April 22, 2002VMASC Board Meeting Issues in Enhancing Simulation Model Reuse C. Michael Overstreet...
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Transcript of April 22, 2002VMASC Board Meeting Issues in Enhancing Simulation Model Reuse C. Michael Overstreet...
April 22, 2002 VMASC Board Meeting
Issues in Enhancing Simulation Model Reuse
C. Michael [email protected]
Richard E. [email protected]
Osman [email protected]
April 22, 2002 VMASC Board Meeting 2
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
April 22, 2002 VMASC Board Meeting 3
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
April 22, 2002 VMASC Board Meeting 4
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.
April 22, 2002 VMASC Board Meeting 5
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.
April 22, 2002 VMASC Board Meeting 6
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
April 22, 2002 VMASC Board Meeting 7
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.
April 22, 2002 VMASC Board Meeting 8
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.
April 22, 2002 VMASC Board Meeting 9
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.
April 22, 2002 VMASC Board Meeting 10
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
April 22, 2002 VMASC Board Meeting 11
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)
April 22, 2002 VMASC Board Meeting 12
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.
April 22, 2002 VMASC Board Meeting 13
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.
April 22, 2002 VMASC Board Meeting 14
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.
April 22, 2002 VMASC Board Meeting 15
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?
April 22, 2002 VMASC Board Meeting 16
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
April 22, 2002 VMASC Board Meeting 17
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
April 22, 2002 VMASC Board Meeting 18
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
April 22, 2002 VMASC Board Meeting 19
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.