计算机科学与技术 外文翻译 外文文献 英文文献 提高字符串处理性能的应用程序Word下载.docx

计算机科学与技术 外文翻译 外文文献 英文文献 提高字符串处理性能的应用程序Word下载.docx

《计算机科学与技术 外文翻译 外文文献 英文文献 提高字符串处理性能的应用程序Word下载.docx》由会员分享，可在线阅读，更多相关《计算机科学与技术 外文翻译 外文文献 英文文献 提高字符串处理性能的应用程序Word下载.docx（28页珍藏版）》请在冰点文库上搜索。

Summary:

MostActiveServerPages（ASP）applicationsrelyonstringconcatenationtobuildHTML-formatteddatathatisthenpresentedtousers.ThisarticlecontainsacomparisonofseveralwaystocreatethisHTMLdatastream,someofwhichprovidebetterperformancethanothersforagivensituation.AreasonableknowledgeofASPandVisualBasicprogrammingisassumed.（11printedpages）

Contents

Introduction

ASPDesign

StringConcatenation

TheQuickandEasySolution

TheStringBuilder

TheBuilt-inMethod

Testing

Results

Conclusion

WhenwritingASPpages,thedeveloperisreallyjustcreatingastreamofformattedtextthatiswrittentotheWebclientviatheResponseobjectprovidedbyASP.YoucanbuildthistextstreaminseveraldifferentwaysandthemethodyouchoosecanhavealargeimpactonboththeperformanceandthescalabilityoftheWebapplication.OnnumerousoccasionsinwhichIhavehelpedcustomerswithperformance-tuningtheirWebapplications,IhavefoundthatoneofthemajorwinshascomefromchangingthewaythattheHTMLstreamiscreated.InthisarticleIwillshowafewofthecommontechniquesandtestwhateffecttheyhaveontheperformanceofasimpleASPpage.

ManyASPdevelopershavefollowedgoodsoftwareengineeringprinciplesandmodularizedtheircodewhereverpossible.Thisdesignnormallytakestheformofanumberofincludefilesthatcontainfunctionsmodelingparticulardiscretesectionsofapage.Thestringoutputsfromthesefunctions,usuallyHTMLtablecode,canthenbeusedinvariouscombinationstobuildacompletepage.SomedevelopershavetakenthisastagefurtherandmovedtheseHTMLfunctionsintoVisualBasicCOMcomponents,hopingtobenefitfromtheextraperformancethatcompiledcodecanoffer.

Althoughthisiscertainlyagooddesignpractice,themethodusedtobuildthestringsthatformthesediscreteHTMLcodecomponentscanhavealargebearingonhowwelltheWebsiteperformsandscales—regardlessofwhethertheactualoperationisperformedfromwithinanASPincludefileoraVisualBasicCOMcomponent.

ConsiderthefollowingcodefragmenttakenfromafunctioncalledWriteHTML.TheparameternamedDataissimplyanarrayofstringscontainingsomedatathatneedstobeformattedintoatablestructure（datareturnedfromadatabase,forinstance）.

CopyCode

FunctionWriteHTML（Data）

DimnRep

FornRep=0to99

sHTML=sHTML&

vbcrlf_

&

"

<

TR>

TD>

"

（nRep+1）&

/TD>

_

Data（0,nRep）&

Data（1,nRep）&

Data（2,nRep）&

Data（3,nRep）&

Data（4,nRep）&

Data（5,nRep）&

/TR>

Next

WriteHTML=sHTML

EndFunction

ThisistypicalofhowmanyASPandVisualBasicdevelopersbuildHTMLcode.ThetextcontainedinthesHTMLvariableisreturnedtothecallingcodeandthenwrittentotheclientusingResponse.Write.Ofcourse,thiscouldalsobeexpressedassimilarcodeembeddeddirectlywithinthepagewithouttheindirectionoftheWriteHTMLfunction.TheproblemwiththiscodeliesinthefactthatthestringdatatypeusedbyASPandVisualBasic,theBSTRorBasicString,cannotactuallychangelength.Thismeansthateverytimethelengthofthestringischanged,theoriginalrepresentationofthestringinmemoryisdestroyed,andanewoneiscreatedcontainingthenewstringdata:

thisresultsinamemoryallocationoperationandamemoryde-allocationoperation.Ofcourse,inASPandVisualBasicthisisalltakencareofforyou,sothetruecostisnotimmediatelyapparent.Allocatingandde-allocatingmemoryrequirestheunderlyingruntimecodetotakeoutexclusivelocksandthereforecanbeexpensive.Thisisespeciallyapparentwhenstringsgetbigandlargeblocksofmemoryarebeingallocatedandde-allocatedinquicksuccession,ashappensduringheavystringconcatenation.Whilethismaypresentnomajorproblemsinasingleuserenvironment,itcancauseseriousperformanceandscalabilityissueswhenusedinaserverenvironmentsuchasinanASPapplicationrunningonaWebserver.

Sobacktothecodefragmentabove:

howmanystringallocationsarebeingperformedhere?

Infacttheansweris16.Inthissituationeveryapplicationofthe'

&

'

operatorcausesthestringpointedtobythevariablesHTMLtobedestroyedandrecreated.Ihavealreadymentionedthatstringallocationisexpensive,becomingincreasinglymoresoasthestringgrows;

armedwiththisknowledge,wecanimproveuponthecodeabove.

Therearetwowaystomitigatetheeffectofstringconcatenations,thefirstistotryanddecreasethesizeofthestringsbeingmanipulatedandthesecondistotryandreducethenumberofstringallocationoperationsbeingperformed.LookattherevisedversionoftheWriteHTMLcodeshownbelow.

（vbcrlf_

）

Atfirstglanceitmaybedifficulttospotthedifferencebetweenthispieceofcodeandtheprevioussample.ThisonesimplyhastheadditionofparenthesesaroundeverythingaftersHTML=sHTML&

.Thisactuallyreducesthesizeofstringsbeingmanipulatedinmostofthestringconcatenationoperationsbychangingtheorderofprecedence.IntheoriginalcodesampletheASPcomplierwilllookattheexpressiontotherightoftheequalssignandjustevaluateitfromlefttoright.Thisresultsin16concatenationoperationsperiterationinvolvingsHTMLwhichisgrowingallthetime.Inthenewversionwearegivingthecompilerahintbychangingtheorderinwhichitshouldcarryouttheoperations.Nowitwillevaluatetheexpressionfromlefttorightbutalsoinsideout,i.e.insidetheparenthesesfirst.Thistechniqueresultsin15concatenationoperationsperiterationinvolvingsmallerstringswhicharenotgrowingandonlyonewiththelarge,andgrowing,sHTML.Figure1showsanimpressionofthememoryusagepatternsofthisoptimizationagainstthestandardconcatenationmethod.

Figure1 

Comparisonofmemoryusagepatternbetweenstandardandparenthesizedconcatenation

Usingparenthesescanmakequiteamarkeddifferenceinperformanceandscalabilityincertaincircumstances,asIwilldemonstratelaterinthisarticle.

Wehaveseenthequickandeasysolutiontothestringconcatenationproblem,andformanysituationsthismayprovidethebesttrade-offbetweenperformanceandefforttoimplement.Ifwewanttogetseriousaboutimprovingtheperformanceofbuildinglargestrings,however,thenweneedtotakethesecondalternative,whichistocutdownthenumberofstringallocationoperations.InordertoachievethisaStringBuilderisrequired.Thisisaclassthatmaintainsaconfigurablestringbufferandmanagesinsertionsofnewpiecesoftextintothatbuffer,causingstringreallocationonlywhenthelengthofthetextexceedsthelengthofthestringbuffer.TheMicrosoft.NETframeworkprovidessuchaclassforfree（System.Text.StringBuilder）thatisrecommendedforallstringconcatenationoperationsinthatenvironment.IntheASPandclassicVisualBasicworldwedonothaveaccesstothisclass,soweneedtobuildourown.BelowisasampleStringBuilderclasscreatedusingVisualBasic6.0（error-handlingcodehasbeenomittedintheinterestofbrevity）.

OptionExplicit

defaultinitialsizeofbufferandgrowthfactor

PrivateConstDEF_INITIALSIZEAsLong=1000

PrivateConstDEF_GROWTHAsLong=1000

buffersizeandgrowth

Privatem_nInitialSizeAsLong

Privatem_nGrowthAsLong

bufferandbuffercounters

Privatem_sTextAsString

Privatem_nSizeAsLong

Privatem_nPosAsLong

PrivateSubClass_Initialize（）

'

setdefaultsforsizeandgrowth

m_nInitialSize=DEF_INITIALSIZE

m_nGrowth=DEF_GROWTH

initializebuffer

InitBuffer

EndSub

settheinitialsizeandgrowthamount

PublicSubInit（ByValInitialSizeAsLong,ByValGrowthAsLong）

IfInitialSize>

0Thenm_nInitialSize=InitialSize

IfGrowth>

0Thenm_nGrowth=Growth

initializethebuffer

PrivateSubInitBuffer（）

m_nSize=-1

m_nPos=1

growthebuffer

PrivateSubGrow（OptionalMinimimGrowthAsLong）

initializebufferifnecessary

Ifm_nSize=-1Then

m_nSize=m_nInitialSize

m_sText=Space$（m_nInitialSize）

Else

justgrow

DimnGrowthAsLong

nGrowth=IIf（m_nGrowth>

MinimimGrowth,

m_nGrowth,MinimimGrowth）

m_nSize=m_nSize+nGrowth

m_sText=m_sText&

Space$（nGrowth）

EndIf

trimthebuffertothecurrentlyusedsize

PrivateSubShrink（）

Ifm_nSize>

m_nPosThen

m_nSize=m_nPos-1

m_sText=RTrim$（m_sText）

addasingletextstring

PrivateSubAppendInternal（ByValTextAsString）

If（m_nPos+Len（Text））>

m_nSizeThenGrowLen（Text）

Mid$（m_sText,m_nPos,Len（Text））=Text

m_nPos=m_nPos+Len（Text）

addanumberoftextstrings

PublicSubAppend（ParamArrayText（））

DimnArgAsLong

FornArg=0ToUBound