Add Modeling Algorithm

Model vs Modeling Algorithm

It is useful to first make the distinction between the model type and the modeling algorithm (= hyperparameter optimization algorithm).

The model type is the kind of model. For example: a polynomial, a radial basis function, a reduced order model for a particular application, etc. Each model type is fully characterized by a number of parameters. For example: a polynomial is determined by its degree n and the n+1 coefficients; a neural network is determined by the number of neurons in each layer and the connection weights between them. So different model types have different numbers of parameters that need to be set. This is where the difficulty usually lies. Setting correct values requires finding the right balance between the bias/variance tradeoff and in many cases this is more of an art than a science.

Finding the best model parameters given the data can be seen as a (constrained) optimization problem over the parameter space. This is where the modeling algorithm, comes in. The modeling algorithm is a particular optimization/search algorithm over the model parameter space. The toolbox comes with a number of modeling algorithms but others can be easily added. Adding support for a new model type therefore boils down to implementing the model type itself and providing an implementation of an optimization algorithm. How this is done in code is discussed below.

Adding a new Modeling algorithm

Some of the modeling algorithms (= hyperparameter optimization algorithms) available in the SUMO-Toolbox are:

• SequentialModelBuilder: a kind of hillclimber, it keeps a sliding window of the past n models and builds new models, one by one based on this history window.
• BatchModelBuilder: here you start with an initial batch of n models and each iteration that batch is replaced by a new batch.
• GeneticModelBuilder: a genetic algorithm is used to search the parameter space. Given an initial population the algorithm uses crossover and mutation operators to search through the parameter space. (requires the Matlab Direct Search Toolbox (GADS) toolbox)
• PatternSearchModelBuilder: uses a pattern search algorithm to optimize the model parameters (requires the Matlab Direct Search Toolbox (GADS) toolbox)
• OptimToolboxModelBuilder: Uses the algorithms from the Matlab Optimization toolbox
• PSOModelBuilde: uses particle swarm optimization
• OptimizationModelBuilder: can work with any algorithm implemented as part of the Optimizer hierarcy.
• SimAnnealingModelBuilder: Uses the simulated annealing algorithms from the Matlab Direct Search Toolbox (GADS) toolbox
• RandomModelBuilder: this is solely useful as a baseline comparison search algorithm.
• FixedModelBuilder: this will build whatever model you tell it to, it uses the same fixed set of model parameters for every model

The algorithms themselves are implemented in the src/matlab/modelbuilder directory (always check this directory for the most up to date list). If you would like to add a new optimization algorithm for optimizing the model parameters, you need to create a new ModelBuilder class in src/matlab/modelbuilders (alternatively you could just add a new optimizer and use the OptimizerModelBuilder). This new class should derive from AdaptiveModelBuilder and provide the following methods:

• a constructor that reads out the xml configuration and configures the modeling algorithm (MyModelBuilder.m)
• runLoop.m : this is the main loop of your algorithm, it gets the current available samples and builds new models according to your optimization algorithm

Make sure you think hard about the interfacing with the different model types. Try to keep the number of methods that model types need to implement as small as possible.

Once your algorithm is implemented you can then write the necessary interfaces for the model types you wish to use. For the gradient descent example, say you wanted to use RBF functions, you would implement a RBFGradientDescentInterface class. See below for more information.

Adding a modeling algorithm implementation

In order to implement one or more of the modeling algorithms for your specific model type you need to provide an MyModelSomeAlgorithmInterface class (eg: MyModelBatchInterface). In this subclass you will need to provide the methods that allow the modeling algorithms to interact with your type. These methods are explained below.

SequentialModelBuilder

• a constructor: that reads in the configuration (eg. MyModelSequentialInterface.m)
• create.m: based on a history of the past n models, return the next model in the search

See the existing implementations for details.

BatchModelBuilder

• a constructor
• createBatch: given a batch of models, return a new, improved batch

See the existing implementations for details.

GeneticModelBuilder

• a constructor
• a population creation function
• a mutation operator function
• a crossover operator function

See the existing implementations for details. NB: you also have to derive from GeneticInterface.

PatternSearchModelBuilder

• a constructor
• a createModelFromIndividual method: given a parameter vector it should return the corresponding model object
• a getInitialPoint method: return an initial point within the parameter space from where to start the search
• a getBounds method: specifies the bounds on the parameter values

See the existing implementations for details.

OptimToolboxModelBuilder

• exactly the same methods as the PatternSearchModelBuilder

See the existing implementations for details.

SimAnnealingModelBuilder

• exactly the same methods as the PatternSearchModelBuilder

See the existing implementations for details.

RandomModelBuilder

• a constructor
• a createRandomModel method that takes no argument and returns a model with random values for its model parameters.

FixedModelBuilder

• a constructor
• a createFixedModel method that takes no argument and returns a model with a fixed set of parameters.

This is useful to see how much better your optimization algorithm is compared to a random search.

See the existing implementations for details.

Notes

If you are planning to implement different modelbuilders you should place methods needed for more than one model builder in your ModelInterface base class. In that case you dont need a separate class/constructor for each model builder.

Using your model builder

Having written the implementations you then have to update your xml configuration file to use you new model type, run 'go'.