Overview of hyperparameter tuning

Hyperparameter tuning takes advantage of the processing infrastructure of Google Cloud to test different hyperparameter configurations when training your model. It can give you optimized values for hyperparameters, which maximizes your model's predictive accuracy.

What's a hyperparameter?

Hyperparameters contain the data that govern the training process itself.

Your training application handles three categories of data as it trains your model:

  • Your input data (also called training data) is a collection of individual records (instances) containing the features important to your machine learning problem. This data is used during training to configure your model to accurately make predictions about new instances of similar data. However, the values in your input data never directly become part of your model.

  • Your model's parameters are the variables that your chosen machine learning technique uses to adjust to your data. For example, a deep neural network (DNN) is composed of processing nodes (neurons), each with an operation performed on data as it travels through the network. When your DNN is trained, each node has a weight value that tells your model how much impact it has on the final prediction. Those weights are an example of your model's parameters. In many ways, your model's parameters are the model—they are what distinguishes your particular model from other models of the same type working on similar data.

  • Your hyperparameters are the variables that govern the training process itself. For example, part of designing a DNN is deciding how many hidden layers of nodes to use between the input and output layers, and how many nodes each hidden layer should use. These variables are not directly related to the training data. They are configuration variables. Note that parameters change during a training job, while hyperparameters are usually constant during a job.

Your model parameters are optimized (you could say "tuned") by the training process: you run data through the operations of the model, compare the resulting prediction with the actual value for each data instance, evaluate the accuracy, and adjust until you find the best values. Hyperparameters are tuned by running your whole training job, looking at the aggregate accuracy, and adjusting. In both cases, you are modifying the composition of your model to find the best combination to handle your problem.

Without an automated technology like Vertex AI hyperparameter tuning, you need to make manual adjustments to the hyperparameters over the course of many training runs to arrive at the optimal values. Hyperparameter tuning makes the process of determining the best hyperparameter settings easier and less tedious.

How hyperparameter tuning works

Hyperparameter tuning works by running multiple trials of your training application with values for your chosen hyperparameters, set within limits you specify. Vertex AI keeps track of the results of each trial and makes adjustments for subsequent trials. When the job is finished, you can get a summary of all the trials along with the most effective configuration of values according to the criteria you specify.

Hyperparameter tuning requires explicit communication between the Vertex AI and your training application. Your training application defines all the information that your model needs. You define the hyperparameters (variables) that you want to adjust, and target variables that are used to evaluate each trial.

Learn more about Bayesian optimization for hyperparameter tuning.

In addition to Bayesian optimization, Vertex AI optimizes across hyperparameter tuning jobs. If you are doing hyperparameter tuning against similar models, changing only the objective function or adding a new input column, Vertex AI is able to improve over time and make the hyperparameter tuning more efficient.

What hyperparameter tuning optimizes

Hyperparameter tuning optimizes target variables that you specify, called hyperparameter metrics. Model accuracy, as calculated from an evaluation pass, is a common metric. Metrics must be numeric.

When configuring a hyperparameter tuning job, you define the name and goal of each metric. The goal specifies whether you want to tune your model to maximize or minimize the value of this metric.

How Vertex AI gets your metrics

Use the cloudml-hypertune Python package to pass metrics to Vertex AI. This library provides helper functions for reporting metrics to Vertex AI.

Learn more about reporting hyperparameter metrics.

The flow of hyperparameter values

Without hyperparameter tuning, you can set your hyperparameters by whatever means you like in your training application. For example, you can configure the hyperparameters by passing command-line arguments to your main application module, or feed them to your application in a configuration file.

When you use hyperparameter tuning, you must use the following procedure to set the values of the hyperparameters that you're using for tuning:

  • Define a command-line argument in your main training module for each tuned hyperparameter.

  • Use the value passed in those arguments to set the corresponding hyperparameter in your application's code.

When you configure a hyperparameter tuning job, you define each hyperparameter to tune, its data type, and the range of values to try. You identify each hyperparameter using the same name as the corresponding argument you defined in your main module. The training service includes command-line arguments using these names when it runs your application.

Learn more about the requirements for parsing command-line arguments.

Select hyperparameters to tune

There is little universal advice to give about how to choose which hyperparameters you should tune. If you have experience with the machine learning technique that you're using, you may have insight into how its hyperparameters behave. You may also be able to find advice from machine learning communities.

However you choose them, it's important to understand the implications. Every hyperparameter that you choose to tune has the potential to increase the number of trials required for a successful tuning job. When you run a hyperparameter tuning job on Vertex AI, the amount you are charged is based on the duration of the trials initiated by your hyperparameter tuning job. A careful choice of hyperparameters to tune can reduce the time and cost of your hyperparameter tuning job.

Hyperparameter data types

In a ParameterSpec object, you specify the hyperparameter data type as an instance of a parameter value specification. The following table lists the supported parameter value specifications.

Type Data type Value ranges Value data
DoubleValueSpec DOUBLE minValue & maxValue Floating-point values
IntegerValueSpec INTEGER minValue & maxValue Integer values
CategoricalValueSpec CATEGORICAL categoricalValues List of category strings
DiscreteValueSpec DISCRETE discreteValues List of values in ascending order

Scale hyperparameters

In a ParameterSpec object, you can specify that scaling should be performed on this hyperparameter. Scaling is recommended for the DOUBLE and INTEGER data types. The available scaling types are:

  • SCALE_TYPE_UNSPECIFIED: No scaling is applied to this hyperparameter.
  • UNIT_LINEAR_SCALE: Scales the feasible space linearly 0 through 1.
  • UNIT_LOG_SCALE: Scales the feasible space logarithmically 0 through 1. The entire feasible space must be strictly positive.
  • UNIT_REVERSE_LOG_SCALE: Scales the feasible space "reverse" logarithmically 0 through 1. The result is that values close to the top of the feasible space are spread out more than points near the bottom. The entire feasible space must be strictly positive.

Conditional hyperparameters

The ConditionalParameterSpec object lets you add hyperparameters to a trial when the value of its parent hyperparameter matches a condition that you specify.

For example, you could define a hyperparameter tuning job with the goal of finding an optimal model using either linear regression or a deep neural network (DNN). To let your tuning job specify the training method, you define a categorical hyperparameter named training_method with the following options: LINEAR_REGRESSION and DNN. When the training_method is LINEAR_REGRESSION, your tuning job must specify a hyperparameter for the learning rate. When the training_method is DNN, your tuning job must specify parameters for the learning rate and the number of hidden layers.

Since the number of hidden layers is applicable only when a trial's training_method is DNN, you define a conditional parameter that adds a hyperparameter named num_hidden_layers when the training_method is DNN.

Since the learning rate is used by both training_method options, you must decide if this conditional hyperparameter should be shared. If the hyperparameter is shared, the tuning job uses what it has learned from LINEAR_REGRESSION and DNN trials to tune the learning rate. In this case, it makes more sense to have separate learning rates for each training_method, since the learning rate for training a model using LINEAR_REGRESSION should not affect the learning rate for training a model using DNN. So you define the following conditional hyperparameters:

  • A hyperparameter named learning_rate that is added when the training_method is LINEAR_REGRESSION.
  • A hyperparameter named learning_rate that is added when the training_method is DNN.

Conditional hyperparameters let you define the hyperparameters for your tuning job as a graph. This lets you tune your training process using different training techniques, each with their own hyperparameter dependencies.

Search algorithms

You can specify a search algorithm in the StudySpec object. If you do not specify an algorithm, your job uses the default Vertex AI algorithm. The default algorithm applies Bayesian optimization to arrive at the optimal solution with a more effective search over the parameter space.

Available values are:

  • ALGORITHM_UNSPECIFIED: Results in the same behavior as when you don't specify a search algorithm. Vertex AI uses a default algorithm, which applies Bayesian optimization to search the space of possible hyperparameter values, resulting in the most effective technique for your set of hyperparameters.

  • GRID_SEARCH: A simple grid search within the feasible space. This option is particularly useful if you want to specify a quantity of trials that is greater than the number of points in the feasible space. In such cases, if you do not specify a grid search, the Vertex AI default algorithm may generate duplicate suggestions. To use grid search, all parameters must be of type INTEGER, CATEGORICAL, or DISCRETE.

  • RANDOM_SEARCH: A simple random search within the feasible space.

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