high-performance-web-sites2.pdf

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1、PerformanceResearch,Part2:BrowserCacheUsage-Exposed!»Yahoo!UserInterf...Page1of14Yahoo!UserInterfaceBlogòBlogòAboutsearchinblog...PerformanceResearch,Part2:BrowserCacheUsage-Exposed!January4,2007at12:24pmbyTenniTheurer

2、InDevelopment

3、Thisisthesecondinaseriesofarticlesdescribingexperimentsconductedtol

4、earnmoreaboutoptimizingwebpageperformance.Youmaybewonderingwhyyou’rereadingaperformancearticleontheYUIBlog.Itturnsoutthatmostofwebpageperformanceisaffectedbyfront-endengineering,thatis,theuserinterfacedesignanddevelopment.Inanearlierpost,IdescribedWhatthe80/20RuleTellsUsaboutReducingHTTPRequests.Sin

5、cebrowsersspend80%ofthetimefetchingexternalcomponentsincludingscripts,stylesheetsandimages,reducingthenumberofHTTPrequestshasthebiggestimpactonreducingresponsetime.Butshouldn’teverythingbesavedinthebrowser’scacheanyway?Whydoescachematter?It’simportanttodifferentiatebetweenenduserexperiencesforanempt

6、yversusafullcachepageview.An“emptycache”meansthebrowserbypassesthediskcacheandhastorequestallthecomponentstoloadthepage.A“fullcache”meansall(oratleastmost)ofthecomponentsarefoundinthediskcacheandthecorrespondingHTTPrequestsareavoided.Themainreasonforanemptycachepageviewisbecausetheuserisvisitingthep

7、ageforthefirsttimeandthebrowserhastodownloadallthecomponentstoloadthepage.Otherreasonsinclude:òTheuservisitedthepagepreviouslybutclearedthebrowsercache.òThebrowsercachewasautomaticallycleared,basedonthebrowser’ssettings.òTheuserreloadedthepageinawaythatcausedthecachetobebypassed.Forexample,thebrowse

8、rwillbypassthecacheifyouholddownthecontrol-shiftkeywhileclickingtheRefreshbuttoninInternetExplorer.Strategiessuchascombiningscripts,stylesheets,orimagesreducethenumberofHTTPrequestsforbothanemptyandafullcachepageview.ConfiguringcomponentstohaveanExpiresheaderwithadateinthefuturereducesthenumberofHTT

9、Prequestsforonlythefullcachepageview.Previously,weobservedwherethetimeisspentwhenauserrequestshttp://www.yahoo.comwithanemptycache.Whenauserloadsthepage,thebrowserdownloadsapproximately30components(se