# optuna.pruners.SuccessiveHalvingPruner¶

class optuna.pruners.SuccessiveHalvingPruner(min_resource: Union[str, int] = 'auto', reduction_factor: int = 4, min_early_stopping_rate: int = 0)[source]

Pruner using Asynchronous Successive Halving Algorithm.

Successive Halving is a bandit-based algorithm to identify the best one among multiple configurations. This class implements an asynchronous version of Successive Halving. Please refer to the paper of Asynchronous Successive Halving for detailed descriptions.

Note that, this class does not take care of the parameter for the maximum resource, referred to as $$R$$ in the paper. The maximum resource allocated to a trial is typically limited inside the objective function (e.g., step number in simple.py, EPOCH number in chainer_integration.py).

Please refer to report().

Example

We minimize an objective function with SuccessiveHalvingPruner.

import numpy as np
from sklearn.linear_model import SGDClassifier
from sklearn.model_selection import train_test_split

import optuna

X_train, X_valid, y_train, y_valid = train_test_split(X, y)
classes = np.unique(y)

def objective(trial):
alpha = trial.suggest_uniform('alpha', 0.0, 1.0)
clf = SGDClassifier(alpha=alpha)
n_train_iter = 100

for step in range(n_train_iter):
clf.partial_fit(X_train, y_train, classes=classes)

intermediate_value = clf.score(X_valid, y_valid)
trial.report(intermediate_value, step)

if trial.should_prune():
raise optuna.TrialPruned()

return clf.score(X_valid, y_valid)

study = optuna.create_study(direction='maximize',
pruner=optuna.pruners.SuccessiveHalvingPruner())
study.optimize(objective, n_trials=20)

Parameters
• min_resource

A parameter for specifying the minimum resource allocated to a trial (in the paper this parameter is referred to as $$r$$). This parameter defaults to ‘auto’ where the value is determined based on a heuristic that looks at the number of required steps for the first trial to complete.

A trial is never pruned until it executes $$\mathsf{min}\_\mathsf{resource} \times \mathsf{reduction}\_\mathsf{factor}^{ \mathsf{min}\_\mathsf{early}\_\mathsf{stopping}\_\mathsf{rate}}$$ steps (i.e., the completion point of the first rung). When the trial completes the first rung, it will be promoted to the next rung only if the value of the trial is placed in the top $${1 \over \mathsf{reduction}\_\mathsf{factor}}$$ fraction of the all trials that already have reached the point (otherwise it will be pruned there). If the trial won the competition, it runs until the next completion point (i.e., $$\mathsf{min}\_\mathsf{resource} \times \mathsf{reduction}\_\mathsf{factor}^{ (\mathsf{min}\_\mathsf{early}\_\mathsf{stopping}\_\mathsf{rate} + \mathsf{rung})}$$ steps) and repeats the same procedure.

Note

If the step of the last intermediate value may change with each trial, please manually specify the minimum possible step to min_resource.

• reduction_factor

A parameter for specifying reduction factor of promotable trials (in the paper this parameter is referred to as $$\eta$$). At the completion point of each rung, about $${1 \over \mathsf{reduction}\_\mathsf{factor}}$$ trials will be promoted.

• min_early_stopping_rate

A parameter for specifying the minimum early-stopping rate (in the paper this parameter is referred to as $$s$$).

__init__(min_resource: Union[str, int] = 'auto', reduction_factor: int = 4, min_early_stopping_rate: int = 0)None[source]

Initialize self. See help(type(self)) for accurate signature.

Methods

 __init__([min_resource, reduction_factor, …]) Initialize self. prune(study, trial) Judge whether the trial should be pruned based on the reported values.
prune(study: optuna.study.Study, trial: optuna.trial._frozen.FrozenTrial)bool[source]

Judge whether the trial should be pruned based on the reported values.

Note that this method is not supposed to be called by library users. Instead, optuna.trial.Trial.report() and optuna.trial.Trial.should_prune() provide user interfaces to implement pruning mechanism in an objective function.

Parameters
• study – Study object of the target study.

• trial – FrozenTrial object of the target trial. Take a copy before modifying this object.

Returns

A boolean value representing whether the trial should be pruned.