPRODUCTIONMODELCARFROMTOINCHES4HE
REASONFORMAKINGTHECHANGEMIGHTHAVEBEENTOIMPROVETHERIDE/NFURTHERANALY-
SISTHISCHANGELEADSTOMANYOTHERCHANGESREDESIGNINGTHEWHEELWELLSENLARGING
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Our life is frittered away by detail . . . simplicity, simplicity, simplicity!
— Henry David Thoreau, Walden; or, Life in the Woods (1854)
5
1.1 Systems and Complexity
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By undue profundity we perplex and enfeeble thought.
— Edgar Allan Poe, “The Murders in the Rue Morgue” (1841)
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