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1、天津大学2007届本科生毕业设计（论文）外文资料ABSTRACTCrawlingthewebisdeceptivelysimple:thebasicalgorithmis(a)Fetchapage(b)ParseittoextractalllinkedURLs(c)ForalltheURLsnotseenbefore,repeat(a)–(c).However,thesizeoftheweb(estimatedatover4billionpages)anditsrateofchange(estimatedat7%perweek)movethisplanfrom
2、atrivialprogrammingexercisetoaseriousalgorithmicandsystemdesignchallenge.Indeed,thesetwofactorsaloneimplythatforareasonablyfreshandcompletecrawloftheweb,step(a)mustbeexecutedaboutathousandtimespersecond,andthusthemembershiptest(c)mustbedonewellovertenthousandtimespersecondagainstase
3、ttoolargetostoreinmainmemory.Thisrequiresadistributedarchitecture,whichfurthercomplicatesthemembershiptest.Acrucialwaytospeedupthetestistocache,thatis,tostoreinmainmemorya(dynamic)subsetofthe“seen”URLs.ThemaingoalofthispaperistocarefullyinvestigateseveralURLcachingtechniquesforwebcr
4、awling.Weconsiderbothpracticalalgorithms:randomreplacement,staticcache,LRU,andCLOCK,andtheoreticallimits:clairvoyantcachingandinfinitecache.Weperformedabout1,800simulationsusingthesealgorithmswithvariouscachesizes,usingactuallogdataextractedfromamassive33daywebcrawlthatissuedoverone
5、billionHTTPrequests.Ourmainconclusionisthatcachingisveryeffective–inoursetup,acacheofroughly50,000entriescanachieveahitrateofalmost80%.Interestingly,thiscachesizefallsatacriticalpoint:asubstantiallysmallercacheismuchlesseffectivewhileasubstantiallylargercachebringslittleadditionalbe
6、nefit.Weconjecturethatsuchcriticalpointsareinherenttoourproblemandventureanexplanationforthisphenomenon.1.INTRODUCTIONArecentPewFoundationstudy[31]statesthat“SearchengineshavebecomeanindispensableutilityforInternetusers”andestimatesthatasofmid-2002,slightly20天津大学2007届本科生毕业设计（论文）over
7、50%ofallAmericanshaveusedwebsearchtofindinformation.Hence,thetechnologythatpowerswebsearchisofenormouspracticalinterest.Inthispaper,weconcentrateononeaspectofthesearchtechnology,namelytheprocessofcollectingwebpagesthateventuallyconstitutethesearchenginecorpus.Searchenginescollectpag
8、esinmanyways,amongt
