docs/ml-tuning.md

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title: "ML Tuning"
displayTitle: "ML Tuning: model selection and hyperparameter tuning"
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This section describes how to use MLlib's tooling for tuning ML algorithms and Pipelines.
Built-in Cross-Validation and other tooling allow users to optimize hyperparameters in algorithms and Pipelines.

**Table of contents**

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# Model selection (a.k.a. hyperparameter tuning)

An important task in ML is *model selection*, or using data to find the best model or parameters for a given task.  This is also called *tuning*.
Tuning may be done for individual `Estimator`s such as `LogisticRegression`, or for entire `Pipeline`s which include multiple algorithms, featurization, and other steps.  Users can tune an entire `Pipeline` at once, rather than tuning each element in the `Pipeline` separately.

MLlib supports model selection using tools such as [`CrossValidator`](api/scala/index.html#org.apache.spark.ml.tuning.CrossValidator) and [`TrainValidationSplit`](api/scala/index.html#org.apache.spark.ml.tuning.TrainValidationSplit).
These tools require the following items:

* [`Estimator`](api/scala/index.html#org.apache.spark.ml.Estimator): algorithm or `Pipeline` to tune
* Set of `ParamMap`s: parameters to choose from, sometimes called a "parameter grid" to search over
* [`Evaluator`](api/scala/index.html#org.apache.spark.ml.evaluation.Evaluator): metric to measure how well a fitted `Model` does on held-out test data

At a high level, these model selection tools work as follows:

* They split the input data into separate training and test datasets.
* For each (training, test) pair, they iterate through the set of `ParamMap`s:
  * For each `ParamMap`, they fit the `Estimator` using those parameters, get the fitted `Model`, and evaluate the `Model`'s performance using the `Evaluator`.
* They select the `Model` produced by the best-performing set of parameters.

The `Evaluator` can be a [`RegressionEvaluator`](api/scala/index.html#org.apache.spark.ml.evaluation.RegressionEvaluator)
for regression problems, a [`BinaryClassificationEvaluator`](api/scala/index.html#org.apache.spark.ml.evaluation.BinaryClassificationEvaluator)
for binary data, or a [`MulticlassClassificationEvaluator`](api/scala/index.html#org.apache.spark.ml.evaluation.MulticlassClassificationEvaluator)
for multiclass problems. The default metric used to choose the best `ParamMap` can be overridden by the `setMetricName`
method in each of these evaluators.

To help construct the parameter grid, users can use the [`ParamGridBuilder`](api/scala/index.html#org.apache.spark.ml.tuning.ParamGridBuilder) utility.
By default, sets of parameters from the parameter grid are evaluated in serial. Parameter evaluation can be done in parallel by setting `parallelism` with a value of 2 or more (a value of 1 will be serial) before running model selection with `CrossValidator` or `TrainValidationSplit`.
The value of `parallelism` should be chosen carefully to maximize parallelism without exceeding cluster resources, and larger values may not always lead to improved performance.  Generally speaking, a value up to 10 should be sufficient for most clusters.

# Cross-Validation

`CrossValidator` begins by splitting the dataset into a set of *folds* which are used as separate training and test datasets. E.g., with `$k=3$` folds, `CrossValidator` will generate 3 (training, test) dataset pairs, each of which uses 2/3 of the data for training and 1/3 for testing.  To evaluate a particular `ParamMap`, `CrossValidator` computes the average evaluation metric for the 3 `Model`s produced by fitting the `Estimator` on the 3 different (training, test) dataset pairs.

After identifying the best `ParamMap`, `CrossValidator` finally re-fits the `Estimator` using the best `ParamMap` and the entire dataset.

**Examples: model selection via cross-validation**

The following example demonstrates using `CrossValidator` to select from a grid of parameters.

Note that cross-validation over a grid of parameters is expensive.
E.g., in the example below, the parameter grid has 3 values for `hashingTF.numFeatures` and 2 values for `lr.regParam`, and `CrossValidator` uses 2 folds.  This multiplies out to `$(3 \times 2) \times 2 = 12$` different models being trained.
In realistic settings, it can be common to try many more parameters and use more folds (`$k=3$` and `$k=10$` are common).
In other words, using `CrossValidator` can be very expensive.
However, it is also a well-established method for choosing parameters which is more statistically sound than heuristic hand-tuning.

<div class="codetabs">

<div data-lang="scala" markdown="1">

Refer to the [`CrossValidator` Scala docs](api/scala/index.html#org.apache.spark.ml.tuning.CrossValidator) for details on the API.

{% include_example scala/org/apache/spark/examples/ml/ModelSelectionViaCrossValidationExample.scala %}
</div>

<div data-lang="java" markdown="1">

Refer to the [`CrossValidator` Java docs](api/java/org/apache/spark/ml/tuning/CrossValidator.html) for details on the API.

{% include_example java/org/apache/spark/examples/ml/JavaModelSelectionViaCrossValidationExample.java %}
</div>

<div data-lang="python" markdown="1">

Refer to the [`CrossValidator` Python docs](api/python/pyspark.ml.html#pyspark.ml.tuning.CrossValidator) for more details on the API.

{% include_example python/ml/cross_validator.py %}
</div>

</div>

# Train-Validation Split

In addition to  `CrossValidator` Spark also offers `TrainValidationSplit` for hyper-parameter tuning.
`TrainValidationSplit` only evaluates each combination of parameters once, as opposed to k times in
 the case of `CrossValidator`. It is, therefore, less expensive,
 but will not produce as reliable results when the training dataset is not sufficiently large.

Unlike `CrossValidator`, `TrainValidationSplit` creates a single (training, test) dataset pair.
It splits the dataset into these two parts using the `trainRatio` parameter. For example with `$trainRatio=0.75$`,
`TrainValidationSplit` will generate a training and test dataset pair where 75% of the data is used for training and 25% for validation.

Like `CrossValidator`, `TrainValidationSplit` finally fits the `Estimator` using the best `ParamMap` and the entire dataset.

**Examples: model selection via train validation split**

<div class="codetabs">

<div data-lang="scala" markdown="1">

Refer to the [`TrainValidationSplit` Scala docs](api/scala/index.html#org.apache.spark.ml.tuning.TrainValidationSplit) for details on the API.

{% include_example scala/org/apache/spark/examples/ml/ModelSelectionViaTrainValidationSplitExample.scala %}
</div>

<div data-lang="java" markdown="1">

Refer to the [`TrainValidationSplit` Java docs](api/java/org/apache/spark/ml/tuning/TrainValidationSplit.html) for details on the API.

{% include_example java/org/apache/spark/examples/ml/JavaModelSelectionViaTrainValidationSplitExample.java %}
</div>

<div data-lang="python" markdown="1">

Refer to the [`TrainValidationSplit` Python docs](api/python/pyspark.ml.html#pyspark.ml.tuning.TrainValidationSplit) for more details on the API.

{% include_example python/ml/train_validation_split.py %}
</div>

</div>