lr_scheduler.py

"""
    LR scheduler from BasicSR https://github.com/xinntao/BasicSR
"""
import math
from collections import Counter
from torch.optim.lr_scheduler import _LRScheduler


class MultiStepRestartLR(_LRScheduler):
    """ MultiStep with restarts learning rate scheme.
    Args:
        optimizer (torch.nn.optimizer): Torch optimizer.
        milestones (list): Iterations that will decrease learning rate.
        gamma (float): Decrease ratio. Default: 0.1.
        restarts (list): Restart iterations. Default: [0].
        restart_weights (list): Restart weights at each restart iteration.
            Default: [1].
        last_epoch (int): Used in _LRScheduler. Default: -1.
    """
    def __init__(self,
                 optimizer,
                 milestones,
                 gamma=0.1,
                 restarts=(0, ),
                 restart_weights=(1, ),
                 last_epoch=-1):
        self.milestones = Counter(milestones)
        self.gamma = gamma
        self.restarts = restarts
        self.restart_weights = restart_weights
        assert len(self.restarts) == len(
            self.restart_weights), 'restarts and their weights do not match.'
        super(MultiStepRestartLR, self).__init__(optimizer, last_epoch)

    def get_lr(self):
        if self.last_epoch in self.restarts:
            weight = self.restart_weights[self.restarts.index(self.last_epoch)]
            return [
                group['initial_lr'] * weight
                for group in self.optimizer.param_groups
            ]
        if self.last_epoch not in self.milestones:
            return [group['lr'] for group in self.optimizer.param_groups]
        return [
            group['lr'] * self.gamma**self.milestones[self.last_epoch]
            for group in self.optimizer.param_groups
        ]


def get_position_from_periods(iteration, cumulative_period):
    """Get the position from a period list.
    It will return the index of the right-closest number in the period list.
    For example, the cumulative_period = [100, 200, 300, 400],
    if iteration == 50, return 0;
    if iteration == 210, return 2;
    if iteration == 300, return 2.
    Args:
        iteration (int): Current iteration.
        cumulative_period (list[int]): Cumulative period list.
    Returns:
        int: The position of the right-closest number in the period list.
    """
    for i, period in enumerate(cumulative_period):
        if iteration <= period:
            return i


class CosineAnnealingRestartLR(_LRScheduler):
    """ Cosine annealing with restarts learning rate scheme.
    An example of config:
    periods = [10, 10, 10, 10]
    restart_weights = [1, 0.5, 0.5, 0.5]
    eta_min=1e-7
    It has four cycles, each has 10 iterations. At 10th, 20th, 30th, the
    scheduler will restart with the weights in restart_weights.
    Args:
        optimizer (torch.nn.optimizer): Torch optimizer.
        periods (list): Period for each cosine anneling cycle.
        restart_weights (list): Restart weights at each restart iteration.
            Default: [1].
        eta_min (float): The mimimum lr. Default: 0.
        last_epoch (int): Used in _LRScheduler. Default: -1.
    """
    def __init__(self,
                 optimizer,
                 periods,
                 restart_weights=(1, ),
                 eta_min=1e-7,
                 last_epoch=-1):
        self.periods = periods
        self.restart_weights = restart_weights
        self.eta_min = eta_min
        assert (len(self.periods) == len(self.restart_weights)
                ), 'periods and restart_weights should have the same length.'
        self.cumulative_period = [
            sum(self.periods[0:i + 1]) for i in range(0, len(self.periods))
        ]
        super(CosineAnnealingRestartLR, self).__init__(optimizer, last_epoch)

    def get_lr(self):
        idx = get_position_from_periods(self.last_epoch,
                                        self.cumulative_period)
        current_weight = self.restart_weights[idx]
        nearest_restart = 0 if idx == 0 else self.cumulative_period[idx - 1]
        current_period = self.periods[idx]

        return [
            self.eta_min + current_weight * 0.5 * (base_lr - self.eta_min) *
            (1 + math.cos(math.pi * (
                (self.last_epoch - nearest_restart) / current_period)))
            for base_lr in self.base_lrs
        ]