VISUALSLX AN OPEN USER SHELL FOR HIGH-PERFORMANCE MODELING AND SIMULATION. Thomas Wiedemann

Proceedigs of the 2000 Witer Simulatio Coferece J. A. Joies, R. R. Barto, K. Kag, ad P. A. Fishwick, eds. VISUALSLX AN OPEN USER SHELL FOR HIGH-PERFORMANCE MODELING AND SIMULATION Thomas Wiedema Techical Uiversity of Berli Frakli Strasse 28/29, FR 5-5 0587 Berli, GERMANY ABSTRACT SLX by Wolverie software is actually oe of the fastest simulatio laguages. Besides the high performace the SLX-compiler ca be exteded very easily by user specific sytax rules ad ew basic fuctios. This pyramid power of SLX is used to build a ew system for modelig ad simulatio VisualSLX. This system is a shell atop the stadard SLX-compiler ad the rutime system. All model ad simulatio data are stored i a uiversal database. VisualSLX could be used for a comfortable, rapid visual modelig ad for remote modelig ad simulatio through the iteret without ay kowledge of the SLX-sytax ad modelig paradigms. This paper reveals the architecture ad the uderlyig data structures of the system. Additioal requiremets ad iterfaces are caused by the applicatio of VisalSLX as a web-based modelig ad simulatio system. INTRODUCTION I the last te years simulatio methods were successful itroduced i almost all areas of sciece ad busiess (Wiedewitsch ad Heusma 995, Roberts ad Dessouky 998). The mai algorithms ad mathematical basics are well defied ad efficiet. At the momet the focus of research is shifted from the kerel simulatio fuctios to more peripheral tasks like graphical modelig, 3D aimatio i the Web with VRML, itelliget result aalysis with Data-Miig tools ad optimizatio methods. However especially i the area of optimizatio with simulatio models we see a revival of a old critical factor - the performace of the simulatio system. It seems to be a paradox that almost 35 years old simulatio laguages like GPSS are sigificatly faster tha moder simulatio laguages ad systems. 2 SIMULATION PERFORMANCE At first glace simulatio performace is cosidered to be solved by the help of Moore s law ad his physical realizatio i Giga-Hertz processors. If oly oe experimet with oe simulatio ru is take ito accout, there is o practical differece betwee 2 secods or miute of oe ru. But if the same model is itegrated i a optimizatio cycle with oe thousad experimets ad 20 rus per experimet it eeds 46 miutes i the first case ad early 4 hours i the secod case. Thus with the icreasig usage of optimizatio methods i simulatio the performace of the simulatio system becomes oe of the most critical parameters. The performace of simulatio systems is iflueced by the followig factors: The most critical performace killers i moder systems are all visualizatio ad reportig fuctios durig ru-time. It ca be show very easily, that eve small ad iexpesive 2D-bitmap aimatios ad parameter idicators cosume about 90% of the processor power. 3D-aimatios eve itesify the problem. Moder 3D-graphic adapters with hardware support for aimatios could defuse the situatio, but the ecessary amout of iformatio iterchage to move a object i the virtual space will sometimes exceed the amout for the abstract movemet iside the simulatio model. A lot of moder simulators like TAYLOR II cotai proprietary script laguages for defiig applicatio specific strategies for schedulig ad routig. Regrettably there are o available details about implemetatio of this script laguages, however the scripts seem to be iterpreted or executed i a iterpreter-like mode. It is almost well kow from the roots of computer sciece that besides a lot of advatages the performace is the weak side of iterpreters. Therefore their 865

usage i simulatio tools is very critical uder performace costraits. The performace of older simulatio systems, like GPSS/H is ofte better tha the performace of moder object orieted simulatio systems. Oe reaso for this very astoishig fact is the immoderate ad sometimes eedless usage of iheritace techiques. Especially if virtual class methods are used, for each call of a object method the processor must fetch the object data poiter, the it has to calculate a idexed poiter to the table of virtual fuctios ad the it receives the correct address of the fuctio. Compared to oe sigle call of a fuctio this procedure eeds 3-20 times more processor cycles. The highly sophisticated cache ad queuig buffers of the moder processors could be kocked out by this procedure ad i result the processor falls back to the sigle speed of the mai memory. As a result of the discussed performace problems, the applicability for optimizatio of actual simulators differs widely. So the paradox situatio has occurred, that for very expasive ad complex simulatio experimets with optimizatio tasks GPSS is still used by large compaies. The historical merits of GPSS ad its performace are well accepted, but the mai sytax ad sematics are still at the level of 964. Therefore it was registered with iterest, whe Wolverie Software, the vedor of GPSS/H released a successor for GPSS the SLX system. 3 THE PYRAMID POWER OF SLX I differece to GPSS the ew SLX system uses a moder C-like sytax. The source text is well structured ad supports subprograms ad iclude files. The first importat feature of SLX is its performace. As metioed i (Herikse 995) this is achieved by very efficiet list structures for the future evet list ad the geeratio of ative assembler code by the SLX compiler. SLX is probably still oe of the last moder simulatio laguages which optimizes the geerated machie code at the assembler level. The results are impressive SLX is 3 to 20 times faster tha other kow simulatio tools like TAYLOR or SIMPLE++. The secod importat feature of SLX is its extesibility. The compiler itself ca be exteded at compile time with ew sytax rules ad commads. The power of this feature outstrips all kow C++-macro defiitios. A excellet example is the H5-extesio, which makes SLX uderstadig GPSS programs (Herikse 999). Although the performace ad extesibility of SLX are excellet, there are also some deficits: - Due to the traditioally text based user iterface the readability of a SLX-model is lower tha models i Area or Taylor. This is especially true for iexperieced modelers. - The SLX sytax is more powerful tha GPSS but the required code for stadard model objects like buffers ad machies is loger ad more variable defiitios are required. I geeral SLX obtais very good perspectives due to its performace ad flexibility. It seems to be useful to add some higher modelig level to the basic SLX system. Oe optio - a database user iterface is preseted at the followig pages. This system allows a very comfortable modelig similar to Taylor, but with the performace of SLX. 4 THE VISUAL SLX SYSTEM The regularity of the SLX sytax ad its well structured source code allow a automated geeratio of SLX programs from a high-level modelig workbech. As a result of earlier projects we decided to realize this system ot as a hard coded program geerator, but as a ope ad flexible code geerator based o meta-models. The term meta model i this cotext does ot deote a uiversal, world referece model, but a relative simple template for a class of applicatio specific models like productio plaig, logistics or etwork models. Each specific metamodel defies:! a set of applicatio specific simulatio objects like machies, buffers, cars or etwork routers;! a set of parameters for every object with type iformatio, default values ad hadlig optios;! the code template for the used simulatio laguage, i this case SLX. The user of the modelig system defies his istaces of the meta-model objects ad the ecessary parameters. All data is stored i the database. A model maagemet system allows differet versios of a model ad stores the history of the model. I order to simulate the defied model the user model data is combied with the code templates of the metamodel objects. The SLX program is compiled by the SLXcompiler ad executed by the ru-time system i commad-lie mode. Results are stored as text files by the SLX system. They ca be imported to the database for further aalysis. A secod optio of result aalysis is give by traditioal programs like Excel or future Data Miig tools (Wiedema 998). 866

VisualSLX Meta model Meta - Objects Meta-Parameters Code templates SLX compiler ad rutime system Simulatio - results Figure : The Mai System Architecture 4. The Modelig Workbech User Model Objects Parameters Code geerator SLX - Model result aalysis A commo user of the modelig shell uses the followig operatios: create or select model create, select or copy versio create or select model object edit object parameters. code geeratio ad start of the simulatio result aalysis. A first workig applicatio of this modelig workbech was realized with Access. This approach was used for rapid prototypig the data structures ad the ecessary algorithms. The fial VisualSLX Workbech will be realized as a web-based iterface (see details of implemetatio). I the first Access-Workbech all user operatios are simply doe by editig the database etries. The user does ot eed ay kowledge about the SLX system or the related meta-model. The data of the meta-model are used for defiig object types i the user model. Almost all labels ad pull-dow-lists i the model istaces are geerated from the related meta-model defiitios. Durig a model edit sessio the meta-data defiitio ca be chaged by the admiistrator of the VisualSLX-shell. This is especially useful i the early state of a meta-model, whe the meta-model itself is created ad tested. The mai form for defiig a model of the VisualSLX user shell is show i fig. 2. The ceter of the scree show all existig model objects ad the type of the object. The subform below shows the parameters of the selected object. The umber of parameters is ulimited ad depeds oly o the used meta-model. Each parameter field ca comprise ot oly a sigle umber but also complex defiitios like fuctios calls (e.g. r_uiform(service,0,30) which calculates a radom value i SLX). If the user shell is used for other simulatio systems, such complex defiitios must be trasformed from a commo represetatio ito the laguage specific sytax. Otherwise the model will be laguage specific. 4.2 Details of Code Geeratio The templates for code geeratio are stored as text fields i the database. The legth of each database rages from sigle words to some lies (see fig. 3). Figure 2: The Actual Modelig Form 867 Figure 3: Code Templates

All fields for user data must be marked with variable ames ad special symbols like # as markers. So the operatio time for a machie object is defied as advace #optime# where advace is the regular commad of SLX for delayig the movig object ad #optime# is the parameter which is take the user model durig code geeratio. I result of the well structured SLX sytax the code geerator writes code to differet sectios of the SLX source code file. I queuig models typical sectios are: global declaratio area for variable ad objects local declaratio area for local variables mai process model cotrol structures. The target sectio of each template lie is specified i the database etry of the code geerator. The umber ad ames of sectios are ot limited ad are defied also i the meta-model. i database tables for fast adjustmets of the VisualSLX- System. The simulatio system is started from the user iterface with the shell-commad of the operatig system. I case of sytax errors i the source code of the model (which is possible, if a ew code template is tested), the SLX compiler returs a error listig. I ay case the cotrol returs back to the user iterface. Whe SLX is started as miimized ico the user does ot recogize this exteral procedure, but works oly with the user shell. I geeral the simulatio model cotais report fuctios for writig a trace file. The curret format of the trace-file is defied as: Time, Evet-type, StaticObjectID, DyObjectID 00,,, 300 2, 300, 2, With this trace file each evet i the model reports the actual time, e.g. the affected machie ad product ad the type of evet (Etry, Exit, Breakdow-Start,..). The trace file is imported by the user shell after the simulatio. database order Program start codes dest 2 Program start codes dest Object type (A) code dest 6 code Object type (A) code dest 5 Object type (B) code dest 6 Object type (B) code dest 5 oe code istace for each object of class A for each object of class B 4.4 Model data hadlig The proprietary data structures of curret simulators cause tremedous problems for all users. The solutio for this problem should be the same as it has bee for busiess systems years before - a database. Thus i the preseted user shell all simulatio data is stored i a database, which icludes: model data with all parameters ad defiitios of the model behavior, experimet parameters ad optimizatio methods, simulatio results ad statistical values. Program ed codes dest 9 Program start codes dest 8 ed up 2 5 6 8 9 Temporary Output buffers (..20) Figure 4: The Code Geeratio Process 4.3 Simulatio processig oe code istace fial program The geerated source file is stored i a local directory ad SLX is called from the user shell i commad lie mode with optios for output of all importat data to files. Both, the local directory ad the commad lie optios are stored 868 The mai problem of this simulatio database cosists i the high complexity of real processes. The database structure has to allow a high flexibility of relatioships betwee all objects of a simulatio model. As it cocers other iformatio systems, there is a serious discussio about the usage of relatioal or object-orieted databases. Regardig that relatioal databases have a strog performace ad well defied iterfaces, we use a relatioal database. This should ot be cosidered as a geeral decisio agaist object-orieted databases. The Etity-Relatioship-Model (ERM) of the simulatio database is show i fig. 5. This database structure is of the same kid like as the SIMCO system, which was preseted i Widema (998).

Model cotais Versio Rus have Results Labels cotais refers to refers to The attributes of the database-tables are divided ito two groups: Attributes for admiistratio ad for buildig the relatioships betwee the differet levels of the model objects (objectid, paretobjectid, datatype, subtype, uique database ID). A set of attributes for the storage of the object data themselves. Curretly we have 3 iteger fields (i,j,k), 3 double fields (d,e,f) ad two strig attributes (s,c). This data structure is curretly used for all model etities. The most importat relatioship is created by the objectidparetid relatio. Each object ca have N child-objects or parameters. Sets of parameters ca be stored as a list of sub-parameters. There is o restrictio cocerig the depth of the parameter hierarchy. We are limited oly by icreasig access time, because each additioal level requires oe more memory-access. The iteral data ames like i,j,k are covered by dyamic chagig labels i the user iterface. The defiitios of the labels are stored i the meta-model. The storage of all model ad meta-model data i a database provides exteral applicatios with access to all simulatio data. With SQL ad a ODBC-driver a cliet system is able to ask for iformatio by usig SELECTstatemets or is capable to make chages by applyig UPDATE-commads for all model ad experimet parameters i the simulatio database. cotais Objects cotais Parameters has subpar. Figure 5: The ERM-Model of the Database 5 VISUALSLX IN THE WEB The user shell ca work i three modes: as a traditioal, stad-aloe system, as a multi-user database i a local etwork, ad as a real cliet server system i Itraet or Iteret eviromets. The first two modes are realized by the database forms show. New or applicatio specific forms ca be developed i a short time by usig latest techologies of assistat supported database desig (e.g. i Microsoft Access). The third mode requires a additioal module for iterfacig to the web. Due to existig powerful software compoets for iteret applicatios, this iterface is realized as combiatio of the VisualSLX code geerator ad a web-server (see fig. 6). A CGI-iterface or similar techology does ot exist. Database related requests from the web are received from the Wisock-compoet i the VisualSLX/WEB-applicatio ad are aswered immediately. Advatages of this web-server itegratio are: a very high performace i result of direct dataexchage ad ope database tables i stad-bymode, a log-time coectio betwee the cliet ad the server with cotiuous data flow durig simulatio processig or result processig. There is almost o differece betwee the geeratio of a SLX source file for simulatio ad geeratig a HTMLform for a web based user iterface! Due this fact we ca use the same code geerator for HTML geeratio. There are o SLX or HTML specific sub programs i the code geerator itself. Thus the code geerator ca be used for almost all text based simulatio laguages or text based user iterfaces.. Database forms HTML for local etwork Browser access Web Webserver for static pages VisualSLX WIN- Web-server SOCK (Code/HTML.OCX geeratio ) VisualSLX Database Figure 6: The Access Modes of the Shell 869

6 WEB SPECIFIC REQUIREMENTS I result of specific characteristics of the actual Iteret techologies some typical problems have ecoutered. 6. Web Performace Outside a itraet the badwidth is very ofte critical ad is rapidly chagig. For this problem we see a solutio i parallelly editig more tha oe etry. For example all objects ad their parameters could be offered i form fields at the same time. Checkig operatios are doe with oe coect to the web server ad all problems are reported to the user at the same time. A secod optio is the usage of more tha oe browser widow ad a iterleave iteractio mode of the user. The best solutio would be a Java-based user-iterface which performs all major operatios at the cliet side. For time reasos this solutio is plaed for a future versio. Due to the delay of iformatio trasfer from the server to the Web, the state of the simulatio system ad the visualizatio o the web based cliet user iterface may differ withi some secods. So the feedback for user iteractios like Break or Stoppig the simulatio will double the iterchage time (from the cliet to the server ad back), of up to 0 secods. The mai solutio is give by the further techical improvemet of the Web or a usage iside a Itraet with guarateed quality of service. A simulatio at the cliet side is ot very useful due to the curret low performace of Java. 6.2 Multi User Problems I a multi-user local database mode a database record is locked, while a user edits the cotet. Durig this time the record is visible to other users, but ca ot be edited. The Web is a system without defied sessios. Thus a user ca switch off the computer or close the browser durig a edit operatio ad the database does ot receive ay iformatio about the loss of the coectio. Similar to moder cliet server system this problem ca be solved by a timeout of the lock mode. I the curret database this lockig operatio ad the check for timeout is explicitly doe by VisualSLX. 6.3 Licese ad Security Costraits Web-based simulatio also creates ew requiremets for software licesig ad project maagemet. Traditioal software liceses of simulatio packages oly allow a sigle place usage. I geeral a web-based system must have a etwork licese. The paymet of the simulatio customers ca be doe per project or by time used. A very critical fact also cosist i data security. If a compay uses a web-based simulatio system possibly sesible data will be stored i a exteral database. I order to provide a safe simulatio study some secret data could be ecrypted with a public key from the compay. A special decrypt-dll will be icluded i the import routies of the simulatio model. The private key for decryptio is directly trasfered betwee the customer ad the decryptio module. If the source code of the decryptio-dll is validated by a exteral istitutio, the security of the private iput data for simulatio will be very high. 7 FUTURE DEVELOPMENT The perspectives of SLX ad the preseted VisualSLX system are very iterestig. Thus the further developmet of this cocept will eclose the followig activities i the ear future (please look at <http://www.aedv.cs. tuberli.de/simco> for actual iformatio): olie-aimatio with VRML or Flash, result aalysis with database ad data miig tools, itegratio of the SLX-HLA iterface, which was developed by the Techical Uiversity of Magdeburg, parallel ad hyper-computig with a distributed versio of VisualSLX ad ru-time versios of SLX. 8 SUMMARY This paper has preseted a shell for the SLX simulatio system. The most importat features of the system are: a flexible ad ope meta model for defiitio of applicatio specific classes of simulatio models; all model, meta model ad simulatio data are stored i databases: SQL as a set-orieted laguage for the maagemet of the model ad simulatio experimets, a uiversal code geerator for the SLX source code ad the HTML code for a web based user iterface. Flexible architectures for data-iterchage ad bidirectioal cotrol with other, complex iformatio systems, Flexible ad powerful iterfaces to related software packages cocerig statistical result aalysis, presetatio, optimizatio, Data-miig ad kowledge reasoig. By the help of the system the eeded kowledge for makig a successful simulatio with SLX is reduced sigificatly. By usig the shell a user eeds oly commo kowledge about modelig ad simulatio ad o 870

programmig practice i SLX. With additioal user iterfaces i Excel, Access or i the Web modelig with SLX will be available also for maagers ad decisio makers. At the first level of meta-model maagemet a VisualSLX admiistrator ca modify predefied examples by their used labels ad parameters. This approach is especially useful for model domais with differet amig covetios but equal simulatio algorithms. At the secod ad highest level of meta-model maagemet a VisualSLX admiistrator is able to create ew meta-models. I geeral this requires a very good kowlegde of SLX. The VisualSLX shell could be see as a additioal level of the SLX-pyramid (see Figure 7 ad Herikse (999)). The ew level adds a high level modelig approach to the powerful SLX base pyramid. The VisualSLX system is curretly tested ad will be available as a add-o product to SLX. The usage of the system as a user shell for other text based simulatio systems is possible. Eve the first prototype of the ew simulatio eviromet allows very iterestig applicatios i the area of cliet-server-simulatio ad hyper-computig. Thus the preseted cocept could be a iterestig perspective for the future developmet of modelig ad simulatio. The uderlyig uiversal database structure could be see as a first attempt to a commo iterchage format i the area of modelig ad simulatio. Together with the strog performace of SLX the preseted cocept could be see as a iterestig step towards a really ew cocept of flexibility ad performace i simulatio. Applicatio specific, highlevel models with database support Applicatio specific meta-models REFERENCES Herikse, J. 995. A itroductio to SLX. I Proceedigs of the 995 Witer Simulatio Coferece, ed. C. Alexopoulos, K. Kag, W. Lilegoo, D. Goldsma, 502-509. Istitute of Electrical ad Electroics Egieers, Piscataway, New Jersey. Herikse, J. 999. SLX: Pyramid Power. I Proceedigs of the 999 Witer Simulatio Coferece, ed. P. Farrigto, H. Nembhard, D. Sturrock, G. Evas, 67-75. Istitute of Electrical ad Electroics Egieers, Piscataway, New Jersey. Roberts C. ad C. Y. Dessouky. 998. C. A overview of object-orieted simulatio. SIMULATION 70(6). Wiedewitsch J. ad J. Heusma J. 995. Future directios of modelig ad simulatio i the Departmet of Defese. I Proceedigs of the SCSC 95, Ottawa, Otario, Caada. Wiedema, T. 997. Perspectives of compoet-based modelig ad simulatio. I Proceedigs of the Wold Cogress o Systems Simulatio, Sigapore. Wiedema. T. 998. SIM-MINING AND SIMSQL - A DATABASE ORIENTED APPROACH FOR COMPONENT-BASED AND DISTRIBUTED SIMULATION, I Proceedig of Summer Simulatio Coferece, Reo Nevada. AUTHOR BIOGRAPHY DR. THOMAS WIEDEMANN is a scietific assistat at the Departmet of Computer Sciece at the Techical Uiversity of Berli. He has fiished a study at the Techical Uiversity Sofia ad a Ph.D. study at the Humboldt-Uiversity of Berli. His research iterests iclude simulatio methodology, tools ad eviromets i distributed simulatio ad maufacturig processes. His teachig areas iclude also itraet solutios ad database applicatios. Applicatio specific SLX - packages SLX-Extesios SLX- Kerel Figure 7: The Exteded SLX Pyramid (Herikse 999) 87