数据库系统处理死锁一般采用哪两类方法?简述每类方法的基本原理
解决死锁一般采用死锁预防和死锁检测与恢复两类方法。死锁预防利用死锁预防协议, 通过破坏死锁产生的必要条件来避免系统进入死锁状态,防止死锁发生。死锁检测与恢复则是允许系统进入死锁状态,并且定期检查系统是否发生死锁,当发现系统发生死锁后,采取相应的恢复机制使系统摆脱死锁状态。
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Thefirstparagraphisintendedto________.
Passage4
Questions16to20arebasedonthefollowingpassage:
Successfulinnovationshavedrivenmanyoldertechnologiestoextinctionandhaveresultedinhigherproductivity,greaterconsumptionofenergy,increaseddemandofrawmaterials,acceleratedflowofmaterialsthroughtheeconomyandincreasedquantitiesofmetalsandothersubstancesinuseeachperson.Thehistoryofindustrialdevelopmentaboundswithexamples.
In1870,horsesandmulesweretheprimesourceofpoweronU.S.farms.Onehorseormulewasrequiredtosupportfourhumanbeingsaratiothatremainedalmostconstantformanydecades.Atthattime,hadanationalcommissionbeenaskedtoforecastthepopulationfor1970,itsanswerprobablywouldhavedependedonwhetheritsconsultantswereofaneconomicortechnologicalturnofmind.Hadtheybeen“economists”,theywouldprobablyhaveprojectedthe1970horsesormulepopulationtobemorethan50million.Hadtheybeen“technologists”,theywouldhaverecognizedthatthepowerofsteamhadalreadybeenharnessedtoindustryandtolearnandoceantransport.Theywouldhaverecognizedfurtherthatitwouldbetheprimesourceofpoweronthefarm.Itwouldhavebeendifficultforthemtoavoidtheconclusionthatthehorseandmulepopulationwoulddeclinerapidly.
16.Accordingtothepassage,whatsuppliedmostofthepoweronU.S.farmsin1870?
A.AnimalsB.HumansC.EnginesD.Water
17.WhichofthefollowingisNOTmentionedbytheauthorasaconsequenceofnewtechnologicaldevelopments?
A.Oldertechnologiesdieaway.
B.ThequalityoflifeisImproved.
C.Overallproductivityincrease.
D.Morerawmaterialsbecomenecessary.
18.Itcanbeinferredfromthepassagethatby1870.
A.technologybegantobemoreeconomical
B.thesteamenginehadbeeninvented
C.theU.S.horsepopulationwasabout10million
D.anationalcommissionwasabout10million
19.Inthesecondparagraph,theauthorsuggeststhat“economists”would.
A.plantheeconomythroughyearlyforecasts
B.failtoconsidertheinfluenceoftechnologicalinnovation
C.valuetheeconomiccontributionoffarmanimals
D.consultthenationalcommissionontheeconomy
20.Whatistheauthor’sattitudetowardchangesbroughtonbytechnologicalinnovations?
A.Heisexcitedaboutthem.
B.Heacceptthemasnatural.
C.Heisdisturbedbythem.
D.Hequestionstheirusefulness.