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Source code for ignite.contrib.handlers.lr_finder

# coding: utf-8
import contextlib
import logging
import tempfile
import warnings
from pathlib import Path
from typing import Any, Callable, Dict, List, Mapping, Optional, Union

import torch
from torch.optim import Optimizer
from torch.optim.lr_scheduler import _LRScheduler

from ignite.contrib.handlers.param_scheduler import LRScheduler, PiecewiseLinear
from ignite.engine import Engine, Events
from ignite.handlers import Checkpoint


[docs]class FastaiLRFinder: """Learning rate finder handler for supervised trainers. While attached, the handler increases the learning rate in between two boundaries in a linear or exponential manner. It provides valuable information on how well the network can be trained over a range of learning rates and what can be an optimal learning rate. Examples: .. code-block:: python from ignite.contrib.handlers import FastaiLRFinder trainer = ... model = ... optimizer = ... lr_finder = FastaiLRFinder() to_save = {"model": model, "optimizer": optimizer} with lr_finder.attach(trainer, to_save=to_save) as trainer_with_lr_finder: trainer_with_lr_finder.run(dataloader) # Get lr_finder results lr_finder.get_results() # Plot lr_finder results (requires matplotlib) lr_finder.plot() # get lr_finder suggestion for lr lr_finder.lr_suggestion() Note: When context manager is exited all LR finder's handlers are removed. Note: Please, also keep in mind that all other handlers attached the trainer will be executed during LR finder's run. Note: This class may require `matplotlib` package to be installed to plot learning rate range test: .. code-block:: bash pip install matplotlib References: Cyclical Learning Rates for Training Neural Networks: https://arxiv.org/abs/1506.01186 fastai/lr_find: https://github.com/fastai/fastai """ def __init__(self) -> None: self._diverge_flag = False self._history = {} # type: Dict[str, List[Any]] self._best_loss = None self._lr_schedule = None # type: Optional[Union[LRScheduler, PiecewiseLinear]] self.logger = logging.getLogger(__name__) def _run( self, trainer: Engine, optimizer: Optimizer, output_transform: Callable, num_iter: int, end_lr: float, step_mode: str, smooth_f: float, diverge_th: float, ) -> None: self._history = {"lr": [], "loss": []} self._best_loss = None self._diverge_flag = False # attach LRScheduler to trainer. if num_iter is None: num_iter = trainer.state.epoch_length * trainer.state.max_epochs else: max_iter = trainer.state.epoch_length * trainer.state.max_epochs # type: ignore[operator] if num_iter > max_iter: warnings.warn( f"Desired num_iter {num_iter} is unreachable with the current run setup of {max_iter} iteration " f"({trainer.state.max_epochs} epochs)", UserWarning, ) if not trainer.has_event_handler(self._reached_num_iterations): trainer.add_event_handler(Events.ITERATION_COMPLETED, self._reached_num_iterations, num_iter) # attach loss and lr logging if not trainer.has_event_handler(self._log_lr_and_loss): trainer.add_event_handler( Events.ITERATION_COMPLETED, self._log_lr_and_loss, output_transform, smooth_f, diverge_th ) self.logger.debug(f"Running LR finder for {num_iter} iterations") # Initialize the proper learning rate policy if step_mode.lower() == "exp": self._lr_schedule = LRScheduler(_ExponentialLR(optimizer, end_lr, num_iter)) else: start_lr = optimizer.param_groups[0]["lr"] self._lr_schedule = PiecewiseLinear( optimizer, param_name="lr", milestones_values=[(0, start_lr), (num_iter, end_lr)] ) if not trainer.has_event_handler(self._lr_schedule): trainer.add_event_handler(Events.ITERATION_COMPLETED, self._lr_schedule, num_iter) def _reset(self, trainer: Engine) -> None: self.logger.debug("Completed LR finder run") trainer.remove_event_handler(self._lr_schedule, Events.ITERATION_COMPLETED) # type: ignore[arg-type] trainer.remove_event_handler(self._log_lr_and_loss, Events.ITERATION_COMPLETED) trainer.remove_event_handler(self._reached_num_iterations, Events.ITERATION_COMPLETED) def _log_lr_and_loss(self, trainer: Engine, output_transform: Callable, smooth_f: float, diverge_th: float) -> None: output = trainer.state.output loss = output_transform(output) lr = self._lr_schedule.get_param() # type: ignore[union-attr] self._history["lr"].append(lr) if trainer.state.iteration == 1: self._best_loss = loss else: if smooth_f > 0: loss = smooth_f * loss + (1 - smooth_f) * self._history["loss"][-1] if loss < self._best_loss: self._best_loss = loss self._history["loss"].append(loss) # Check if the loss has diverged; if it has, stop the trainer if self._history["loss"][-1] > diverge_th * self._best_loss: # type: ignore[operator] self._diverge_flag = True self.logger.info("Stopping early, the loss has diverged") trainer.terminate() def _reached_num_iterations(self, trainer: Engine, num_iter: int) -> None: if trainer.state.iteration > num_iter: trainer.terminate() def _warning(self, _: Any) -> None: if not self._diverge_flag: warnings.warn( "Run completed without loss diverging, increase end_lr, decrease diverge_th or look" " at lr_finder.plot()", UserWarning, ) def _detach(self, trainer: Engine) -> None: """ Detaches lr_finder from trainer. Args: trainer: the trainer to detach form. """ if trainer.has_event_handler(self._run, Events.STARTED): trainer.remove_event_handler(self._run, Events.STARTED) if trainer.has_event_handler(self._warning, Events.COMPLETED): trainer.remove_event_handler(self._warning, Events.COMPLETED) if trainer.has_event_handler(self._reset, Events.COMPLETED): trainer.remove_event_handler(self._reset, Events.COMPLETED)
[docs] def get_results(self) -> Dict[str, List[Any]]: """ Returns: dictionary with loss and lr logs fromm the previous run """ return self._history
[docs] def plot(self, skip_start: int = 10, skip_end: int = 5, log_lr: bool = True) -> None: """Plots the learning rate range test. This method requires `matplotlib` package to be installed: .. code-block:: bash pip install matplotlib Args: skip_start: number of batches to trim from the start. Default: 10. skip_end: number of batches to trim from the start. Default: 5. log_lr: True to plot the learning rate in a logarithmic scale; otherwise, plotted in a linear scale. Default: True. """ try: from matplotlib import pyplot as plt except ImportError: raise RuntimeError( "This method requires matplotlib to be installed. " "Please install it with command: \n pip install matplotlib" ) if not self._history: raise RuntimeError("learning rate finder didn't run yet so results can't be plotted") if skip_start < 0: raise ValueError("skip_start cannot be negative") if skip_end < 0: raise ValueError("skip_end cannot be negative") # Get the data to plot from the history dictionary. Also, handle skip_end=0 # properly so the behaviour is the expected lrs = self._history["lr"] losses = self._history["loss"] if skip_end == 0: lrs = lrs[skip_start:] losses = losses[skip_start:] else: lrs = lrs[skip_start:-skip_end] losses = losses[skip_start:-skip_end] # Plot loss as a function of the learning rate plt.plot(lrs, losses) if log_lr: plt.xscale("log") plt.xlabel("Learning rate") plt.ylabel("Loss") plt.show()
[docs] def lr_suggestion(self) -> Any: """ Returns: learning rate at the minimum numerical gradient """ if not self._history: raise RuntimeError("learning rate finder didn't run yet so lr_suggestion can't be returned") loss = self._history["loss"] grads = torch.tensor([loss[i] - loss[i - 1] for i in range(1, len(loss))]) min_grad_idx = grads.argmin() + 1 return self._history["lr"][int(min_grad_idx)]
[docs] @contextlib.contextmanager def attach( self, trainer: Engine, to_save: Mapping, output_transform: Callable = lambda output: output, num_iter: Optional[int] = None, end_lr: float = 10.0, step_mode: str = "exp", smooth_f: float = 0.05, diverge_th: float = 5.0, ) -> Any: """Attaches lr_finder to a given trainer. It also resets model and optimizer at the end of the run. Usage: .. code-block:: python to_save = {"model": model, "optimizer": optimizer} with lr_finder.attach(trainer, to_save=to_save) as trainer_with_lr_finder: trainer_with_lr_finder.run(dataloader)` Args: trainer: lr_finder is attached to this trainer. Please, keep in mind that all attached handlers will be executed. to_save: dictionary with optimizer and other objects that needs to be restored after running the LR finder. For example, `to_save={'optimizer': optimizer, 'model': model}`. All objects should implement `state_dict` and `load_state_dict` methods. output_transform: function that transforms the trainer's `state.output` after each iteration. It must return the loss of that iteration. num_iter: number of iterations for lr schedule between base lr and end_lr. Default, it will run for `trainer.state.epoch_length * trainer.state.max_epochs`. end_lr: upper bound for lr search. Default, 10.0. step_mode: "exp" or "linear", which way should the lr be increased from optimizer's initial lr to `end_lr`. Default, "exp". smooth_f: loss smoothing factor in range `[0, 1)`. Default, 0.05 diverge_th: Used for stopping the search when `current loss > diverge_th * best_loss`. Default, 5.0. Returns: trainer_with_lr_finder (trainer used for finding the lr) Note: lr_finder cannot be attached to more than one trainer at a time. """ if not isinstance(to_save, Mapping): raise TypeError(f"Argument to_save should be a mapping, but given {type(to_save)}") Checkpoint._check_objects(to_save, "state_dict") Checkpoint._check_objects(to_save, "load_state_dict") if "optimizer" not in to_save: raise ValueError("Mapping to_save should contain 'optimizer' key") if not isinstance(to_save["optimizer"], torch.optim.Optimizer): raise TypeError( f"Object to_save['optimizer'] should be torch optimizer, but given {type(to_save['optimizer'])}" ) if smooth_f < 0 or smooth_f >= 1: raise ValueError("smooth_f is outside the range [0, 1]") if diverge_th < 1: raise ValueError("diverge_th should be larger than 1") if step_mode not in ["exp", "linear"]: raise ValueError(f"step_mode should be 'exp' or 'linear', but given {step_mode}") if num_iter is not None: if not isinstance(num_iter, int): raise TypeError(f"if provided, num_iter should be an integer, but give {num_iter}") if num_iter <= 0: raise ValueError(f"if provided, num_iter should be positive, but give {num_iter}") # store to_save with tempfile.TemporaryDirectory() as tmpdirname: obj = {k: o.state_dict() for k, o in to_save.items()} # add trainer obj["trainer"] = trainer.state_dict() cache_filepath = Path(tmpdirname) / "ignite_lr_finder_cache.pt" torch.save(obj, cache_filepath.as_posix()) optimizer = to_save["optimizer"] # Attach handlers if not trainer.has_event_handler(self._run): trainer.add_event_handler( Events.STARTED, self._run, optimizer, output_transform, num_iter, end_lr, step_mode, smooth_f, diverge_th, ) if not trainer.has_event_handler(self._warning): trainer.add_event_handler(Events.COMPLETED, self._warning) if not trainer.has_event_handler(self._reset): trainer.add_event_handler(Events.COMPLETED, self._reset) yield trainer self._detach(trainer) # restore to_save and reset trainer's state obj = torch.load(cache_filepath.as_posix()) trainer.load_state_dict(obj["trainer"]) for k, o in obj.items(): if k in to_save: to_save[k].load_state_dict(o)
class _ExponentialLR(_LRScheduler): """Exponentially increases the learning rate between two boundaries over a number of iterations. Args: optimizer: wrapped optimizer. end_lr: the initial learning rate which is the lower boundary of the test. Default: 10. num_iter: the number of iterations over which the test occurs. Default: 100. last_epoch: the index of last epoch. Default: -1. """ def __init__(self, optimizer: Optimizer, end_lr: float, num_iter: int, last_epoch: int = -1): self.end_lr = end_lr self.num_iter = num_iter super(_ExponentialLR, self).__init__(optimizer, last_epoch) def get_lr(self) -> List[float]: # type: ignore curr_iter = self.last_epoch + 1 # type: ignore[attr-defined] r = curr_iter / self.num_iter return [base_lr * (self.end_lr / base_lr) ** r for base_lr in self.base_lrs] # type: ignore[attr-defined]

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