1英文翻譯：本文出自ComputerNetwork第四版AndrewS.Tanenbaum著NetworkPerformanceMeasurementWhenanetworkperformspoorly,itsusersoftencomplaintothefolksrunningit,demandingimprovements.Toimprovetheperformance,theoperatorsmustfirstdetermineexactlywhatisgoingon.Tofindoutwhatisreallyhappening,theoperatorsmustmakemeasurements.Inthissectionwewilllookatnetworkperformancemeasurements.ThediscussionbelowisbasedontheworkofMogul(1993).Thebasicloopusedtoimprovenetworkperformancecontainsthefollowingsteps:1.Measuretherelevantnetworkparametersandperformance.2.Trytounderstandwhatisgoingon.3.Changeoneparameter.Thesestepsarerepeateduntiltheperformanceisgoodenoughoritisclearthatthelastdropofimprovementhasbeensqueezedout.Measurementscanbemadeinmanywaysandatmanylocations(bothphysicallyandintheprotocolstack).Themostbasickindofmeasurementistostartatimerwhenbeginningsomeactivityandseehowlongthatactivitytakes.Forexample,knowinghowlongittakesforaTPDUtobeacknowledgedisakeymeasurement.Othermeasurementsaremadewithcountersthatrecordhowoftensomeeventhashappened(e.g.,numberoflostTPDUS).Finally,oneisofteninterestedinknowingtheamountofsomething,suchasthenumberofbytesprocessedinacertaintimeinterval.Measuringnetworkperformanceandparametershasmanypotentialpitfalls.Belowwelistafewofthem.Anysystematicattempttomeasurenetworkperformanceshouldbecarefultoavoidthese.MakeSureThattheSampleSizeIsLargeEnoughDonotmeasurethetimetosendoneTPDU,butrepeatthemeasurement,say,onemilliontimesandtaketheaverage.Havingalargesamplewillreducetheuncertaintyinthemeasuredmeanandstandarddeviation.Thisuncertaintycanbecomputedusingstandardstatisticalformulas.MakeSureThattheSamplesAreRepresentative2Ideally,thewholesequenceofonemillionmeasurementsshouldberepeatedatdifferenttimesofthedayandtheweektoseetheeffectofdifferentsystemloadsonthemeasuredquantity.Measurementsofcongestion,forexample,areoflittleuseiftheyaremadeatamomentwhenthereisnocongestion.Sometimestheresultsmaybecounterintuitiveatfirst,suchasheavycongestionat10,11,1,and2oclock,butnocongestionatnoon(whenalltheusersareawayatlunch).BeCarefulWhenUsingaCoarse-GrainedClockComputerclocksworkbyincrementingsomecounteratregularintervals.Forexample,amillisecondtimeradds1toacounterevery1msec.Usingsuchatimertomeasureaneventthattakeslessthan1msecispossible,butrequiressomecare.(Somecomputershavemoreaccurateclocks,ofcourse.)TomeasurethetimetosendaTPDU,forexample,thesystemclock(say,inmilliseconds)shouldbereadoutwhenthetransportlayercodeisenteredandagainwhenitisexited.IfthetrueTPDUsendtimeis300sec,thedifferencebetweenthetworeadingswillbeeither0or1,bothwrong.However,ifthemeasurementisrepeatedonemilliontimesandthetotalofallmeasurementsaddedupanddividedbyonemillion,themeantimewillbeaccuratetobetterthan1sec.BeSureThatNothingUnexpectedIsGoingOnduringYourTestsMakingmeasurementsonauniversitysystemthedaysomemajorlabprojecthastobeturnedinmaygivedifferentresultsthanifmadethenextday.Likewise,ifsomeresearcherhasdecidedtorunavideoconferenceoveryournetworkduringyourtests,youmaygetabiasedresult.Itisbesttoruntestsonanidlesystemandcreatetheentireworkloadyourself.Eventhisapproachhaspitfallsthough.Whileyoumightthinknobodywillbeusingthenetworkat3A.M.,thatmightbepreciselywhentheautomaticbackupprogrambeginscopyingallthediskstotape.Furthermore,theremightbeheavytrafficforyourwonderfulWorldWideWebpagesfromdistanttimezones.CachingCanWreakHavocwithMeasurementsTheobviouswaytomeasurefiletransfertimesistoopenalargefile,readthewholething,closeit,andseehowlongittakes.Thenrepeatthemeasurementmanymoretimestogetagoodaverage.Thetroubleis,thesystemmaycachethefile,soonlythefirstmeasurementactuallyinvolvesnetworktraffic.Therestarejustreadsfromthe3localcache.Theresultsfromsuchameasurementareessentiallyworthless(unlessyouwanttomeasurecacheperformance).Oftenyoucangetaroundcachingbysimplyoverflowingthecache.Forexample,ifthecacheis10MB,thetestloopcouldopen,read,andclosetwo10-MBfilesoneachpass,inanattempttoforcethecachehitrateto0.Still,cautionisadvisedunlessyouareabsolutelysureyouunderstandthecachingalgorithm.Bufferingcanhaveasimilareffect.OnepopularTCP/IPperformanceutilityprogramhasbeenknowntoreportthatUDPcanachieveaperformancesubstantiallyhigherthanthephysicallineallows.Howdoesthisoccur?AcalltoUDPnormallyreturnscontrolassoonasthemessagehasbeenacceptedbythekernelandaddedtothetransmissionqueue.Ifthereissufficientbufferspace,timing1000UDPcallsdoesnotmeanthatallthedatahavebeensent.Mostofthemmaystillbeinthekernel,buttheperformanceutilitythinkstheyhaveallbeentransmitted.UnderstandWhatYouAreMeasuringWhenyoumeasurethetimetoreadaremotefile,yourmeasurementsdependonthenetwork,theoperatingsystemsonboththeclientandserver,theparticularhardwareinterfaceboardsused,theirdrivers,andotherfactors.Ifthemeasurementsaredonecarefully,youwillultimatelydiscoverthefiletransfertimefortheconfigurationyouareusing.Ifyourgoalistotunethisparticularconfiguration,thesemeasurementsarefine.However,ifyouaremakingsimilarmeasurementsonthreedifferentsystemsinordertochoosewhichnetworkinterfaceboardtobuy,yourresultscouldbethrownoffcompletelybythefactthatoneofthenetworkdriversistrulyawfulandisonlygetting10percentoftheperformanceoftheboard.4網絡性能的測量當一個網絡的運行效果很差的時候，它的用戶通常會向網絡運行商抱怨并要求提高網絡的質量。為了改善網絡的性能，網絡操作人員首先必須確定發生了什么問題。為了找出真正的問題所在，操作人員必須進行測量工作。在這一小節中，我們來看一看網絡性能的測量問題。下面的討論以Mogul(1993)的工作為基礎。用來改善網絡性能的基本循環過程包括以下步驟：（1）測量有關的網絡參數和性能。（2）試圖理解當前的網絡狀況。（3）改變一個參數。這些步驟不斷重復，直到網絡的性能已經足夠好，或者改善性能的全部空間都已經被發掘出來了。測量工作可以有許多做法，也可以在許多地點或場所進行（既指物理位置，也指協議棧中的位置）。最基本的一種測量手段是：在開始某一個動作的時候啟動一個定時器，然后確定該需要多長時間。例如，知道一個TPDU需要多長時間才被確認是一個很關鍵的測量指標。其他有一些測量指標可以通過計數器來完成，即記錄某種事件發生的次數，比如丟失的TPDU的數量。最后，人們通常對于某些事物的數量比較感興趣，比如在特定的時間間隔內所處理的字節數。測量網絡的性能和參數有許多潛在的陷阱。以下我們列出其中一部分。任何一種系統化的網絡性能測量