[农林牧渔]分布式存储及应用系统架构分析

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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2021-8-6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2021-8-61.1Memcache实现分析理解1.1.1实现结构SpeedOfcourse,thepr

2、imarymotivationforcachingisspeed,soMemcachedisdesignedtobeasfastaspossible.TheinitialprototypeofMemcachedwaswritteninPerl.AlthoughIlovePerl,theprototypewaslaughablyslowandbloated.Perltradesoffmemoryusageforeverything,soalotofpreciousmemorywaswasted,andPerlcan'thandletonsofn

3、etworkconnectionsatonce.ThecurrentversioniswritteninCasasingle-process,single-threaded,asynchronousI/O,event-basedd?mon.Forportabilityandspeed,weuselibevent(seetheon-lineResourcessection)foreventnotification.Theadvantageoflibeventisthatitpicksthebestavailablestrategyfordeal

4、ingwithfiledescriptorsatruntime.Forexample,itchooseskqueueonBSDandepollonLinux2.6,whichareefficientwhendealingwiththousandsofconcurrentconnections.Onothersystems,libeventfallsbacktothetraditionalpollandselectmethods.InsideMemcached,allalgorithmsareO(1).Thatis,theruntimeofth

5、ealgorithmsandCPUusednevervarieswiththenumberofconcurrentclients,atleastwhenusingkqueueorepoll,orwiththesizeofthedataoranyotherfactor.Ofnote,Memcachedusesaslaballocatorformemoryallocation.EarlyversionsofMemcachedusedthemallocfromglibcandendedupfallingontheirfacesafterabouta

6、week,eatingupalotofCPUspaceduetoaddressspacefragmentation.Aslaballocatorallocatesonlylargechunksofmemory,slicingthemupintolittlechunksforparticularclassesofitems,thenmaintaining龙兴平(MSN:lxp8@sina.com)第83页共83页2021-8-6freelistsforeachclasswheneveranobjectisfreed.SeetheBonwickp

7、aperinResourcesformoredetails.Memcachedcurrentlygeneratesslabclassesforallpower-of-twosizesfrom64bytesto1MB,anditallocatesanobjectofthesmallestsizethatcanholdasubmitteditem.Asaresultofusingaslaballocator,wecanguaranteeperformanceoveranylengthoftime.Indeed,we'vehadproduction

8、Memcachedserversupfor4–5monthsatatime,averaging7,000queries/second,withoutproblems