1. April 22, 2002VMASC Board Meeting Issues in Enhancing Simulation Model Reuse C. Michael Overstreet cmo@cs.odu.edu Richard E. Nance nance@vt.edu Osman Balci balci@vt.edu

2. April 22, 2002VMASC Board Meeting2 Motivations for Simulation Model Reuse: To reduce life-cycle costs model specification code specification & implementation V&V plans & execution accreditation To reduce time until new simulation is available near instantaneous construction of new simulations To improve quality of new simulations based on trusted or efficient components

3. April 22, 2002VMASC Board Meeting3 Perspective/terminology A simulation typically consists of A collection of interacting sim. models An infrastructure enabling interaction of those sim. models Mechanisms for observing or characterizing selected behaviors Mechanisms for user interaction with simulation

4. April 22, 2002VMASC Board Meeting4 Fundamental assertions - 1: Each simulation is constructed to meet a concrete set of objectives, such as: Improve system performance planning, design Improve understanding scientific modeling; manager’s intuition Reduce training time, improved quality “correctness” of some aspects may not be important Build a fun game laws of physics might be intentionally ignored Generate believable behaviors movies, background for training simulations Different objectives can imply different behaviors, correctness, accuracy, and performance requirements for the same object or situation.

5. April 22, 2002VMASC Board Meeting5 Fundamental assertions - 2: Objectives determine desired behaviors of models. Desired behaviors determine model content. Models are based on abstractions and assumptions. Appropriateness of abstractions depends on desired behaviors. The models used in simulations reflect sometimes subtle tradeoffs among speed, accuracy, included features, costs.

6. April 22, 2002VMASC Board Meeting6 Thus: Simulation model reuse must take both original and new objectives into consideration; valid reuse requires consistency between the two sets of objectives. Similarly for model assumptions and constraints

7. April 22, 2002VMASC Board Meeting7 Occam’s view of simulation: The simplest, minimal model is best: Ease of understanding Quicker implementation Reduced debugging Often most run-time efficient Improved reuse potential (perhaps) easier modification, if needed Bias towards elegant, simple Thus models should be just barely good enough to meet objectives.

8. April 22, 2002VMASC Board Meeting8 Desirability of minimalist view? Does this enhance or impede reuse? Does this reflect an inappropriate 1950’s view of computing: It’s a sin to waste a cycle.

9. April 22, 2002VMASC Board Meeting9 Economic facts of simulation: Costs are in development & CPU cycles are free. Tyranny of better software and cheaper hardware: User “needs” are often quite elastic; if it’s not too expensive, it’s a requirement. Faster, cheaper hardware results in unanticipated new uses (e.g., real-time decision support) Many of today’s cutting-edge simulations will be perceived as inadequate tomorrow.

10. April 22, 2002VMASC Board Meeting10 Conflicting user needs Create “total immersion” interactive environment Create believable environment Create new simulations on demand Create simulations cheaply Incorrect behavior unacceptable Some incorrectness required Games Tutorials Execution efficiency vital

11. April 22, 2002VMASC Board Meeting11 Example levels of reuse Plug ‘n play: no changes necessary ModSAF a successful example Existing model “easily” altered to provide new or modified behaviors Can result in significant cost benefit Modeling approach useful in new domain Reuse concepts, architecture, designs, etc. Infrastructure reused (e.g., HLA)

12. April 22, 2002VMASC Board Meeting12 Impossible goal: automated reuse of arbitrary models? Page & Opper showed that deciding if a collection of models meets a set of objectives is NP-complete. Overstreet & Nance showed that deciding if two models are equivalent is unsolvable.

13. April 22, 2002VMASC Board Meeting13 Feasible goal: automated reuse of specially constructed models ModSAF (OneSAF): can build “new” simulation by combining existing library of models as needed. Each model is built from consistent set of objectives so that it will interact with other models correctly. Adding a new model to library requires that it be built in conformance to these objectives. Still, a slight change in objectives can mean that reuse of these models is undesirable.

14. April 22, 2002VMASC Board Meeting14 Key reuse issues: research needed - 1 Determining how to locate potentially reusable models Detecting incompatible objectives and assumptions among selected models Building models in such a way that reuse potential is enhanced Determining the level of granularity that best enhances reuse potential.

15. April 22, 2002VMASC Board Meeting15 Key reuse issues: research needed - 2 Capturing and representing the objectives, constraints and assumptions of each model. Determining if constraints (such as speed, memory) will be met with selected collection of models. If individual models are valid, what does this imply about a new combination?

16. April 22, 2002VMASC Board Meeting16 Comments on issues Some of these issues are well know to designers of Simulation Programming Languages, for example, granularity: GPSS (and many current simulation programming languages) consists of a collection of reusable models, each easily parameterized. But building a new simulation is like writing a new program from scratch. Use of high level components results in faster development but loss of flexibility

17. April 22, 2002VMASC Board Meeting17 No single solution Execution overhead: Some models are run once and thrown away Some model executions must meet real-time deadlines Some are execution intensive but not real-time Some models need only be suggestive (wake of a ship at sea); others must be highly precise (fluid flow about a supersonic wing). A solution should be less expensive than the problem it solves we need both quick & dirty simulations and well- documented, highly reusable simulations

18. April 22, 2002VMASC Board Meeting18 Summary - 1 Reuse is, in large part, motivated by economics. The changing costs of computing changes the models we choose to build. The changing costs of computing changes the economics of reuse: Faster hardware makes execution inefficiencies due to reuse irrelevant

19. April 22, 2002VMASC Board Meeting19 Summary - 2 Key to reuse is the capturing of objectives, assumptions and constraints. Models can be designed for reuse, but it appears feasible only when objectives are well-understood and stable. Completely automated reuse appears scientifically infeasible. Automated support is more likely economical.