同步高级sync-advanced25合成型.pptx

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同步高级sync-advanced25合成型.pptx

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Synchronization:Advanced

15-213/18-213:IntroductiontoComputerSystems

25thLecture,Nov.21,2013Instructors:RandyBryant,DaveO’Hallaron,andGregKesden

Review:SemaphoresSemaphore:non-negativeglobalintegersynchronizationvariableManipulatedbyPandVoperations:P(s):[while(s==0)wait();s--;]DutchforProberen(test)V(s):[s++;]DutchforVerhogen(increment)OSkernelguaranteesthatoperationsbetweenbrackets[]areexecutedindivisiblyOnlyonePorVoperationatatimecanmodifys.WhenwhileloopinPterminates,onlythatPcandecrementsSemaphoreinvariant:(s=0)

Review:UsingsemaphorestoprotectsharedresourcesviamutualexclusionBasicidea:Associateauniquesemaphoremutex,initially1,witheachsharedvariable(orrelatedsetofsharedvariables)Surroundeachaccesstothesharedvariable(s)withP(mutex)and V(mutex)operationsmutex=1P(mutex)cnt++V(mutex)

TodayUsingsemaphorestoschedulesharedresourcesProducer-consumerproblemReaders-writersproblemOtherconcurrencyissuesThreadsafetyRacesDeadlocks

UsingSemaphorestoCoordinateAccesstoSharedResourcesBasicidea:ThreadusesasemaphoreoperationtonotifyanotherthreadthatsomeconditionhasetrueUsecountingsemaphorestokeeptrackofresourcestate.Usebinarysemaphorestonotifyotherthreads.Twoclassicexamples:TheProducer-ConsumerProblemTheReaders-WritersProblem

Producer-ConsumerProblemCommonsynchronizationpattern:Producerwaitsforemptyslot,insertsiteminbuffer,andnotifiesconsumerConsumerwaitsforitem,removesitfrombuffer,andnotifiesproducerExamplesMultimediaprocessing:ProducercreatesMPEGvideoframes,consumerrendersthemEvent-drivengraphicaluserinterfacesProducerdetectsmouseclicks,mousemovements,andkeyboardhitsandinsertscorrespondingeventsinbufferConsumerretrieveseventsfrombufferandpaintsthedisplayproducerthreadsharedbufferconsumerthread

Producer-Consumeron1-elementBuffer#in