Impact of task-level concurrency transformations on the MPEG4 IM1 player for weakly paralle

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Impactoftask-levelconcurrencytransformationsontheMPEG4IM1playerforweaklyparallelprocessorplatforms

PaulMarchal(,**)ChunWong(,**)AggelikiPrayati(,)NathalieCossement(*)FranckyCatthoor(,***)RudyLauwereins(*)DiederikVerkest()HugoDeMan(,***)

IMEC,Kapeldreef75,Leuven,Belgium()tel:32-16-281202,fax:32-16-281515,email:catthoor@imecDepartmentofElectricalandComputerEngineering,UniversityofPatras,Greece()KatholiekeUniversiteitLeuven-ESAT,Leuven,Belgium(*)alsoPh.D.studentofK.U.Leuven-ESAT(**)alsoprofessorofK.U.Leuven-ESAT(***)

AbstractThispaperaddressestheconcurrenttaskmanagementofcomplexmulti-mediasystems,liketheMPEG4IM1player,withemphasisonsystemleveltransformationstoimprovethetaskconcurrencybehavior.Startingfromtheorigi-nal“standard”specification,wehaveextractedthecon-currencyoriginallyhiddenbyimplementationdecisionsina“grey-box”model.Thenwehaveappliedtwohigh-leveltransformationsonthismodeltoimprovethetask-levelconcurrency.Finally,byschedulingthetransformedtask-graph,wehaveimprovedtheperformanceoftheIM1player,whilesimultaneouslyloweringthesystemenergycost.Atwo-processortargetplatformisusedintheexper-iment,combiningprocessorsrunningatahighVddandalowVddrespectively.

Keywords:embeddedsystems,concurrency,schedul-ing,transformations,cost-efficiency

1MotivationandcontextToday,anewheterogeneousarchitecturaldesignparadigmisemerging,usuallycalleda“platform”,includ-ingoneormoreprogrammablecomponents,augmentedwithsomespecializeddatapathsorco-processors(acceler-ators).Theprogrammablecomponentsrunsoftwarecom-ponents[4],beingslowtomediumspeedalgorithms,whiletime-criticalpartsareexecutedondedicatedhardwareac-celerators.Withthisevolution,embeddedprocessorsbe-comeubiquitousandanewroleforembeddedsoftwareincontemporaryandfutureASIC(applicationspecificIC)systemsisreserved.Whenlookingatexistingdesignprac-ticesformappingsoftware(andhardware)onsuchaplat-form,onecanonlyconcludethatthesesystemsnowadaysaredesignedinaveryadhocmanner.Thedesigntrajec-torystartsbyidentifyingtheglobalspecificationentitiesthatfunctionallybelongtogether,calledtasksorprocesses.Thisstepisfollowedbyamanual“hardware-softwarepar-titioning”.Becauseofseparateimplementationofthedif-ferenttasksandofthesoftwareandhardware,afterwardsasystemintegrationstepisinevitable.Thisstepperformsthe“system/softwareembedding”andsynthesizestheinterfacehardware,whichclosesthegapbetweenthesoftwareandthehardwarecomponent.Themaingoalofsystem/softwareembeddingistoen-capsulatetheconcurrenttasksinacontrolshellwhichtakescareofthetaskscheduling(softwareschedulinginthere-strictedsense)andtheinter-taskcommunication.Thelatterbecomesbettersupportedbyemerginginterfacesynthesistools.Incontrast,taskschedulingisanerror-proneprocesswhichalsorequiresmorecomputerassistancetoconsiderthemanyinteractionsbetweenconstraints.Unfortunately,currentdesignpracticesforreactivereal-timesystemsareadhocandnotveryretargetable.Fromthis,onecanconcludethataneedexistsforasys-tematicmethodologyonthesystemlevelfortheco-designofhardwareandsoftwareincludingespeciallythemanage-mentof(dynamicallycreated)concurrenttasks,whichisstillmissinginpracticaltechniques.Therefore,weproposeamethodologythatstartsfromaninitial,unifiedspecifi-cationmodelthatiscapableofrepresentingsystem-levelabstractionliketaskconcurrency,inter-taskcommunica-DecodingBufferData

Channel

Presenter

Synchron-izationLayerCompressionLayerCompositionLayerCompositionBufferDataChannelDataUnitDataUnitBIFSODDecodersDeliveryLayer

part under investigatiom

Figure1.SystemlevelmodulesoftheMPEG4IM1playertionandsynchronization,andreal-timeconstraints[18,5].Wethensystematicallytransformthismodelinordertoin-creasetheamountofconcurrencyavailableintheapplica-tion[16].Basedonthisimprovedmodelwethencanbuildmoreoptimalschedulesfordifferenttime-budgets.Finally,werepresenttheseschedulesinaParetocurve,tradingoffprocessingcost(e.g.energy)versusperformance(time-budgetofthesubsystem).Inthispaperwewillmainlyfocusonexamplesoffor-malizabletransformationsthatintroduceextraconcurrencyintheapplicationbyremovinginefficientdatadependen-cies.AsaresultnewandbetterscheduleswillpopupintheParetocurvesallowingthedesignertomakemoreandmoreinterestingtrade-offs.Aweaklyparallelprocessortargetplatformisusedintheexperiments,combiningtwoproces-sorswithdifferentVdds.2TargetapplicationThetargetapplicationsofourtask-levelsystemsynthesisapproachareadvancedreal-timemulti-media.Theseappli-cationsinvolveacombinationofcomplexdata-andcontrol-flowwherecomplexdatatypesaremanipulatedandtrans-ferred.Mostoftheseapplicationsareimplementedwithportabledevices,puttingstringentconstraintsonthedegreeofintegrationandontheirpowerconsumption.Secondly,thesesystemsareextremelyheterogeneousinnatureandcombinehighperformancedataprocessing(e.g.datapro-cessingontransmissiondatainput)aswellasslowratecon-trolprocessing(e.g.systemcontrolfunctions),synchronousaswellasasynchronousparts,analogversusdigital,andsoon.Thirdly,time-to-markethasbecomeacriticalfactorinthedesignphase.Finally,thesesystemsaresubjectedtostringentreal-timeconstraints(bothhardandsoftdeadlinesarepresent),complicatingtheirimplementationconsider-ably.ThemaindriverforourresearchistheIM1player[1].ThisplayerisbasedontheMPEG4standard,whichspec-ifiesasystemforthecommunicationofinteractiveaudio-visualscenescomposedofobjects.Theplayercanbepar-titionedintofourlayers(seeFig.1):thedeliverylayer,thesynchronizationlayer,thecompressionlayerandthecom-positionlayer.Thedeliverylayerisagenericmechanismthatconveysstreamingdatatotheplayer.Inputstreamsflowthroughthedifferentlayersandaregraduallyinter-preted.Wemainlyfocusonthecompressionlayer.Thislayercontainsseveraldecoders.TwoimportantonesaretheBIFSandtheODdecoder.TheBIFSdecoderinter-pretsthedatapacketsthatdescribethecompositionofthesceneobjects.Theoutputofthisdecoderisagraph.Eachnodeinthisgraphpresentseitheranobjectinthesceneordescribestherelationbetweenthedifferentobjects.How-ever,torepresentsomeofthesceneobjectsextraAV-dataisneeded.Howthisdataneedstobeconveyedtothecom-pressionlayerandhowitneedstobedecodedisdescribedinObjectDescriptors(OD).TheseODsareencodedinoneormorestreamsenteringthecompressionlayerandarede-codedbytheODdecoder.TheODsarelinkedwiththenodesrequiringtheextraAV-data.