川丹甫制冷压缩机股份有限公司招(I)

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1、OutlineforTodayObjective:Tocontinuetalkingaboutthecriticalsectionproblemandgetmorepracticethinkingaboutpossibleinterleavings.Starttalkingaboutsynchronizationprimitives.Introduceother“classic”concurrencyproblemsAdministrativedetails:LookonthewebforTAs’officehoursorc

2、hecknewsgroupforUTAs’officehoursforassignment1.Oncollectingproblemsets…SemaphoresWell-knownsynchronizationabstractionDefinedasanon-negativeintegerwithtwoatomicoperationsP(s)-[waituntils>0;s--]V(s)-[s++]Theatomicityandthewaitingcanbeimplementedbyeitherbusywaitingorb

3、lockingsolutions.Reminder:notation[]=atomicSemaphoreUsageBinarysemaphorescanprovidemutualexclusion(solutionofcriticalsectionproblem)Countingsemaphorescanrepresentaresourcewithmultipleinstances(e.g.solvingproducer/consumerproblem)Signalingevents(persistenteventsthat

4、stayrelevantevenifnobodylisteningrightnow)while(1){...otherstuff...criticalsection}TheCriticalSectionProblemP(mutex)V(mutex)Semaphore:mutexinitially1FillintheboxesProducer/ConsumerProducer:while(whatever){locallygenerateitemfillemptybufferwithitem}Consumer:while(wh

5、atever){getitemfromfullbufferuseitem}Producer/ConsumerProducer:while(whatever){locallygenerateitemfillemptybufferwithitem}Consumer:while(whatever){getitemfromfullbufferuseitem}P(emptybuf);V(fullbuf);P(fullbuf);V(emptybuf);Semaphores:emptybufinitiallyN;fullbufinitia

6、lly0;TweedledumandTweedledeeSeparatethreadsexecutingtheirrespectiveprocedures.TheideatocausethemtoforevertaketurnsexchanginginsultsthroughthesharedvariableXinstrictalternation.TheSleep()andWakeup()routinesoperateasfollows:Sleepblocksthecallingthread,Wakeupunblocksa

7、specificthreadifthatthreadisblocked,otherwiseitsbehaviorisunpredictableThecodeshownaboveexhibitsawell-knownsynchronizationflaw.Outlineascenarioinwhichthiscodewouldfail,andtheoutcomeofthatscenariovoidTweedledum(){while(1){Sleep();x=Quarrel(x);Wakeup(Tweedledee);}}vo

8、idTweedledee(){while(1){x=Quarrel(x);Wakeup(Tweedledum);Sleep();}}Ifdeegoesfirsttosleep,thewakeupislost(sincedumisn’tsleepingyet).Bothsleepforeve