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1、分布式存储及应用系统架构分析文件更改摘要：日期修订说明修订人批准人2010-4-4创建龙兴平修订记录龙兴平(MSN:lxp8@sina.com)第83页共83页2021-7-11目录1Memcached1.1Memcached架构memcached是高性能的分布式内存缓存服务器。一般的使用目的是，通过缓存数据库查询结果，减少数据库访问次数，以提高动态Web应用的速度、提高可扩展性。龙兴平(MSN:lxp8@sina.com)第83页共83页2021-7-111.1Memcache实现分析理解1.1.1实现结构SpeedOfcourse,theprimarymotivationforcachi
2、ngisspeed,soMemcachedisdesignedtobeasfastaspossible.TheinitialprototypeofMemcachedwaswritteninPerl.AlthoughIlovePerl,theprototypewaslaughablyslowandbloated.Perltradesoffmemoryusageforeverything,soalotofpreciousmemorywaswasted,andPerlcan'thandletonsofnetworkconnectionsatonce.Thecurrentversioniswritt
3、eninCasasingle-process,single-threaded,asynchronousI/O,event-basedd?mon.Forportabilityandspeed,weuselibevent(seetheon-lineResourcessection)foreventnotification.Theadvantageoflibeventisthatitpicksthebestavailablestrategyfordealingwithfiledescriptorsatruntime.Forexample,itchooseskqueueonBSDandepollon
4、Linux2.6,whichareefficientwhendealingwiththousandsofconcurrentconnections.Onothersystems,libeventfallsbacktothetraditionalpollandselectmethods.InsideMemcached,allalgorithmsareO(1).Thatis,theruntimeofthealgorithmsandCPUusednevervarieswiththenumberofconcurrentclients,atleastwhenusingkqueueorepoll,orw
5、iththesizeofthedataoranyotherfactor.Ofnote,Memcachedusesaslaballocatorformemoryallocation.EarlyversionsofMemcachedusedthemallocfromglibcandendedupfallingontheirfacesafteraboutaweek,eatingupalotofCPUspaceduetoaddressspacefragmentation.Aslaballocatorallocatesonlylargechunksofmemory,slicingthemupintol
6、ittlechunksforparticularclassesofitems,thenmaintaining龙兴平(MSN:lxp8@sina.com)第83页共83页2021-7-11freelistsforeachclasswheneveranobjectisfreed.SeetheBonwickpaperinResourcesformoredetails.Memcachedcurrentlygeneratesslabclassesforallpower-of-twosizesfrom64bytesto1MB,anditallocatesanobjectofthesmallestsize
7、thatcanholdasubmitteditem.Asaresultofusingaslaballocator,wecanguaranteeperformanceoveranylengthoftime.Indeed,we'vehadproductionMemcachedserversupfor4–5monthsatatime,averaging7,000queries/second,withoutprobl
