最新The Structuring of Systems Using Upcalls教学讲义ppt.ppt

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1、TheStructuringofSystemsUsingUpcallsAQuickReviewofLayersManydesignsareorganizedintermsofmultiplelayersofabstractionByconvention,wetalkaboutlayerslikethey’restackedontopofeachotherThemostbasiclayerisonthebottomExample(Libraries)LayersasModules(1)So,putthemodulesinasingleaddressspace,asdemon

2、stratedin“Swift”,theirresearchO.S.Forsafety,theywroteSwiftinCLU:Strongcompile-timetypingDynamicboundscheckingGarbagecollectionLayersasModules(2)Modulessharethesamespace,socallsbetweenthemarejustprocedurecallsModuleshavetobemultithreaded:Assigningasinglethreadtoeachmoduleandasynchronouslyh

3、andingoffatboundariesisslowSoasinglethreadfollowsajobthroughthelayersSoeachlayerhastohandlemultiplejobsatonceIsthereadisciplineaboutwhocallswhom?DownwardsControlFlowNormally,wethinkofcontrolflowingfromthetopdown(downcalls)TheapplicationcallsalibraryThelibrarycallsthesystemJustanormalproce

4、durecall:thelowerlevelexecutesforawhile,thenreturnstotheupperlevelUpwardsControlFlowSometimes,it’smorenaturalforcontroltoflowupwardsOnejobsendsasignaltoanotherThesystemgetsanetworkpacketJustlikeanormalprocedurecall:theupperlevelexecutesforawhile,thenreturnstothelowerlevelDuality(1)Upcalls

5、aren’trequired:Theupperlevelcanpollthelowerlevel,askingifanyinterestingasynchronouseventshaveoccurredButthisisanasynchronoushandofffromthelow-leveleventtothepollingjobAndthere’sinherentoverheadinpollingformultipleeventsatonceDuality(2)Downcallsaren’trequiredeither!Asystemwithonlyupcallswo

6、uldhandleasynchronouseventsefficientlyButlowerlayerswouldhavetopolltoseeifhigherlayershaveanyrequestsSo,they’redualnotionsWhenshouldweuseupcalls?Whenshouldweusedowncalls?Arethereinherentdangerswithupcalls?Downcall-basedSocketsdowncall:createandbindasocketdowncall:receivedataifpossibledown

7、call:senddataifpossibledowncall:tellmewhenIcansendorreceivemoredataonanyofthesesocketsUpcall-basedSocketsdowncall:createandbindasocketWealsoneedtotellthesocketlayerwhichprocedurestouseasupcallsupcall:somedatajustarrivedupcall:doyouhaveanydatatosend?downcall:callmeba