# Manual¶

This manual shows how to use several aspects of auto-sklearn. It either references the examples where possible or explains certain configurations.

## Examples¶

auto-sklearn comes with the following examples which demonstrate several aspects of its usage:

## Time and memory limits¶

A crucial feature of auto-sklearn is limiting the resources (memory and time) which the scikit-learn algorithms are allowed to use. Especially for large datasets, on which algorithms can take several hours and make the machine swap, it is important to stop the evaluations after some time in order to make progress in a reasonable amount of time. Setting the resource limits is therefore a tradeoff between optimization time and the number of models that can be tested.

While auto-sklearn alleviates manual hyperparameter tuning, the user still has to set memory and time limits. For most datasets a memory limit of 3GB or 6GB as found on most modern computers is sufficient. For the time limits it is harder to give clear guidelines. If possible, a good default is a total time limit of one day, and a time limit of 30 minutes for a single run.

Further guidelines can be found in auto-sklearn/issues/142.

## Restricting the Searchspace¶

Instead of using all available estimators, it is possible to restrict auto-sklearn‘s searchspace. The following shows an example of how to exclude all preprocessing methods and restrict the configuration space to only random forests.

>>> import autosklearn.classification
>>> automl = autosklearn.classification.AutoSklearnClassifier(
>>>     include_estimators=["random_forest", ], exclude_estimators=None,
>>>     include_preprocessors=["no_preprocessing", ], exclude_preprocessors=None)
>>> automl.fit(X_train, y_train)
>>> predictions = automl.predict(X_test)


Note: The strings used to identify estimators and preprocessors are the filenames without .py.

For a full list please have a look at the source code (in autosklearn/pipeline/components/):

### Turning off preprocessing¶

Preprocessing in auto-sklearn is divided into data preprocessing and feature preprocessing. Data preprocessing includes One-Hot encoding of categorical features, imputation of missing values and the normalization of features or samples. These steps currently cannot be turned off. Feature preprocessing is a single transformer which implements for example feature selection or transformation of features into a different space (i.e. PCA). This can be turned off by setting include_preprocessors=["no_preprocessing"] as shown in the example above.

## Resampling strategies¶

Examples for using holdout and cross-validation can be found in auto-sklearn/examples/

## Parallel computation¶

auto-sklearn supports parallel execution by data sharing on a shared file system. In this mode, the SMAC algorithm shares the training data for it’s model by writing it to disk after every iteration. At the beginning of each iteration, SMAC loads all newly found data points. An example can be found in the example directory.

In it’s default mode, auto-sklearn already uses two cores. The first one is used for model building, the second for building an ensemble every time a new machine learning model has finished training. The file example_sequential .py in the example directory describes how to run these tasks sequentially to use only a single core at a time.

Furthermore, depending on the installation of scikit-learn and numpy, the model building procedure may use up to all cores. Such behaviour is unintended by auto-sklearn and is most likely due to numpy being installed from pypi as a binary wheel (see here). Executing export OPENBLAS_NUM_THREADS=1 should disable such behaviours and make numpy only use a single core at a time.

## Model persistence¶

auto-sklearn is mostly a wrapper around scikit-learn. Therefore, it is possible to follow the persistence example from scikit-learn.

## Vanilla auto-sklearn¶

In order to obtain vanilla auto-sklearn as used in Efficient and Robust Automated Machine Learning set ensemble_size=1 and initial_configurations_via_metalearning=0:

>>> import autosklearn.classification
>>> automl = autosklearn.classification.AutoSklearnClassifier(
>>>     ensemble_size=1, initial_configurations_via_metalearning=0)


An ensemble of size one will result in always choosing the current best model according to its performance on the validation set. Setting the initial configurations found by meta-learning to zero makes auto-sklearn use the regular SMAC algorithm for suggesting new hyperparameter configurations.