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[WIP] Add chosen metric argument to clarify early stopping behaviour #6424

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[WIP] Add chosen metric argument to clarify early stopping behaviour #6424

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4ce23bd
feat: add chosen_metric attribute in early stopping callback class
sami-ka Apr 20, 2024
dbdc17c
feat: add chosen_metric_early_stopping parameter to create early stop…
sami-ka Apr 20, 2024
53698f7
feat: add chosen_metric_early_stopping to the list of parameters
sami-ka Apr 20, 2024
44fcae2
refactor: replace argument name and add chekcs +warning
sami-ka Apr 26, 2024
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3 changes: 3 additions & 0 deletions include/LightGBM/config.h
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Expand Up @@ -397,6 +397,9 @@ struct Config {
// desc = LightGBM allows you to provide multiple evaluation metrics. Set this to ``true``, if you want to use only the first metric for early stopping
bool first_metric_only = false;

// desc = LightGBM allows you to provide multiple evaluation metrics. Set this to a specific metric name, if you want to use only this metric for early stopping
std::string chosen_metric_early_stopping;
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Thinking about this some more... I don't think we should add this as a parameter in LightGBM's C API.

Right now, LightGBM (across all its interfaces), has this mix of behaviors:

  • you can provide multiple metrics via the metric parameter
  • if you set early_stopping_rounds > 0 and provide any validation sets, LightGBM will try to perform early stopping based on all metrics and all validation sets
    • ... unless you set first_metric_only = true, in which case LightGBM will perform early stopping on only 1 metric (but still for all validation sets)

related: #6360

Two types of behavior rely on that metric parameter:

  • which metrics should be computed and logged/recorded during training?
  • which metrics should be used for early stopping?

We still want to provide the ability to record multiple metrics during training.

In addition, the CLI and C API don't have a concept of "callbacks", so a parameter metric_name that only accepts a single metric wouldn't be sufficient for them if they want to perform early stopping on the basis of multiple metrics.

We also have to think carefully about what breaking changes (if any) to make to LightGBM's existing behavior of automatically performing early stopping on all metrics if you enable early stopping at all.

I'm not sure what direction to set you on... need to think about this for a few days.

This is has been a quite complicated part of LightGBM's interface, I'd like to simplify it and give people finer control, but also do that in a way that minimizes the number of breaking changes made.

For example, maybe we could turn off the "automatically add the early stopping callback based on params" behavior if any lgb.early_stopping callbacks are passed through callbacks.

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Thanks for your explanations, I now also realize all the implications of this change, adjusting the python part with others!

I also understand that being able to specify the metric_name in the parameters dict would be preferable, as other early stopping parameters can be specified here as well. However feel free to tell me to undo the changes outside of the Callback class, if it helps to split this in different PRs.
My tests with the callback API changes alone have the expected behaviour.

I will try to take a bit more look at the C API and give you my 2cents during the week-end about how the change could be implemented. I don't expect to come up with the solution, but I guess it could help you to take a decision.

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Sure, thanks! I just want to be sure we're respectful of your time and limit how often we ask you to do something and then to undo it.

This is a part of LightGBM (and the Python package) that has to be handled with extreme care. Early stopping is a very important part of training GBDTs, and lots of existing code out in the world relies on the existing behavior.

If you want some more background on that,you might find this discussion useful: #5808


// alias = max_tree_output, max_leaf_output
// desc = used to limit the max output of tree leaves
// desc = ``<= 0`` means no constraint
Expand Down
39 changes: 37 additions & 2 deletions python-package/lightgbm/callback.py
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Expand Up @@ -279,16 +279,29 @@ def __init__(
first_metric_only: bool = False,
verbose: bool = True,
min_delta: Union[float, List[float]] = 0.0,
metric_name: Optional[str] = None,
) -> None:
self.enabled = _should_enable_early_stopping(stopping_rounds)

# Test if both parameters are used
if first_metric_only and (metric_name is not None):
error_msg = """
Only one of 'first_metric_only' and 'chosen_metric' should be used"""
raise ValueError(error_msg)

# If metric_name is used, min_delta must be a scalar
if isinstance(min_delta, list) and (metric_name is not None):
error_msg = "Use a scalar value for 'min_delta' when using 'chosen_metric'."
raise ValueError(error_msg)

self.order = 30
self.before_iteration = False

self.stopping_rounds = stopping_rounds
self.first_metric_only = first_metric_only
self.verbose = verbose
self.min_delta = min_delta
self.metric_name = metric_name

self._reset_storages()

Expand Down Expand Up @@ -345,7 +358,13 @@ def _init(self, env: CallbackEnv) -> None:

self._reset_storages()

n_metrics = len({m[1] for m in env.evaluation_result_list})
set_metrics = {m[1] for m in env.evaluation_result_list}
if (self.metric_name is not None) and (self.metric_name not in set_metrics):
error_message = f"""Chosen callback metric:{self.metric_name} is not in the evaluation list.
The set of available metrics for early stopping is : {set_metrics}."""
raise ValueError(error_message)

n_metrics = len(set_metrics)
n_datasets = len(env.evaluation_result_list) // n_metrics
if isinstance(self.min_delta, list):
if not all(t >= 0 for t in self.min_delta):
Expand All @@ -367,7 +386,7 @@ def _init(self, env: CallbackEnv) -> None:
else:
if self.min_delta < 0:
raise ValueError("Early stopping min_delta must be non-negative.")
if self.min_delta > 0 and n_metrics > 1 and not self.first_metric_only and self.verbose:
if self.min_delta > 0 and n_metrics > 1 and not self.first_metric_only and (self.metric_name is None) and self.verbose:
_log_info(f"Using {self.min_delta} as min_delta for all metrics.")
deltas = [self.min_delta] * n_datasets * n_metrics

Expand All @@ -391,6 +410,8 @@ def _final_iteration_check(self, env: CallbackEnv, eval_name_splitted: List[str]
)
if self.first_metric_only:
_log_info(f"Evaluated only: {eval_name_splitted[-1]}")
if self.metric_name is not None:
_log_info(f"Evaluated only: {self.metric_name}")
raise EarlyStopException(self.best_iter[i], self.best_score_list[i])

def __call__(self, env: CallbackEnv) -> None:
Expand Down Expand Up @@ -418,6 +439,8 @@ def __call__(self, env: CallbackEnv) -> None:
eval_name_splitted = env.evaluation_result_list[i][1].split(" ")
if self.first_metric_only and self.first_metric != eval_name_splitted[-1]:
continue # use only the first metric for early stopping
if (self.metric_name is not None) and self.metric_name != eval_name_splitted[-1]:
continue # use only the first metric for early stopping
if self._is_train_set(
ds_name=env.evaluation_result_list[i][0],
eval_name=eval_name_splitted[0],
Expand All @@ -432,6 +455,8 @@ def __call__(self, env: CallbackEnv) -> None:
_log_info(f"Early stopping, best iteration is:\n[{self.best_iter[i] + 1}]\t{eval_result_str}")
if self.first_metric_only:
_log_info(f"Evaluated only: {eval_name_splitted[-1]}")
if self.metric_name is not None:
_log_info(f"Evaluated only: {self.metric_name}")
raise EarlyStopException(self.best_iter[i], self.best_score_list[i])
self._final_iteration_check(env, eval_name_splitted, i)

Expand All @@ -453,6 +478,7 @@ def early_stopping(
first_metric_only: bool = False,
verbose: bool = True,
min_delta: Union[float, List[float]] = 0.0,
metric_name: Optional[str] = None,
) -> _EarlyStoppingCallback:
"""Create a callback that activates early stopping.

Expand Down Expand Up @@ -487,9 +513,18 @@ def early_stopping(
callback : _EarlyStoppingCallback
The callback that activates early stopping.
"""

if first_metric_only:
warning_message = """
'first_metric_only' parameter is deprecated.
It will be removed in a future release of lightgbm.
"""
_log_warning(warning_message)

return _EarlyStoppingCallback(
stopping_rounds=stopping_rounds,
first_metric_only=first_metric_only,
verbose=verbose,
min_delta=min_delta,
metric_name=metric_name
)
16 changes: 15 additions & 1 deletion python-package/lightgbm/engine.py
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Expand Up @@ -191,7 +191,13 @@ def train(
if params["early_stopping_round"] is None:
params.pop("early_stopping_round")
first_metric_only = params.get("first_metric_only", False)

chosen_metric_early_stopping = params.get("chosen_metric_early_stopping", None)
# Test if both parameters are used
if (first_metric_only + (chosen_metric_early_stopping is not None)) == 2:
error_message = """
Only one of first_metric_only and chosen_metric_early_stopping parameters should be used"""
raise ValueError(error_message)

predictor: Optional[_InnerPredictor] = None
if isinstance(init_model, (str, Path)):
predictor = _InnerPredictor.from_model_file(model_file=init_model, pred_parameter=params)
Expand Down Expand Up @@ -241,6 +247,7 @@ def train(
callback.early_stopping(
stopping_rounds=params["early_stopping_round"], # type: ignore[arg-type]
first_metric_only=first_metric_only,
chosen_metric=chosen_metric_early_stopping,
verbose=_choose_param_value(
main_param_name="verbosity",
params=params,
Expand Down Expand Up @@ -716,6 +723,12 @@ def cv(
if params["early_stopping_round"] is None:
params.pop("early_stopping_round")
first_metric_only = params.get("first_metric_only", False)
chosen_metric_early_stopping = params.get("chosen_metric_early_stopping", None)
# Test if both parameters are used
if (first_metric_only + (chosen_metric_early_stopping is not None)) == 2:
error_message = """
Only one of first_metric_only and chosen_metric_early_stopping parameters should be used"""
raise ValueError(error_message)

if isinstance(init_model, (str, Path)):
predictor = _InnerPredictor.from_model_file(
Expand Down Expand Up @@ -765,6 +778,7 @@ def cv(
callback.early_stopping(
stopping_rounds=params["early_stopping_round"], # type: ignore[arg-type]
first_metric_only=first_metric_only,
chosen_metric=chosen_metric_early_stopping,
verbose=_choose_param_value(
main_param_name="verbosity",
params=params,
Expand Down
1 change: 1 addition & 0 deletions src/io/config_auto.cpp
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Expand Up @@ -815,6 +815,7 @@ const std::unordered_map<std::string, std::vector<std::string>>& Config::paramet
{"extra_seed", {}},
{"early_stopping_round", {"early_stopping_rounds", "early_stopping", "n_iter_no_change"}},
{"first_metric_only", {}},
{"chosen_metric_early_stopping", {}},
{"max_delta_step", {"max_tree_output", "max_leaf_output"}},
{"lambda_l1", {"reg_alpha", "l1_regularization"}},
{"lambda_l2", {"reg_lambda", "lambda", "l2_regularization"}},
Expand Down
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