TN-05_ExcaliburNeptuneBoutMultiﬂuidImplementation
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   :description: technical note

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Excalibur-Neptune,report,2047356-TN-05-2,Task,4.2,BOUT++,multiﬂuid,implementation,Ben,Dudson,,Peter,Hill,,Ed,Higgins,,David,Dickinson,,and,Steven,Wright,University,of,York,David,Moxey,University,of,Exeter,October,28,,2021,Contents,1,Executive,summary,2,Introduction,3,Subclassing,3.1,Architecture,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,3.1.1,Species,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,3.1.2,Reactions,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,3.2,Advantages,and,disadvantages,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,4,State-Command,pattern,4.1,Architecture,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,1,2,4,4,5,6,7,8,9,4.2,Advantages,and,disadvantages,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,10,5,Alternatives,12,5.1,Entity,Component,Systems,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,13,5.2,Task,graphs,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,.,13,6,Conclusions,7,References,1,Executive,summary,14,15,In,this,report,BOUT++,[1],is,used,as,a,platform,to,test,design,patterns,which,enable,plasma,simulations,models,to,be,extended,from,single,ion,species,to,multiple,species.,The,aims,are,to:,1,1.,Evaluate,code,designs,which,enable,single-species,plasma,simulations,to,be,extended,to,multiple,species,,in,terms,of,their,ﬂexibility,and,complexity,of,implementation.,This,can,inform,the,design,of,models,built,on,other,frameworks,,including,ﬁnite,element,and,spectral,libraries.,2.,Explore,Domain,Speciﬁc,Languages,(DSLs),for,specifying,complex,multi-,ﬂuid,models,,and,provide,a,platform,for,further,development.,2,Introduction,An,important,feature,of,tokamak,edge,plasmas,is,that,they,consist,of,a,mixture,of,diﬀerent,species.,Physics,studies,of,some,phenomena,,such,as,plasma,turbu-,lence,,have,frequently,treated,the,plasma,as,if,it,consisted,of,a,single,ion,species,(eg.,deuterium).,In,order,to,make,predictions,of,phenomena,important,to,the,performance,and,design,of,the,divertor,,models,must,capture,the,interactions,between,plasma,and,neutral,gas,species,(deuterium,and,tritium,in,a,reactor),,as,well,as,radiation,from,impurity,species,(eg.,Be,,C,,Ne,,Ar,,W).,Eﬃcient,pumping,of,helium,“ash”,is,also,an,important,aspect,of,the,design.,In,many,cases,none,of,these,species,can,be,treated,as,“trace”,,but,are,all,coupled,,so,that,all,should,be,considered,self-consistently,in,the,same,simulation.,For,each,of,these,atomic,species,,the,density,and,dynamics,of,multiple,states,may,need,to,be,considered,,for,example,the,ground,state,,multiple,ionisation,stages,,and,molecular,species.,In,some,cases,it,may,also,be,necessary,to,track,metastable,states,,such,as,vibrationally,excited,states,or,charged,molecules.,The,result,is,a,potentially,large,number,of,species,types,which,must,be,solved,for,,together,with,a,complex,set,of,reactions,between,them.,If,the,number,of,species,states,to,be,solved,for,is,relatively,small,,such,as,electrons,,ions,and,a,single,atomic,species,as,in,Hermes-2,[2],and,recent,versions,of,STORM,[3],,then,code,can,be,written,for,each,species.,Unfortunately,with,this,design,the,size,of,the,code,(number,of,lines),grows,linearly,with,the,number,of,species,states,,becoming,increasingly,error,prone,,diﬃcult,to,test,,and,hard,to,maintain,,as,the,number,of,species,is,increased.,Experience,with,Hermes-2,indicated,that,this,was,not,a,viable,path,to,multi-species,simulations.,Some,desirable,features,of,a,software,design,for,this,problem,are:,2,1.,The,number,and,type,of,species,to,be,solved,for,,and,the,reactions,be-,tween,them,,should,be,customisable.,Either,at,run-time,with,a,user-,friendly,conﬁguration,language,(DSL),,or,at,compile,time,with,minimal,code,changes.,2.,The,code,must,be,modular,such,that,it,can,be,tested,in,small,pieces.,This,makes,ﬁnding,and,ﬁxing,errors,more,eﬃcient,than,tests,which,run,the,whole,of,a,large,model.,3.,Code,modules,must,be,reusable:,Where,the,evolution,equations,for,two,species,are,the,same,,the,same,code,should,be,used,to,perform,those,calculations.,This,is,needed,to,limit,growth,of,code,size,with,number,of,species,,and,reduce,the,amount,of,code,which,needs,to,be,tested.,4.,The,composition,of,these,modules,should,be,reliable:,The,conﬁguration,method,(e.g.,input,DSL),should,prevent,unphysical,or,erroneous,combi-,nations,of,components,,or,these,should,be,caught,quickly,at,run-time.,In,particular,the,system,design,should,prevent,the,following,scenarios:,•,Incorrect,use:,A,value,is,used,in,a,way,which,is,inconsistent,,for,example,the,units,or,normalisations,are,diﬀerent.,•,Set,after,use:,Calculating,a,quantity,after,it,is,used,,so,that,it,is,either,assumed,to,be,zero,(missing),or,incorrect,values,are,used,(e.g,uninitialised,values,,or,values,from,a,previous,timestep).,•,Update,after,use:,Similar,to,set,after,use,,a,term,or,quantity,(e.g,collision,rate),is,modiﬁed,after,it,has,been,used.,The,result,is,inconsistent,with,what,the,user,likely,intended.,•,Unused,values:,A,term,which,the,user,intends,to,be,used,,such,as,a,source,or,sink,,is,calculated,but,never,used.,In,this,report,we,describe,two,multi-ﬂuid,implementations,,both,built,on,top,of,BOUT++:,In,section,3,a,1D,model,developed,from,SD1D,is,described,,which,uses,inheritance,to,deﬁne,species,behaviour,,and,a,centralised,system,to,coordinate,interactions,between,species.,In,section,4,a,more,ﬂexible,code,design,is,presented,,which,is,capable,of,1,2,or,3-dimensional,multi-ﬂuid,simulations,,and,coordinates,inputs,and,outputs,of,code,components,through,a,state,object.,The,advantages,and,disadvantages,of,these,approaches,will,be,discussed.,Finally,in,section,5.1,the,Entity,Component,System,(ECS),approach,to,solving,similar,problems,is,discussed,,which,was,originally,developed,for,game,development.,3,3,Subclassing,In,this,section,we,describe,an,approach,to,building,a,multi-ﬂuid,simulation,model,using,a,class,hierarchy,,where,object-oriented,inheritance,is,used,to,adapt,and,specialise,behaviour.,This,development,started,from,the,SD1D,single-ﬂuid,code,[4].,3.1,Architecture,In,this,design,,the,model,is,divided,into,Species,,which,represent,particular,evolving,atomic,or,ion,species,,and,Reactions,,which,represent,all,interactions,between,species.,Species,hold,the,data,representing,the,state,of,the,system,(density,,temperatures,etc.),,and,have,no,knowledge,of,other,species,or,inter-,actions,between,them;,all,interactions,are,represented,by,externally,speciﬁed,sources,and,sinks,of,particles,,momentum,and,energy.,Reactions,here,repre-,sent,all,interactions,between,species,,including,eﬀects,such,as,ionisation,and,also,things,like,thermal,forces,which,would,not,normally,be,considered,to,be,“reac-,tions”.,The,interaction,between,Species,and,Reactions,objects,is,orchestrated,by,a,central,object,representing,the,whole,system,,which,creates,and,contains,lists,of,the,participating,species,and,reactions,objects.,This,is,illustrated,in,ﬁgure,1,Figure,1:,Subclassing,architecture,,in,which,Species,objects,describe,plasma,species,,and,Reactions,represent,interactions,between,them.,All,interactions,are,carried,out,through,exchange,of,sources.,4,SpeciesReactiongetDensity()getTemperature()getVelocity()addParticleSource()addMomentumSource()addEnergySource()updateSpecies(species[])getParticleSources()getMomentumSources()getEnergySources()nTvstate,An,implementation,of,this,design,is,provided,in,SD1D-multiﬂuid,available,at,https://github.com/boutproject/SD1D/tree/new-multifluid.,The,evalu-,ation,of,the,time,derivatives,is,done,in,the,following,steps:,1.,Call,the,evolve(),method,for,each,species.,The,species,updates,its,own,density,,velocity,and,temperature,by,taking,values,from,the,BOUT++,time,integrator,,which,advances,the,entire,system,of,all,species,syncronously.,2.,Call,the,updateSpecies(),method,for,each,reaction,,passing,in,the,state,of,all,species,in,the,simulation.,The,reaction,updates,its,own,rates,,using,the,state,of,all,the,species,3.,The,mass,,momentum,and,energy,sources,and,sinks,are,requested,from,each,reaction,,and,added,to,the,required,species,by,passing,the,sources/sinks,to,the,species,objects.,Details,of,these,steps,are,given,below,,starting,with,a,description,of,the,Species,class.,3.1.1,Species,Every,species,has,an,atomic,mass,number,and,charge,,and,a,density,,tempera-,ture,and,velocity,at,every,point,in,the,domain.,To,distinguish,between,model,equations,for,diﬀerent,types,of,species,,the,Species,class,can,be,subclassed.,The,FluidSpecies,class,evolves,three,equations,for,the,density,,momentum,and,pressure.,These,equations,do,not,include,coupling,to,other,species,,but,are,updated,by,a,call,to,an,evolve(),function,which,is,implemented,for,each,Species,class.,p,(cid:18),3,∂,∂t,2,(cid:0)minv||,∂,∂t,∂n,∂t,(cid:19),=,−∇,·,(cid:2)bv||n(cid:3),=,−∇,·,qe,+,v||∂||p,(cid:1),=,−∇,·,(cid:2)minv||bv||,(cid:3),−,∂||p,qe,=,5,2,pbv||,−,κ||∂||Te,5,(1),(2),(3),(4),As,with,all,BOUT++,models,,the,whole,system,is,evolved,together,in,time,,so,that,the,time,integrator,treats,all,time-evolving,variables,as,part,of,a,single,state,vector.,3.1.2,Reactions,The,evolve(),function,for,each,species,is,not,passed,information,on,the,other,species,present,,and,so,cannot,calculate,interactions,between,species.,The,equa-,tions,1-4,for,each,species,are,therefore,decoupled.,To,couple,species,together,,additional,terms,need,to,be,added,to,the,right,hand,side,of,these,equations.,To,represent,reactions,,collisions,,and,other,interactions,between,species,,a,set,of,Reaction,classes,are,deﬁned.,Each,reaction,deﬁnes,a,method,updateSpecies(),,which,is,passed,a,collection,(a,C++,map),of,all,species,in,the,simulation.,The,intention,is,that,this,function,calculates,all,reaction,rates,,and,exchanges,of,particles,,momentum,and,energy,between,species.,Each,reaction,can,also,implement,methods,which,return,density,,momentum,and,energy,sources,(densitySources(),,momentumSources(),and,energySources()).,These,return,C++,maps,of,species,→,Field3D,,where,a,Field3D,represents,a,spatially,dependent,scalar,ﬁeld,,here,representing,the,net,source,for,the,species,due,to,the,reaction.,In,terms,of,python-like,pseudo-code,,the,main,loop,of,the,SD1D,multiﬂuid,implementation,,which,calculates,the,time,derivatives,of,all,evolving,quantities,,consists,of:,for,species,in,species_list:,species.evolve(),for,reaction,in,reaction_list:,#,Calculate,reaction,rates,using,all,species,reaction.updateSpecies(species_list),for,species_name,,source,in,reaction.densitySources():,#,Check,that,the,species,is,in,species_list,6,species_list[species_name].addDensity(source),for,species_name,,source,in,reaction.momentumSources():,species_list[species_name].addMomentum(source),for,species_name,,source,in,reaction.energySources():,species_list[species_name].addEnergy(source),3.2,Advantages,and,disadvantages,In,terms,of,the,desirable,characteristics,described,in,the,introduction,(section,2,(with,subjective,score)):,1.,Customisable,(medium):,The,species,and,reactions,to,be,created,can,be,speciﬁed,at,run-time:,A,list,of,species,and,a,list,of,reactions,are,built,during,initialisation,,and,iterated,over.,The,interaction,between,species,is,however,limited,to,a,single,round,,meaning,that,some,interactions,which,depend,on,multiple,steps,(e.g.,calculating,electromagnetic,ﬁelds,,or,colli-,sion,rates,,then,using,those,quantities,in,a,further,calculation),are,diﬃcult,to,customise.,2.,Modular,(low):,Each,species,can,be,largely,tested,in,isolation,,because,it,is,self-contained,and,all,interactions,are,through,sources,which,can,be,speciﬁed,as,part,of,the,test.,Reactions,similarly,don’t,depend,on,each,other,,though,do,depend,on,Species.,This,provides,some,coarse,grained,modularity,,but,is,not,suﬃcient,to,simplify,testing:,The,Species,objects,are,however,complex,internally,,and,are,coupled,to,the,time,integrator,,making,them,hard,to,test,with,frameworks,like,GoogleTest.,Furthermore,creating,Species,objects,to,test,Reactions,can,be,quite,complex.,3.,Reusable,(medium):,The,code,for,Species,is,quite,reusable;,the,code,to,solve,the,ﬂuid,moment,equations,is,the,same,for,all,species.,Where,there,are,species-speciﬁc,terms,,these,can,be,added,as,Reactions,(even,though,they,only,involve,one,species).,The,code,for,Reactions,is,however,harder,to,reuse,,and,so,there,are,many,implementations,of,essentially,the,same,reaction,but,using,diﬀerent,rates.,This,is,something,which,could,likely,7,be,improved,,using,inheritance,for,classes,of,reactions,,but,needs,further,design,work.,4.,Reliable,(high):,The,coordination,of,interactions,between,species,in,a,sin-,gle,centralised,place,enables,a,number,of,useful,checks,to,be,performed:,The,code,can,ensure,that,sources,and,sinks,are,always,balanced,between,species.,The,simple,interaction,between,components,involving,a,single,interaction,(species,state,to,reactions,,then,reaction,sources,to,species),means,there,is,little,room,for,unexpected,behaviour,,but,also,limited,abil-,ity,to,incorporate,more,complex,multi-step,calculations.,The,two,main,disadvantages,of,this,structured,approach,are,that,1.,The,dependencies,between,components,are,very,restricted.,Complex,in-,teractions,between,diﬀerent,parts,of,the,code,should,be,minimised,,but,the,scheme,used,here,with,only,two,steps,(species,evolve,,reaction,update),is,overly,restrictive:,There,is,no,way,to,,for,example,,modify,the,calcula-,tion,of,the,collision,frequency,or,electric,ﬁeld,,and,have,that,aﬀect,other,calculations.,2.,The,class,hierarchy,of,Species,isn’t,really,used:,Only,a,single,species,type,(FluidSpecies),is,actually,used,,and,all,modiﬁcations,to,that,species,are,represented,as,reactions.,Thus,composition,of,behaviour,turns,out,to,be,more,useful,than,inheritance.,Building,on,this,experience,,a,diﬀerent,design,which,emphasises,composition,was,developed,for,Hermes-3,,described,in,the,following,section.,4,State-Command,pattern,Following,experience,with,the,inheritance,pattern,used,in,SD1D-multiﬂuid,,a,more,abstract,and,general,software,architecture,has,been,developed.,This,was,done,so,that,a,wider,variety,of,interactions,and,eﬀects,could,be,incorporated,,than,were,possible,with,a,single,pass,from,species,state,to,reactions.,One,of,the,key,aspects,considered,was,how,to,enable,mutual,dependencies,between,code,components,,while,keeping,code,complexity,manageable.,This,design,has,been,8,implemented,in,Hermes-3,[5],,which,has,been,developed,starting,from,an,exist-,ing,Hermes-2,single-ﬂuid,model,[2],with,a,manual,hosted,on,ReadTheDocs,[6],which,describes,in,detail,the,code,implemented.,It,is,capable,of,1-,,2-,,and,3-dimensional,simulations,,and,wide,range,of,plasma,models,,conﬁgured,by,an,input,text,ﬁle.,The,following,sections,describe,the,overall,architecture,(sec,4.1),,and,discuss,the,advantages,and,disadvantages,of,this,approach,(sec,4.2).,4.1,Architecture,The,main,features,of,the,state-command,design,are:,•,A,ﬂexible,store,or,database,,into,which,values,(e.g.,spatially,dependent,ﬁelds,like,densities,,temperatures),can,be,inserted,and,later,retrieved.,In,Hermes-3,this,is,a,nested,dictionary,strucure,(a,tree),,using,C++,variant,to,enable,diﬀerent,data,types,to,be,stored.,A,schema,deﬁnes,a,convention,for,where,values,are,stored,,for,example,state["species"]["h+"]["density"],would,be,the,number,density,of,hydrogen,ions.,•,A,collection,of,composable,model,components,,which,set,and,use,values,in,the,store.,For,example,there,is,a,component,which,evolves,an,equation,for,the,density,,another,component,which,evolves,an,equation,for,pressure.,These,components,are,conﬁgured,when,they,are,created,,so,that,the,same,code,is,used,to,evolve,every,species,which,needs,that,component.,Every,component,can,access,the,whole,state,,so,some,perform,calculations,for,a,single,species,,while,others,perform,calculations,involving,multiple,species,(e.g.,collisions,,sheath,boundary,conditions).,The,ﬂow,of,information,carried,by,a,state,through,a,sequence,of,components,is,shown,in,ﬁgure,2.,An,important,distinction,between,this,design,and,one,with,a,shared,global,state,is,that,the,state,is,an,object,which,is,passed,to,components,in,a,user-deﬁned,order.,This,has,two,advantages:,1.,It,facilitates,unit,testing,,because,the,inputs,to,the,components,can,be,9,Figure,2:,State-Command,pattern:,The,State,of,the,simulation,system,is,passed,through,a,sequence,of,Components,in,two,passes.,In,the,ﬁrst,pass,in,the,second,pass,(transform(),calls),,components,can,modify,the,state;,(finally(),calls),the,state,cannot,be,modiﬁed,,but,is,used,to,update,com-,ponent,internal,states.,precisely,speciﬁed,with,no,hidden,side-channels,or,large,setup/teardown,procedures.,2.,Access,to,the,state,can,be,controlled,,making,the,logic,of,the,program,easier,to,follow.,In,Hermes-3,each,component,is,passed,the,state,twice:,The,ﬁrst,time,the,state,is,mutable,,and,the,component,can,insert,values,into,it.,After,all,components,have,been,called,in,this,way,,the,state,is,“frozen”,,and,passed,to,each,component,again,as,const,and,so,cannot,be,modiﬁed.,Each,component,can,then,use,this,ﬁnal,state,to,update,its,own,internal,state,,such,as,time,derivatives,to,be,passed,to,the,time,integrator.,This,means,that,components,can,depend,on,each,other,,but,all,modiﬁcations,must,be,in,the,ﬁrst,function,(called,transform()),,and,in,the,second,function,(called,finally()),all,components,can,assume,that,the,state,will,not,subsequently,change1.,4.2,Advantages,and,disadvantages,In,terms,of,the,desirable,characteristics,described,in,the,introduction,(section,2),,with,subjective,scoring:,1.,Customisable,(medium):,The,species,and,reactions,to,be,created,can,be,speciﬁed,at,run-time:,A,list,of,components,is,built,at,run-time,,and,can,in,principle,represent,a,system,of,equations,for,an,arbitrary,number,1This,could,be,used,to,enable,all,component,finally(),functions,to,be,called,in,parallel,,but,this,is,not,currently,done,in,Hermes-3.,10,ComponentComponentComponenttransform()transform()transform()ﬁnally()ﬁnally()ﬁnally()DoneState,of,species.,Components,can,interact,with,each,other,in,complex,,though,not,arbitrarily,complex,ways,in,order,to,simplify,debugging.,2.,Modular,(high):,The,inputs,to,the,components,can,be,quite,straightfor-,wardly,created,in,tests,,and,because,the,components,don’t,interact,with,global,variables,or,other,shared,state,,the,number,of,diﬀerent,tests,re-,quired,to,cover,all,possible,code,paths,is,minimised.,As,a,result,of,this,,Hermes-3,has,a,collection,of,50,unit,tests,(using,GoogleTest),which,cover,most,of,the,code.,3.,Reusable,(high):,The,code,is,broken,up,into,smaller,pieces,than,the,Im-,SD1D-multiﬂuid,(subclassing),design,,aiding,reuse,of,components.,provements,to,how,the,components,and,their,conﬁguration,are,speciﬁed,in,Hermes,(a,Domain,Speciﬁc,Language,of,sorts),make,reusing,compo-,nents,for,reactions,quite,straightforward.,4.,Reliable,(low-medium):,The,ﬂexible,data,structure,used,to,store,the,state,does,not,by,itself,provide,many,checks,to,ensure,consistency,and,correctness.,As,implemented,in,Hermes-3,,the,user,could,specify,an,incor-,rect,ordering,of,components,,since,dependencies,are,not,explicitly,speciﬁed,in,a,way,that,would,enable,automatic,ordering.,To,mitigate,some,potential,reliability,issues,(“set,after,use”,and,“update,after,use”),,the,setting,and,getting,of,values,in,the,state,is,further,controlled:,If,a,component,requests,(get),a,value,from,the,state,,then,by,default,that,value,cannot,be,further,modiﬁed.,The,assumption,is,that,the,component,will,treat,the,value,as,ﬁnal,,rather,than,a,preliminary,value,which,might,be,modiﬁed,later,by,another,component.,In,order,to,get,a,value,which,might,be,modiﬁed,later,,the,more,verbose,getNonFinal,function,can,be,used.,An,important,corner,case,is,components,such,as,boundary,conditions,,where,the,value,of,the,boundary,cells,is,modiﬁed.,The,solution,described,above,doesn’t,distinguish,between,cells,in,the,boundary,and,in,the,domain,,and,therefore,doesn’t,catch,modiﬁcations,after,the,boundary,condition,is,applied.,We,there-,fore,need,to,distinguish,between,boundary,conditions,and,values,in,the,domain;,at,any,point,in,the,calculation,one,or,both,of,these,could,be,considered,“ﬁnal”.,The,design,used,in,Hermes-3,is,that,each,value,has,two,ﬂags,which,can,be,set:,final-domain,means,that,the,values,in,the,domain,are,ﬁnal,and,cannot,be,11,modiﬁed;,final,means,that,all,cells,are,ﬁnal,and,cannot,be,modiﬁed.,There,are,three,ways,to,get,values,from,the,store:,get(),means,the,caller,assumes,the,value,everywhere,(including,boundaries),is,ﬁnal;,getNoBoundary(),means,the,boundary,cells,are,not,assumed,ﬁnal,,and,getNonFinal(),means,all,values,may,be,further,modiﬁed,later.,There,are,also,two,ways,to,set,values:,set(),can,modify,any,cells,,and,checks,that,neither,final,nor,final-domain,ﬂags,have,been,set;,setBoundary(),modiﬁes,only,the,boundary,,and,checks,that,final,ﬂag,has,not,been,set.,After,making,these,correctness,improvements,,failure,modes,described,in,sec-,tion,2,are,considered,here:,•,Incorrect,use:,The,meaning,and,units,for,each,quantity,are,not,enforced,automatically,,but,follow,a,documented,schema.,Automatic,enforcement,of,compatible,units,is,usually,diﬃcult,because,the,equations,being,solved,are,made,dimensionless.,Further,improvements,might,be,possible,if,code,and,data,were,annotated,,so,that,the,type,of,data,expected,can,be,checked,(e.g.,is,this,data,a,rate,,a,density,,a,temperature?),•,Set,after,use:,This,failure,is,mostly,prevented,by,marking,values,as,ﬁnal,when,they,are,used.,This,includes,data,which,is,missing,(and,so,may,be,assumed,to,be,zero,in,the,consuming,component),,so,that,setting,a,value,after,it,is,used,results,in,an,error.,•,Update,after,use:,The,same,comments,apply,as,for,“set,after,use”.,•,Unused,values:,This,is,currently,not,prevented.,It,would,be,possible,to,check,after,all,components,have,been,run,that,all,values,have,been,used.,There,are,cases,where,components,can,set,values,which,are,not,used,in,the,particular,(perhaps,simpliﬁed),model,that,the,user,is,solving.,5,Alternatives,In,the,previous,sections,(3,and,4),two,designs,have,been,described,and,im-,plemented,using,the,BOUT++,framework,to,produce,ﬂexible,plasma,models,which,can,be,conﬁgured,at,run-time,based,on,input,settings.,There,are,other,software,designs,which,aim,to,solve,similar,issues,,which,have,been,developed,12,within,scientiﬁc,computing,and,in,other,ﬁelds.,Here,we,brieﬂy,describe,some,of,these.,5.1,Entity,Component,Systems,Entity,Component,Systems,(ECS),are,a,design,pattern,commonly,used,in,game,development.,It,is,intended,to,describe,a,set,of,“Entities”,which,represent,things,which,have,deﬁned,sets,of,behaviour,,and,can,interact,with,each,other.,To,deﬁne,the,behaviour,of,thse,entities,,an,ECS,enables,components,to,be,de-,ﬁned,and,composed,,rather,than,deﬁning,a,class,hierarchy,to,represent,diﬀerent,types,of,behaviour.,These,ECS,systems,also,enable,components,to,be,iter-,ated,over,in,an,eﬃcient,manner,,by,managing,the,mapping,of,components,to,contiguous,memory,structures.,A,prominent,and,high-performance,implementation,of,an,ECS,is,EnTT,[7].,Like,the,state-command,pattern,,it,oﬀers,ﬂexibility,through,composition,rather,than,inheritance,,and,considerable,run-time,conﬁgurability.,For,high,performance,scientiﬁc,simulation,codes,,it,is,debatable,whether,run-,time,conﬁguration,is,essential,or,even,beneﬁcial:,Once,the,simulation,is,set,up,,it,is,performing,the,same,set,of,operations,repeatedly,,calculating,time,derivatives,given,diﬀerent,system,states.,Run-time,conﬁguration,enables,the,user,(scientist),to,modify,the,equations,without,recompiling,,but,means,that,errors,are,only,caught,at,runtime,,which,might,have,been,caught,quicker,at,compile,time.,Compile-time,conﬁguration,of,the,equations,solved,might,enable,more,optimisations,,since,conditionals,can,be,known,and,optimised,out,by,the,compiler.,It,is,possible,that,Just-In-Time,(JIT),compilation,might,oﬀer,the,best,of,both,worlds;,since,the,operations,are,the,same,with,diﬀerent,data,,run-time,analysis,of,the,performance,might,enable,real-time,tuning,to,identify,bottlenecks,and,optimise,throughput.,5.2,Task,graphs,The,majority,of,the,code,lines,and,computation,cost,of,a,simulation,consists,of,calculating,the,time,derivatives,of,a,system,of,equations,,turning,a,PDE,into,an,13,ODE,which,can,be,then,integrated,by,standard,explicit,or,implicit,ODE,solvers.,The,outputs,(the,time,derivatives),depend,on,quantities,such,as,collision,times,and,ﬂows,,which,in,turn,may,depend,on,calculations,of,electromagnetic,ﬁelds,or,boundary,conditions.,The,whole,calculation,can,be,viewed,as,a,task,graph.,Task,graphs,are,a,set,of,tasks,,each,with,inputs,and,outputs;,the,outputs,from,some,tasks,act,as,the,input,to,other,tasks.,This,is,quite,a,general,way,of,think-,ing,about,computations,,and,how,to,break,them,up,into,pieces.,An,advantage,from,a,performance,perspective,is,that,these,tasks,can,be,automatically,sched-,uled,,and,in,some,cases,moved,between,CPU,and,GPU,depending,on,available,resources.,Examples,of,task-based,systems,include,StarPU,[8],and,TaskFlow[9],which,can,use,SYCL,as,an,implementation,language,for,cross-platform,pro-,gramming.,6,Conclusions,Two,multi-ﬂuid,plasma,models,have,been,implemented,on,top,of,the,BOUT++,framework,,using,diﬀerent,architectures,to,achieve,the,ﬂexibility,and,code,reuse,needed,for,practical,multi-ﬂuid,implementations.,The,desirable,qualities,of,such,architectures,have,been,discussed,,and,the,implementations,evaluated,against,them.,Lessons,learned,include,that,composition,is,a,better,model,than,inheri-,tance,,and,that,plasma,models,quite,naturally,ﬁt,into,a,model,of,interconnected,tasks.,Ideas,for,further,reﬁnement,,and,alternative,directions,to,explore,have,been,suggested.,14,7,References,[1],BOUT++,contributors.,BOUT++,manual.,https://bout-dev.,readthedocs.io/.,[2],Ben,Dudson,,Jarrod,Leddy,,Hasan,Muhammed.,Hermes-2,hot,ion,drift-,reduced,model.,https://github.com/bendudson/hermes-2.,[3],L.,Easy,,F.,Militello,,T.,Nicholas,,J.,Omotani,,F.,Riva,,N.,Walkden,,UKAEA.,Hermes-2,hot,ion,drift-reduced,model.,https://github.com/,boutproject/STORM.,[4],Ben,Dudson,et,al.,SD1D,SOL,and,Divertor,model,in,1D.,https://github.com/boutproject/SD1D.,[5],Ben,Dudson.,Hermes-3.,https://github.com/bendudson/hermes-3.,[6],Ben,Dudson.,Hermes-3,manual.,https://hermes3.readthedocs.io.,[7],Michele,Caini.,EnTT:,Gaming,meets,modern,C++.,https://github.com/skypjack/entt,,2017-2021.,[8],INRIA.,StarPU,-,A,Uniﬁed,Runtime,System,for,Heterogeneous,Multicore,Architectures.,https://starpu.gitlabpages.inria.fr/.,[9],Taskﬂow,-,A,General-purpose,Parallel,and,Heterogeneous,Task,Program-,ming,System.,https://github.com/taskﬂow/taskﬂow.,15

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