Spark ML - Tuning

ml-tuning

Description

Perform hyper-parameter tuning using either K-fold cross validation or train-validation split.

Usage

ml_sub_models(model)

ml_validation_metrics(model)

ml_cross_validator(
  x,
  estimator = NULL,
  estimator_param_maps = NULL,
  evaluator = NULL,
  num_folds = 3,
  collect_sub_models = FALSE,
  parallelism = 1,
  seed = NULL,
  uid = random_string("cross_validator_"),
  ...
)

ml_train_validation_split(
  x,
  estimator = NULL,
  estimator_param_maps = NULL,
  evaluator = NULL,
  train_ratio = 0.75,
  collect_sub_models = FALSE,
  parallelism = 1,
  seed = NULL,
  uid = random_string("train_validation_split_"),
  ...
)

Arguments

Arguments	Description
model	A cross validation or train-validation-split model.
x	A spark_connection, ml_pipeline, or a tbl_spark.
estimator	A ml_estimator object.
estimator_param_maps	A named list of stages and hyper-parameter sets to tune. See details.
evaluator	A ml_evaluator object, see ml_evaluator.
num_folds	Number of folds for cross validation. Must be >= 2. Default: 3
collect_sub_models	Whether to collect a list of sub-models trained during tuning. If set to FALSE, then only the single best sub-model will be available after fitting. If set to true, then all sub-models will be available. Warning: For large models, collecting all sub-models can cause OOMs on the Spark driver.
parallelism	The number of threads to use when running parallel algorithms. Default is 1 for serial execution.
seed	A random seed. Set this value if you need your results to be reproducible across repeated calls.
uid	A character string used to uniquely identify the ML estimator.
…	Optional arguments; currently unused.
train_ratio	Ratio between train and validation data. Must be between 0 and 1. Default: 0.75

Details

ml_cross_validator() performs k-fold cross validation while ml_train_validation_split() performs tuning on one pair of train and validation datasets.

Value

The object returned depends on the class of x.

• spark_connection: When x is a spark_connection, the function returns an instance of a ml_cross_validator or ml_traing_validation_split object.

• ml_pipeline: When x is a ml_pipeline, the function returns a ml_pipeline with the tuning estimator appended to the pipeline.

• tbl_spark: When x is a tbl_spark, a tuning estimator is constructed then immediately fit with the input tbl_spark, returning a ml_cross_validation_model or a ml_train_validation_split_model object.

For cross validation, ml_sub_models() returns a nested list of models, where the first layer represents fold indices and the second layer represents param maps. For train-validation split, ml_sub_models() returns a list of models, corresponding to the order of the estimator param maps.

ml_validation_metrics() returns a data frame of performance metrics and hyperparameter combinations.

Examples

sc <- spark_connect(master = "local")
iris_tbl <- sdf_copy_to(sc, iris, name = "iris_tbl", overwrite = TRUE)

# Create a pipeline
pipeline <- ml_pipeline(sc) %>%
  ft_r_formula(Species ~ .) %>%
  ml_random_forest_classifier()

# Specify hyperparameter grid
grid <- list(
  random_forest = list(
    num_trees = c(5, 10),
    max_depth = c(5, 10),
    impurity = c("entropy", "gini")
  )
)

# Create the cross validator object
cv <- ml_cross_validator(
  sc,
  estimator = pipeline, estimator_param_maps = grid,
  evaluator = ml_multiclass_classification_evaluator(sc),
  num_folds = 3,
  parallelism = 4
)

# Train the models
cv_model <- ml_fit(cv, iris_tbl)

# Print the metrics
ml_validation_metrics(cv_model)