【移动应用开发技术】Using Memory Efficiently(Pro Android Apps Performance Optimization)

【移动应用开发技术】UsingMemoryEfficiently(ProAndroidAppsPerformanceOptimization)

UsingMaoemoryEfficiently

高效使用内存4.1AWordOnMemory4.2DataTypes4.2.1ComparingValues值的比较4.2.2OtherAlgorithms4.2.3SortingArrays4.2.4DefiningYourOwnClasses4.3AccessingMemory访问内存4.4

LayingOutYourData排布数据4.5

GarbageCollection垃圾收集4.51MemoryLeaks4.5.2References4.6APIs4.7LowMemory内存少的时候

Applicationsspendasignificantpartoftheirtimedealingwithdatainmemory(应用生命期的绝大部分时间都是用来处理内存中的数据).Whilemanydevelopersareawareoftheneedtotrytouseaslittlememoryaspossibleondeviceslikephonesortablets,notallrealizetheimpactofmemoryusageonperformance(并不是所有人都意识到内存对性能的影响）.Inthischapter,youwilllearnhowchoosingtherightdatatypeandhowarrangingyourdatainmemory如何在内存中排布数据canboostyourapplication’sperformance.Also,wewillreviewabasicyet虽然基本还是oftenoverlookedfeatureofJava:memorymanagementusing.garbagecollectionandreferences垃圾回收和内存管理.4.1AWordOnMemory

Nomatterhowmuchanapplicationisgiven,itcouldalwaysaskformore.TherearetwobigdifferencesbetweenanAndroiddevice,likeaphoneortablet,andatraditionalcomputer:Theamountofphysicalmemory物理内存的大小Theabilitytodovirtualmemoryswapping虚拟内存交换的能力

Typically通常情况下,today’scomputerscomeinstalledwithseveralgigabytesofRAM(fewcomeinstalledwithonly1GBorless),howeverAndroiddevicesoftenhaveamaximumof512MBofRAM.Toaddinsulttoinjury雪上加霜的是,computersuseswapping内存交换能力,whichAndroiddevicesdon’thave,togivethesystemandapplicationstheillusionofmorememory内存可用的假象.Forexample,anapplicationcouldstilladdressup寻址to4GBofmemoryevenwithasystemthathasonly256MBofRAM.YourAndroidapplicationsimplydoesnothavethisluxury特遇,andthereforeyouhavetobemorecarefulabouthowmuchmemoryyourapplicationuses必须小心内存的使用量.NOTE:YoucanfindapplicationsonAndroidMarketthatenableswapping,buttheyrequirerootaccessandadifferentLinuxkernel内核.AssumetheAndroiddevicesyourapplicationswillberunningondonothaveswappingcapabilities假定运行在不具备内存交换功能的设备上.

TheJavalanguagedefinesthesizeofmostprimitivetypes,asshowninTable4–1,togetherwiththeirmatchingnativetypes.IfyouareprimarilyfamiliarwithC/C++code,thenyouneedtopayextraattentiontotwothings:Java’scharis16-bit(UTF-16).Java’slongis64–bit(whileClongisusually32-bitandClonglongisusually64–bit).Table4–1.JavaPrimitiveTypesJavaprimitivetypeNativetypeSizebooleanjboolean8bits(VMdependent)bytejbyte8bitscharjchar16bitsshortjshort16bitsintjint32bitslongjlong64bitsfloatjfloat32bitsdoublejdouble64bits

Agoodruleofthumb翻阅istouseaslittlememoryaspossible,whichisjustcommonsenseasyourAndroiddeviceandapplicationshavealimitedamountofmemory.ButinadditiontoreducingtheriskofanOutOfMemoryErrorexception,usinglessmemorycanalsoincreaseperformance.

Performancedependsmostlyonthreethings:

HowtheCPUcanmanipulateacertaindatatype

Howmuchspaceisneededtostoredataandinstructions

Howdataislaidoutinmemory数据在内存中的布局

Wewillcovereachone逐一讲解oftheseitems,lookingatbothnativecodeandJavacode,measuringexecutiontimes,andcomparingseveralimplementationsofsimplealgorithmstoimproveperformance.4.2DataTypes

Youalreadygotanaperuofthefirstpoint,howtheCPUcanmanipulateacertaindatatype,inChapter2withthenativecodeversionofcomputeIterativelyFaster(),whichusedtwoaddinstructionstoaddtwo64–bitintegers,asshowninListing4–1.Listing4–1.AddingTwo64–bitIntegers448:e0944002addsr4,r4,r244c:e0a55003adcr5,r5,r3

BecausetheARMregistersare32-bitwide,twoinstructionsareneededtoaddtwo64–bitintegers;thelowest32-bitintegersarestoredinoneregister(r4),andthehighest32-bitarestoredinanotherregister(r5).Addingtwo32-bitvalueswouldrequireasingleinstruction.

Let’snowconsiderthetrivial简单Cfunction,showninListing4–2,whichsimplyreturnsthesumoftwo32-bitvaluespassedasparameters.Listing4–2.SumofTwoValuesint32_tadd_32_32(int32_tvalue1,int32_tvalue2){returnvalue1+value2;}TheassemblycodeforthisfunctionisshowninListing4–3.Listing4–3.AssemblyCode000016c8<add_32_32>:16c8:e0810000addr0,r1,r016cc:e12fff1ebxlr

Asexpected,onlyoneinstructionisneededtoaddthetwovalues(andbxlristheequivalentofreturn).Wecannowcreatenewfunctionsthatareverymuchlikeadd_32_32,butwithdifferenttypesforvalue1andvalue2.Forexample,add_16_16addstwoint16_tvalues,asshowninListing4–4,andadd_16_32addsanint16_tvalueandanint32_tvalue,asshowninListing4–5.Listing4–4.add_16_16’sCandAssemblyint16_tadd_16_16(int16_tvalue1,int16_tvalue2){returnvalue1+value2;}000016d0<add_16_16>:16d0:e0810000addr0,r1,r016d4:e6bf0070sxthr0,r016d8:e12fff1ebxlrListing4–5.add_16_32’sCAndAssemblyint32_tadd_16_32(int16_tvalue1,int32_tvalue2){returnvalue1+value2;}000016dc<add_16_32>:16dc:e0810000addr0,r1,r016e0:e12fff1ebxlr

Youcanseethataddingtwo16-valuesrequiredanadditionalinstructioninordertoconvert转换theresultfrom16-bitto32-bit.

Listing4–6showsfivemorefunctions,repeatingbasicallythesamealgorithmbutfor

differentdatatypes.Listing4–6.MoreAssemblyCode000016e4<add_32_64>:16e4:e0922000addsr2,r2,r016e8:e0a33fc0adcr3,r3,r0,asr#3116ec:e1a00002movr0,r216f0:e1a01003movr1,r316f4:e12fff1ebxlr000016f8<add_32_float>:16f8:ee070a10fmsrs14,r016fc:eef87ac7fsitoss15,s141700:ee071a10fmsrs14,r11704:ee777a87faddss15,s15,s141708:eefd7ae7ftosizss15,s15170c:ee170a90fmrsr0,s151710:e12fff1ebxlr00001714<add_float_float>:1714:ee070a10fmsrs14,r01718:ee071a90fmsrs15,r1171c:ee377a27faddss14,s14,s151720:ee170a10fmrsr0,s141724:e12fff1ebxlr00001728<add_double_double>:1728:ec410b16vmovd6,r0,r1172c:ec432b17vmovd7,r2,r31730:ee366b07fadddd6,d6,d71734:ec510b16vmovr0,r1,d61738:e12fff1ebxlr0000173c<add_float_double>:173c:ee060a10fmsrs12,r01740:eeb77ac6fcvtdsd7,s121744:ec432b16vmovd6,r2,r31748:ee376b06fadddd6,d7,d6174c:ec510b16vmovr0,r1,d61750:e12fff1ebxlr

NOTE:Thegeneratednativecodemaydifferfromwhatisshownhereasalotdependsonthecontextoftheaddition加法的上下文而定.(Codethatisinlinemaylookdifferentasthecompilermayreorderinstructionsorchangetheregisterallocation.)

Asyoucansee,usingasmallertypeisnotalwaysbeneficialtoperformanceasitmayactuallyrequiremoreinstructions,asdemonstratedinListing4–4.Besides,ifadd_16_16werecalledwithtwo32-bitvaluesasparameters,thesetwovalueswouldfirsthavetobeconvertedto16-bitvaluesbeforetheactualcall,asshowninListing4–7.Onceagain,thesxthinstructionisusedtoconvertthe32-bitvaluesinto16-bitvaluesbyperforminga“signextend符号扩展”operation.Listing4–7.Callingadd_16_16WithTwo32-BitValues00001754<add_16_16_from_32_32>:1754:e6bf0070sxthr0,r01758:e6bf1071sxthr1,r1175c:eaffffdbb16d0<add_16_16>4.2.1ComparingValues值的比较

Let’snowconsideranotherbasicfunction,whichtakestwoparametersandreturns0or1dependingonwhetherthefirstparameterisgreaterthanthesecondone,asshowninListing4–8.Listing4–8.ComparingTwoValuesint32_tcmp_32_32(int32_tvalue1,int32_tvalue2){return(value1>value2)?1:0;}

Again,wecanseetheassemblycodeforthisfunctionanditsvariants变种inListing4–9.Listing4–9.ComparingTwoValuesInAssembly00001760<cmp_32_32>:1760:e1500001cmpr0,r11764:d3a00000movler0,#0;0x01768:c3a00001movgtr0,#1;0x1176c:e12fff1ebxlr00001770<cmp_16_16>:1770:e1500001cmpr0,r11774:d3a00000movler0,#0;0x01778:c3a00001movgtr0,#1;0x1177c:e12fff1ebxlr00001780<cmp_16_32>:1780:e1500001cmpr0,r11784:d3a00000movler0,#0;0x01788:c3a00001movgtr0,#1;0x1178c:e12fff1ebxlr00001790<cmp_32_64>:1790:e92d0030push{r4,r5}1794:e1a04000movr4,r01798:e1a05fc4asrr5,r4,#31179c:e1550003cmpr5,r317a0:e3a00000movr0,#0;0x017a4:ca000004bgt17bc<cmp_32_64+0x2c>17a8:0a000001beq17b4<cmp_32_64+0x24>17ac:e8bd0030pop{r4,r5}17b0:e12fff1ebxlr17b4:e1540002cmpr4,r217b8:9afffffbbls17ac<cmp_32_64+0x1c>17bc:e3a00001movr0,#1;0x117c0:eafffff9b17ac<cmp_32_64+0x1c>000017c4<cmp_32_float>:17c4:ee070a10fmsrs14,r017c8:eef87ac7fsitoss15,s1417cc:ee071a10fmsrs14,r117d0:eef47ac7fcmpess15,s1417d4:eef1fa10fmstat17d8:d3a00000movler0,#0;0x017dc:c3a00001movgtr0,#1;0x117e0:e12fff1ebxlr000017e4<cmp_float_float>:17e4:ee070a10fmsrs14,r017e8:ee071a90fmsrs15,r117ec:eeb47ae7fcmpess14,s1517f0:eef1fa10fmstat17f4:d3a00000movler0,#0;0x017f8:c3a00001movgtr0,#1;0x117fc:e12fff1ebxlr00001800<cmp_double_double>:1800:ee060a10fmsrs12,r01804:eeb77ac6fcvtdsd7,s121808:ec432b16vmovd6,r2,r3180c:eeb47bc6fcmpedd7,d61810:eef1fa10fmstat1814:d3a00000movler0,#0;0x01818:c3a00001movgtr0,#1;0x1181c:e12fff1ebxlr00001820<cmp_float_double>:1820:ee060a10fmsrs12,r01824:eeb77ac6fcvtdsd7,s121828:ec432b16vmovd6,r2,r3182c:eeb47bc6fcmpedd7,d61830:eef1fa10fmstat1834:d3a00000movler0,#0;0x01838:c3a00001movgtr0,#1;0x1183c:e12fff1ebxlr1840:e3a00001movr0,#1;0x11844:e12fff1ebxlr

Usingthelongtypeappearstobeslowerthanusingtheshortandinttypesbecauseofthehighernumberofinstructionshavingtobeexecuted.Similarly,usingthedoubletypeandmixingfloatanddoubleseemstobeslowerthanusingthefloattypealone.NOTE:Thenumberofinstructionsaloneisnotenoughtodeterminewhethercodewillbeslowerornot.Becausenotallinstructionstakethesameamountoftimetocomplete,andbecauseofthecomplexnatureoftoday’sCPUs,onecannotsimplycountthenumberofinstructionstoknowhowmuchtimeacertainoperationwilltake.4.2.OtherAlgorithms

Nowthatwehaveseenwhatdifferencedatatypescanmakeonthegeneratedcode,itistimetoseehowslightly轻微moresophisticatedalgorithmsperformwhendealingwithmoresignificantamountsofdata.

Listing4–10showsthreesimplemethods:onethatsortsanarraybysimplycallingthestaticArrays.sort()method,onethatfindstheminimumvalueinanarray,andonethataddsalltheelementsinanarray.Listing4–10.Sorting,Finding,andSumminginJavaprivatestaticvoidsort(intarray[]){Arrays.sort(array);}privatestaticintfindMin(intarray[]){intmin=Integer.MAX_VALUE;for(inte:array){if(e<min)min=e;}returnmin;}privatestaticintaddAll(intarray[]){intsum=0;for(inte:array){sum+=e;//thiscouldoverflowbutwe’llignorethat}returnsum;}

Table4–2showshowmanymillisecondseachofthesefunctionstooktocompletewhengivenanarrayofonemillionrandomelements.Inadditiontothese,theresultsforthevariantsofthesemethods(usingshort,long,float,anddoubletypes)arealsoshown.RefertoChapter6formoreinformationonhowtomeasureexecutiontimes.Table4–2.ExecutionTimesWith1,000,000-ElementArrayJavaprimitivetypesortfindMinaddAllshort932725int7533130long1,2405754float1,0804133double1,3585855Wecanmakeacoupleofcommentsontheseresults:Sortingtheshortarrayismuchfasterthansortinganyoftheotherarrays.(sort数组排序永远快于其他数组排序)Workingwith64–bittypes(longordouble)isslowerthanworkingwith32-bittypes.4.2.3SortingArrays

Sortinganarrayof16-bitvaluescanbemuchfasterthansortinganarrayof32-or64–bitvaluessimplybecauseitisusingadifferentalgorithm.WhiletheintandlongarraysaresortedusingsomeversionofQuicksortalgorithm,theshortarraywassortedusingcountingsort,whichsortsinlineartime.Usingtheshorttypeinthatcaseiskillingtwobirdswithonestone:lessmemoryisconsumed(2megabytesinsteadof4forthearrayofintvalues,and8megabytesforthearrayoflongvalues)andperformanceisimproved.NOTE:ManywronglybelieveQuicksortisalwaysthemostefficientsortingalgorithm.YoucanrefertoArrays.javaintheAndroidsourcecodetoseehoweachtypeofarrayissorted.

Alesscommonsolutionistousemultiplearraystostoreyourdata用多个数组储存数据.Forexample,ifyourapplicationneedstostoremanyintegersallintherange0to100,000,thenyoumaybetemptedtoallocateanarrayof32-bitvaluesforstorageastheshorttypecanbeusedonlytostorevaluesfrom-32,768to32,767.Dependingonhowvaluesaredistributed,

youmayendupwithmanyvaluesequaltoorlessthan32,767.Insuchanevent,itmaybebeneficialtousetwoarrays:oneforallthevaluesbetween0and32,767(thatis,anarrayofshortvalues),andoneforallvaluesgreaterthan32,767(anarrayofintvalues).Whilethismostlikelywouldresultinagreatercomplexity复杂,thememorysavingsandpotentialperformanceincreasemaymakethatsolutionworthwhile,andperhapsevenallowyoutooptimizetheimplementationofcertainmethods.Forexample,amethodthatfindsthemaximumelementmayonlyhavetogothroughthearrayofintvalues.(Onlyifthearrayofintvaluesisemptywoulditgothroughthearrayofshortvalues.只有Int数组是空的时候才遍历short数组)

OneofthetypesthatwasnotshowninListing4–9andTable4–2isthebooleantype.Infact,sortinganarrayofbooleanvaluesmakeslittlesense.However,theremaybeoccasionswhereyouneedtostorearatherlargenumberofbooleanvaluesandrefertothembyindex.Forthatpurpose,youcouldsimplycreateanarray.Whilethiswouldwork,thiswouldresultinmanybitsbeingwastedas8bitswouldbeallocatedforeachentryinthearraywhenactuallyabooleanvaluecanonlybetrueorfalse.Inotherwords,onlyonebitisneededtorepresentabooleanvalue.Forthatpurpose,theBitSetclasswasdefined:itallowsyoutostorebooleanvaluesinanarray(andallowsyoutorefertothembyindex)whileatthesametimeusingtheminimumamountofmemoryforthatarray(onebitperentry).IfyoulookatthepublicmethodsoftheBitSetclassanditsimplementationinBitSet.java,youmaynoticeafewthingsthatdeserveyourattention:

BitSet’sbackend后端isanarrayoflongvalues.Youmayachievebetterperformanceusinganarrayofintvalues.(Testsshowedagainofabout10%whenswitchingtoanintarray.)

Somenotesinthecodeindicatesomethingsshouldbechangedforbetterperformance(forexample,seethecommentstartingwithFIXME).

Youmaynotneedallthefeaturesfromthatclass.Forallthesereasons,itwouldbeacceptableforyoutoimplementyourownclass,possiblybasedonBitSet.javatoimproveperformance.4.2.4DefiningYourOwnClasses

Listing4–11showsaverysimpleimplementationthatwouldbeacceptableifthearray

doesnotneedtogrowafterthecreationoftheobject,andiftheonlyoperationsyouneedtoperformaretosetandgetthevalueofacertainbitinthearray,forexampleasyouimplementyourownBloom花filter.WhenusingthissimpleimplementationversusBitSet,testsshowedperformanceimprovedbyabout50%.WeachievedevenbetterperformancebyusingasimplearrayinsteadoftheSimpleBitSetclass:usinganarrayalonewasabout50%fasterthanusingaSimpleBitSetobject(thatis,usinganarraywas4timesfasterthanusingaBitSetobject).Thispracticeactuallygoesagainsttheencapsulation封装principleofobject-orienteddesignandlanguages,soyoushoulddothiswithcare.Listing4–11.DefiningYourOwnBitSet-likeClasspublicclassSimpleBitSet{privatestaticfinalintSIZEOF_INT=32;privatestaticfinalintOFFSET_MASK=SIZEOF_INT-1;//0x1Fprivateint[]bits;SimpleBitSet(intnBits){bits=newint[(nBits+SIZEOF_INT-1)/SIZEOF_INT];}voidset(intindex,booleanvalue){inti=index/SIZEOF_INT;into=index&OFFSET_MASK;if(value){bits[i]|=1<<o;//setbitto1}else{bits[i]&=~(1<<o);//setbitto0}}booleanget(intindex){inti=index/SIZEOF_INT;into=index&OFFSET_MASK;return0!=(bits[i]&(1<<o));}}

Alternatively另外,ifmostbitsaresettothesamevalue,youmaywanttouseaSparseBooleanArraytosavememory(possiblyatthecostofperformance).Onceagain,youcouldusetheStrategypatterndiscussedinChapter2toeasilyselectoneimplementationortheother.

Allinall,theseexamplesandtechniquescanbesummarizedasfollows:Whendealingwithlargeamountsofdata,usethesmallesttype用最小的数据类型possiblethatmeetsyourrequirements.Forexample,chooseanarrayofshortvaluesoveranarrayofintvalues,forbothperformanceandmemoryconsumptionreasons基于性能和空间考量.Usefloatinsteadofdoubleifyoudon’tneedtheextraprecision(anduseFloatMathclassifneeded).Avoidconversionsfromonetypetoanother.Trytobeconsistentanduseasingletypeinyourcomputations,ifpossible.Reinvent重建thewheelifnecessarytoachievebetterperformance如果有必要取得更好性能，推倒重来,butdoitwithcare.

Ofcourse,theserulesarenotsetinstone.Forexample,youmayfindyourselfinasituationwhereconvertingfromonetypetoanothercouldactuallygivebetterperformance,despitetheconversionoverhead类型转换后的性能超过开销.Bepragmatic务实andfixaproblemonlywhenyoudeterminethereisaone.Moreoftenthannot,usinglessmemoryisagoodruletofollow.Inadditiontosimplyleavingmorememoryavailableforothertasks,usinglessmemorycanimproveperformanceasCPUsusecachestoquicklyaccessdataorinstructions.4.3AccessingMemory访问内存

Aswesawearlier,manipulatinglargertypescanbemorecostlybecauseofthehighernumberofinstructionsinvolved指令较多.Intuitively此外,moreinstructionsoftenresultinlowerperformancesimplybecauseoftheextraworktheCPUhastoperformtoexecutethem.Inadditiontothat,codeanddatabothreside驻留inmemory,andaccessingmemoryitselfhasacost访问内存本身也有开销.

Becauseaccessingmemoryisacostlyoperation,aCPUcachesthememorythatwasrecentlyaccessed,whetheritwasmemorythatwasreadfromormemorythatwaswrittento.Infact,aCPUtypicallyusestwocachesorganizedinahierarchy层级:Level1cache(L1)Level2cache(L2)

TheL1cacheisthefasterbutalsothesmallerofthetwo.Forexample,theL1cachecouldbe64kilobytes(32kilobytesfordatacache,and32kilobytesforinstructioncache)whereasanL2cachecouldbe512kilobytes.

NOTE:SomeprocessorsmayalsohaveaLevel3cache,typicallyseveralmegabytes几兆字节insize,butyouwon’tfindthatonembeddeddevicesyet.

Whendataorinstructionscannotbefoundinacache,acachemissoccurs.Thisiswhendataorinstructionsneedtobefetched命中frommainmemory.Thereareseveralkindsofcachemisses:

Readmissininstructioncache

Readmissindatacache

Writemiss写未命中

Thefirsttypeofcachemissisthemostcritical最关键,astheCPUhastowaituntiltheinstructionisreadfrommemorybeforeitcanbeexecuted.Thesecondtypeofcachemisscanbeascriticalasthefirsttype,althoughtheCPUmaystillbeabletoexecuteotherinstructionsthatdonotdependonthedatabeingfetched.Thiseffectivelyresultsinanout-of-orderexecutionoftheinstructions（CPU指令乱序执行时出现）.Thelasttypeofcachemissismuchlesscritical,astheCPUcantypicallycontinueexecutinginstructions.Youwillhavelittlecontroloverwritemisses几乎无法控制写为明证,butyoushouldnotworryaboutitmuch.Yourfocusshouldbeonthefirsttwotypes,whicharethekindsofcachemissesyouwanttoavoid.TheCache’sLineSize缓存行的大小

Besidesitstotalsize,anotherimportantpropertyofacacheisitslinesize.Eachentryinthecacheisaline,whichcontainsseveralbytes.Forexample,acachelineonaCortexA8L1cacheis64bytes(16words).Theideabehindthecacheandcachelineistheprincipleoflocality:ifyourapplicationreadsfromorwritestoacertainaddress,itislikelytoreadfromorwritetothesameaddress,oraclose-enoughaddressinthenearfuture.Forexample,thisbehaviorwasobviousinthe

implementationofthefindMin()andaddAll()methodsinListing4–9.

Thereisnoeasywayforyourapplicationtoknowthesizeofacacheandthesizeofacacheline.However,knowingthecachesexistandhavingsomeknowledgeabouthowcachesworkcanhelpyouwritebettercodeandachievebetterperformance.Thefollowingtipscanhelpyoutakeadvantageofthecachewithouthavingtorecoursetolow-leveloptimization不必诉诸底层的优化手段,asshowninChapter3withthePLDandPLIassemblyinstructions.Toreducethenumberofcachereadmissesfromtheinstructioncache:

CompileyournativelibrariesinThumbmode.ThereisnoguaranteethiswillmakeyourcodefasterthoughasThumbcodecanbeslowerthanARMcode(becausemoreinstructionsmayhavetobeexecuted).RefertoChapter2formoreinformationonhowtocompilelibrariesinThumbmode.

Keepyourcoderelativelydense保持代码相对密集.WhilethereisnoguaranteedenseJavacodewillultimatelyresultindensenativecode,thisisstillquiteoftenatrueassumption.

Toreducethenumberofcachereadmissesfromthedatacache:

Again,usethesmallesttypepossiblewhenstoringalargeamountofdatainarrays大量数据存储在数组中，使用尽可能小的数据类型.

Choosesequentialaccessoverrandomaccess.Thismaximizesthereuseofdataalreadyinthecache,andcanpreventdatafrombeingremovedfromthecacheonlytobeloadedinthecacheagainlater.防止数据中清除后再次载入

NOTE:ModernCPUsarecapableofprefetching预取memoryautomaticallytoavoid,oratleastlimit,

cachemisses.防止缓存未命中的情况

Asusual,applythesetipsonperformance-criticalsectionsofyourapplication在应用性能关键处使用这些代码,whichusuallyisonlyasmallpartofyourcode.Ontheonehand,compilinginThumbmodeisaneasyoptimizationthatdoesnotreallyincreaseyourmaintenanceeffort.Ontheotherhand,writingdensecodemaymakethingsmorecomplicatedinthelongrun.Thereisnoone-size-fits-alloptimization没有一个放之四海皆准的方法,andyouwillhavetheresponsibilityofbalancingthemultipleoptionsyouhave.

Whileyoudon’tnecessarilyhavecontroloverwhatgoesintothecache,howyoustructureanduseyourdatacanhaveanimpactonwhatendsupbeinginthecache,andthereforecanimpactperformance.Insomecases,youmaybeabletoarrangeyourdatainaspecificmannertomaximizecachehits,albeit虽然possiblycreatinggreatercomplexityandmaintenancecost.4.4

LayingOutYourData排布数据

Onceagain,theprincipleofencapsulation封装willbebroken.Let’sassumeyourapplicationneedstostorerecordsofsomesort,andeachrecordcontainstwofields:anidandavalue.Averyobject-orientedapproachistodefineaRecordclass定义一个记录类,asshowninListing4–12.Listing4–12.RecordClasspublicclassRecord{privatefinalshortid;privatefinalshortvalue;//andpossiblymorepublicRecord(shortid,shortvalue){this.id=id;this.value=value;}publicfinalshortgetId(){returnid;}publicfinalshortgetValue(){returnvalue;}publicvoiddoSomething(){//dosomethinghere}}

NowthattheRecordclassisdefined,yourapplicationcouldsimplyallocateanarray,savetherecordsinthatarray,andprovideadditionalmethods,asshowninListing4–13.Listing4–13.SavingRecordspublicclassMyRecords{privateRecord[]records;intnbRecords;publicMyRecords(intsize){records=newRecord[size];}publicintaddRecord(shortid,shortvalue){intindex;if(nbRecords<records.length){index=nbRecords;records[nbRecords]=newRecord(id,value);nbRecords++;}else{index=-1;}returnindex;}publicvoiddeleteRecord(intindex){if(index<0){//throwexceptionhere–invalidargument}if(index<nbRecords){nbRecords--;records[index]=records[nbRecords];records[nbRecords]=null;//don’tforgettodeletereference122CHAPTER4:UsingMemoryEfficiently}}publicintsumValues(intid){intsum=0;for(inti=0;i<nbRecords;i++){Recordr=records[i];if(r.getId()==id){sum+=r.getValue();}}returnsum;}publicvoiddoSomethingWithAllRecords(){for(inti=0;i<nbRecords;i++){records[i].doSomething();}}}

Allofthiswouldworkandwouldresultinaprettycleandesign.However,therearedrawbacks缺陷thatmaynotbevisibleuntilyouactuallyrunthecode:

Anewobjectiscreatedeverysingletimearecordisaddedtothearray.Whileeachobjectislightweight轻量级,memoryallocationsarestillsomewhatcostlyandcouldbeavoided.

CallstogetId()andgetValue()couldbeavoidedifidandvaluewerepublic.

IfyouareallowedtomodifytheRecordclass,makingidandvaluepublicisobviously

trivial简单.TheimplementationofsumValues()wouldthenbeslightlymodified,asshowninListing4–14.Listing4–14.ModifiedsumValues()publicintsumValues(intid){intsum=0;for(Recordr:records){if(r.id==id){sum+=r.value;}}returnsum;}

However,thisalonedoesnotreducethenumberofallocationsatall;recordobjectsstillneedtobecreatedasrecordsareaddedtothearray.NOTE:YoucouldavoidallocationseasilyinC/C++,butinJavaallobjectsareactuallyreferences引用andhavetobecreatedwiththenewoperator操作符.

Sinceallobjectsareallocatedintheheap,andyoucanonlystorereferencestoobjectsinthearray,youcanmodifytheMyRecordsclasstouseanarrayofshortvaluestoremovetheallocations.ThemodifiedclassisshowninListing4–15.Listing4–15.ModifiedMyRecordsClassUsingaShortArraypublicclassMyRecords{privateshort[]records;intnbRecords;publicMyRecords(intsize){records=newshort[size*2];}publicintaddRecord(shortid,shortvalue){intindex;if(nbRecords<records.length){index=nbRecords;records[nbRecords*2]=id;records[nbRecords*2+1]=value;nbRecords++;}else{index=-1;}returnindex;}publicvoiddeleteRecord(intindex){if(index<0){//throwexceptionhere–invalidargument}if(index<nbRecords){nbRecords--;records[index*2]=records[nbRecords*2];records[index*2+1]=records[nbRecords*2+1];}}publicintsumValues(intid){intsum=0;for(inti=0;i<nbRecords;i++){if(records[i*2]==id){sum+=records[i*2+1];}}returnsum;}publicvoiddoSomethingWithAllRecords(){Recordr=newRecord(0,0);for(inti=0;i<nbRecords;i++){r.id=records[i*2];r.value=records[i*2+1];r.doSomething();}}}

Let’simaginethat,lateron,youfindoutthesethingsabouthowtheMyRecordsclassisused:sumValues()iscalledmuchmoreoftenthandoSomethingWillAllRecords().Only