1 CPU scheduling algorithm determines an order for the execution of its scheduled processes Given processes to be scheduled on one processor how many different schedules are possible Give a formula in terms of n Answer n factorial 1 2 1 62 Exp ID: 32580
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CPUSchedulingPracticeExercises-schedulingalgorithmdeterminesanorderfortheexecutionofitsscheduledprocesses.Givenprocessestobescheduledononeprocessor,howmanydifferentschedulesarepossible?Giveaformulaintermsof 0.080.441.01a.Whatistheaverageturnaroundtimefortheseprocesseswiththeschedulingalgorithm?b.Whatistheaverageturnaroundtimefortheseprocesseswiththeschedulingalgorithm?c.Thealgorithmissupposedtoimproveperformance,butnoticethatwechosetorunprocessattime0becausewedidnotknow Chapter6CPUSchedulingthattwoshorterprocesseswouldarrivesoon.Computewhattheaverageturnaroundtimewillbeiftheisleftidleforthe rst1unitandthenschedulingisused.Rememberthatprocessesarewaitingduringthisidletime,sotheirwaitingtimemayincrease.Thisalgorithmcouldbeknownasfuture-knowledgea.10.53b.9.53c.6.86Rememberthatturnaroundtimeis nishingtimeminusarrivaltime,soyouhavetosubtractthearrivaltimestocomputetheturnaroundtimes.is11ifyouforgettosubtractarrivaltime.Whatadvantageisthereinhavingdifferenttime-quantumsizesatdifferentlevelsofamultilevelqueueingsystem?Processesthatneedmorefrequentservicing,forinstance,interactiveprocessessuchaseditors,canbeinaqueuewithasmalltimequantum.Processeswithnoneedforfrequentservicingcanbeinaqueuewithalargerquantum,requiringfewercontextswitchestocompletetheprocessing,andthusmakingmoreef cientuseofthecomputer.-schedulingalgorithmsareparameterized.Forexample,thealgorithmrequiresaparametertoindicatethetimeslice.Multilevelfeedbackqueuesrequireparameterstode nethenumberofqueues,theschedulingalgorithmsforeachqueue,thecriteriausedtomoveprocessesbetweenqueues,andsoon.Thesealgorithmsarethusreallysetsofalgorithms(forexample,thesetofalgorithmsforalltimeslices,andsoon).Onesetofalgorithmsmayincludeanother(forexample,thealgorithmisthewithanin nitetimequantum).What(ifany)relationholdsbetweenthefollowingpairsofalgorithmsets?a.Priorityandb.Multilevelfeedbackqueuesandc.Priorityanda.Theshortestjobhasthehighestpriority.b.Thelowestlevelofgivesthehighestprioritytothejobhavingbeeninexistencethelongest.d.None. PracticeExercisesSupposethataschedulingalgorithm(atthelevelofshort-termscheduling)favorsthoseprocessesthathaveusedtheleastprocessortimeintherecentpast.WhywillthisalgorithmfavorI/Oprogramsandyetnotpermanentlystarve-boundprograms?ItwillfavortheI/O-boundprogramsbecauseoftherelativelyshortburstrequestbythem;however,the-boundprogramswillnotstarvebecausetheI/O-boundprogramswillrelinquishtherelativelyoftentodotheirI/ODistinguishbetweenschedulingisdonelocaltotheprocess.ItishowthethreadlibraryschedulesthreadsontoavailableLWPschedulingisthesituationwheretheoperatingsystemscheduleskernelthreads.Onsystemsusingeithermany-to-oneormany-to-many,thetwoschedulingmodelsarefundamentallydifferent.Onsystemsusingone-to-one,arethesame.Assumethatanoperatingsystemmapsuser-levelthreadstothekernelusingthemany-to-manymodelandthatthemappingisdonethroughtheuseofLWPs.Furthermore,thesystemallowsprogramdeveloperstocreatereal-timethreads.Isitnecessarytobindareal-timethreadtoanLWPYes,otherwiseauserthreadmayhavetocompeteforanavailableLWPpriortobeingactuallyscheduled.BybindingtheuserthreadtoanLWPthereisnolatencywhilewaitingforanavailableLWP;thereal-timeuserthreadcanbescheduledimmediately.Thetraditionalschedulerenforcesaninverserelationshipbetweenprioritynumbersandpriorities:thehigherthenumber,thelowerthepriority.Theschedulerrecalculatesprocessprioritiesoncepersecondusingthefollowingfunction:Priority=(recentusage/2)+basewherebase=60andrecentreferstoavalueindicatinghowoftenaprocesshasusedthesinceprioritieswerelastrecalculated.Assumethatrecentusageforprocessis40,forprocessis18,andforprocessis10.Whatwillbethenewprioritiesforthesethreeprocesseswhenprioritiesarerecalculated?Basedonthisinformation,doesthetraditionalschedulerraiseorlowertherelativepriorityofa-boundprocess?Theprioritiesassignedtotheprocessesare80,69,and65respectively.Theschedulerlowerstherelativepriorityof-boundprocesses.