# Source code for mmengine.optim.scheduler.lr_scheduler

```
# Copyright (c) OpenMMLab. All rights reserved.
from mmengine.registry import PARAM_SCHEDULERS
# yapf: disable
from .param_scheduler import (ConstantParamScheduler,
CosineAnnealingParamScheduler,
CosineRestartParamScheduler,
ExponentialParamScheduler, LinearParamScheduler,
MultiStepParamScheduler, OneCycleParamScheduler,
PolyParamScheduler,
ReduceOnPlateauParamScheduler,
StepParamScheduler)
# yapf: enable
class LRSchedulerMixin:
"""A mixin class for learning rate schedulers."""
def __init__(self, optimizer, *args, **kwargs):
super().__init__(optimizer, 'lr', *args, **kwargs)
[docs]@PARAM_SCHEDULERS.register_module()
class ConstantLR(LRSchedulerMixin, ConstantParamScheduler):
"""Decays the learning rate value of each parameter group by a small
constant factor until the number of epoch reaches a pre-defined milestone:
``end``. Notice that such decay can happen simultaneously with other
changes to the learning rate value from outside this scheduler.
Args:
optimizer (Optimizer or OptimWrapper): Wrapped optimizer.
factor (float): The number we multiply learning rate until the
milestone. Defaults to 1./3.
begin (int): Step at which to start updating the learning rate.
Defaults to 0.
end (int): Step at which to stop updating the learning rate.
Defaults to INF.
last_step (int): The index of last step. Used for resume without state
dict. Defaults to -1.
by_epoch (bool): Whether the scheduled learning rate is updated by
epochs. Defaults to True.
verbose (bool): Whether to print the learning rate for each update.
Defaults to False.
"""
[docs]@PARAM_SCHEDULERS.register_module()
class CosineAnnealingLR(LRSchedulerMixin, CosineAnnealingParamScheduler):
r"""Set the learning rate of each parameter group using a cosine annealing
schedule, where :math:`\eta_{max}` is set to the initial value and
:math:`T_{cur}` is the number of epochs since the last restart in SGDR:
.. math::
\begin{aligned}
\eta_t & = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1
+ \cos\left(\frac{T_{cur}}{T_{max}}\pi\right)\right),
& T_{cur} \neq (2k+1)T_{max}; \\
\eta_{t+1} & = \eta_{t} + \frac{1}{2}(\eta_{max} - \eta_{min})
\left(1 - \cos\left(\frac{1}{T_{max}}\pi\right)\right),
& T_{cur} = (2k+1)T_{max}.
\end{aligned}
Notice that because the schedule
is defined recursively, the learning rate can be simultaneously modified
outside this scheduler by other operators. If the learning rate is set
solely by this scheduler, the learning rate at each step becomes:
.. math::
\eta_t = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1 +
\cos\left(\frac{T_{cur}}{T_{max}}\pi\right)\right)
It has been proposed in
`SGDR: Stochastic Gradient Descent with Warm Restarts`_. Note that this
only implements the cosine annealing part of SGDR, and not the restarts.
Args:
optimizer (Optimizer or OptimWrapper): Wrapped optimizer.
T_max (int): Maximum number of iterations.
eta_min (float): Minimum learning rate. Defaults to None.
begin (int): Step at which to start updating the learning rate.
Defaults to 0.
end (int): Step at which to stop updating the learning rate.
Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled learning rate is updated by
epochs. Defaults to True.
verbose (bool): Whether to print the learning rate for each update.
Defaults to False.
eta_min_ratio (float, optional): The ratio of the minimum parameter
value to the base parameter value. Either `eta_min` or
`eta_min_ratio` should be specified. Defaults to None.
New in version 0.3.2.
.. _SGDR\: Stochastic Gradient Descent with Warm Restarts:
https://arxiv.org/abs/1608.03983
"""
[docs]@PARAM_SCHEDULERS.register_module()
class ExponentialLR(LRSchedulerMixin, ExponentialParamScheduler):
"""Decays the learning rate of each parameter group by gamma every epoch.
Args:
optimizer (Optimizer or OptimWrapper): Wrapped optimizer.
gamma (float): Multiplicative factor of learning rate decay.
begin (int): Step at which to start updating the learning rate.
Defaults to 0.
end (int): Step at which to stop updating the learning rate.
Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled learning rate is updated by
epochs. Defaults to True.
verbose (bool): Whether to print the learning rate for each update.
Defaults to False.
"""
[docs]@PARAM_SCHEDULERS.register_module()
class LinearLR(LRSchedulerMixin, LinearParamScheduler):
"""Decays the learning rate of each parameter group by linearly changing
small multiplicative factor until the number of epoch reaches a pre-defined
milestone: ``end``.
Notice that such decay can happen simultaneously with other changes to the
learning rate from outside this scheduler.
Args:
optimizer (Optimizer or OptimWrapper): Wrapped optimizer.
start_factor (float): The number we multiply learning rate in the
first epoch. The multiplication factor changes towards end_factor
in the following epochs. Defaults to 1./3.
end_factor (float): The number we multiply learning rate at the end
of linear changing process. Defaults to 1.0.
begin (int): Step at which to start updating the learning rate.
Defaults to 0.
end (int): Step at which to stop updating the learning rate.
Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled learning rate is updated by
epochs. Defaults to True.
verbose (bool): Whether to print the learning rate for each update.
Defaults to False.
"""
[docs]@PARAM_SCHEDULERS.register_module()
class MultiStepLR(LRSchedulerMixin, MultiStepParamScheduler):
"""Decays the specified learning rate in each parameter group by gamma once
the number of epoch reaches one of the milestones. Notice that such decay
can happen simultaneously with other changes to the learning rate from
outside this scheduler.
Args:
optimizer (Optimizer or OptimWrapper): Wrapped optimizer.
milestones (list): List of epoch indices. Must be increasing.
gamma (float): Multiplicative factor of learning rate decay.
Defaults to 0.1.
begin (int): Step at which to start updating the learning rate.
Defaults to 0.
end (int): Step at which to stop updating the learning rate.
Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled learning rate is updated by
epochs. Defaults to True.
verbose (bool): Whether to print the learning rate for each update.
Defaults to False.
"""
[docs]@PARAM_SCHEDULERS.register_module()
class StepLR(LRSchedulerMixin, StepParamScheduler):
"""Decays the learning rate of each parameter group by gamma every
step_size epochs. Notice that such decay can happen simultaneously with
other changes to the learning rate from outside this scheduler.
Args:
optimizer (Optimizer or OptimWrapper): Wrapped optimizer.
step_size (int): Period of learning rate decay.
gamma (float): Multiplicative factor of learning rate decay.
Defaults to 0.1.
begin (int): Step at which to start updating the learning rate.
Defaults to 0.
end (int): Step at which to stop updating the learning rate.
Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled learning rate is updated by
epochs. Defaults to True.
verbose (bool): Whether to print the learning rate for each update.
Defaults to False.
"""
[docs]@PARAM_SCHEDULERS.register_module()
class PolyLR(LRSchedulerMixin, PolyParamScheduler):
"""Decays the learning rate of each parameter group in a polynomial decay
scheme.
Notice that such decay can happen simultaneously with other changes to the
parameter value from outside this scheduler.
Args:
optimizer (Optimizer or OptimWrapper): Wrapped optimizer.
eta_min (float): Minimum learning rate at the end of scheduling.
Defaults to 0.
power (float): The power of the polynomial. Defaults to 1.0.
begin (int): Step at which to start updating the parameters.
Defaults to 0.
end (int): Step at which to stop updating the parameters.
Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled parameters are updated by
epochs. Defaults to True.
verbose (bool): Whether to print the value for each update.
Defaults to False.
"""
[docs]@PARAM_SCHEDULERS.register_module()
class OneCycleLR(LRSchedulerMixin, OneCycleParamScheduler):
r"""Sets the learning rate of each parameter group according to the
1cycle learning rate policy. The 1cycle policy anneals the learning
rate from an initial learning rate to some maximum learning rate and then
from that maximum learning rate to some minimum learning rate much lower
than the initial learning rate.
This policy was initially described in the paper `Super-Convergence:
Very Fast Training of Neural Networks Using Large Learning Rates`_.
The 1cycle learning rate policy changes the learning rate after every
batch. `step` should be called after a batch has been used for training.
This scheduler is not chainable.
Note also that the total number of steps in the cycle can be determined in
one of two ways (listed in order of precedence):
#. A value for total_steps is explicitly provided.
#. A number of epochs (epochs) and a number of steps per epoch
(steps_per_epoch) are provided.
In this case, the number of total steps is inferred by
total_steps = epochs * steps_per_epoch
You must either provide a value for total_steps or provide a value for both
epochs and steps_per_epoch.
The default behaviour of this scheduler follows the fastai implementation
of 1cycle, which claims that "unpublished work has shown even better
results by using only two phases". To mimic the behaviour of the original
paper instead, set ``three_phase=True``.
Args:
optimizer (Optimizer): Wrapped optimizer.
eta_max (float or list): Upper parameter value boundaries in the cycle
for each parameter group.
total_steps (int): The total number of steps in the cycle. Note that
if a value is not provided here, then it must be inferred by
providing a value for epochs and steps_per_epoch.
Defaults to None.
pct_start (float): The percentage of the cycle (in number of steps)
spent increasing the learning rate.
Defaults to 0.3
anneal_strategy (str): {'cos', 'linear'}
Specifies the annealing strategy: "cos" for cosine annealing,
"linear" for linear annealing.
Defaults to 'cos'
div_factor (float): Determines the initial learning rate via
initial_param = eta_max/div_factor
Defaults to 25
final_div_factor (float): Determines the minimum learning rate via
eta_min = initial_param/final_div_factor
Defaults to 1e4
three_phase (bool): If ``True``, use a third phase of the schedule to
annihilate the learning rate according to 'final_div_factor'
instead of modifying the second phase (the first two phases will be
symmetrical about the step indicated by 'pct_start').
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled parameters are updated by
epochs. Defaults to True.
verbose (bool): Whether to print the value for each update.
Defaults to False.
.. _Super-Convergence\: Very Fast Training of Neural Networks Using Large Learning Rates:
https://arxiv.org/abs/1708.07120
"""# noqa E501
@PARAM_SCHEDULERS.register_module()
class CosineRestartLR(LRSchedulerMixin, CosineRestartParamScheduler):
"""Sets the learning rate of each parameter group according to the cosine
annealing with restarts scheme. The cosine restart policy anneals the
learning rate from the initial value to `eta_min` with a cosine annealing
schedule and then restarts another period from the maximum value multiplied
with `restart_weight`.
Args:
optimizer (Optimizer or OptimWrapper): optimizer or Wrapped
optimizer.
periods (list[int]): Periods for each cosine anneling cycle.
restart_weights (list[float]): Restart weights at each
restart iteration. Defaults to [1].
eta_min (float): Minimum parameter value at the end of scheduling.
Defaults to None.
eta_min_ratio (float, optional): The ratio of minimum parameter value
to the base parameter value. Either `min_lr` or `min_lr_ratio`
should be specified. Defaults to None.
begin (int): Step at which to start updating the parameters.
Defaults to 0.
end (int): Step at which to stop updating the parameters.
Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled parameters are updated by
epochs. Defaults to True.
verbose (bool): Whether to print the value for each update.
Defaults to False.
"""
[docs]@PARAM_SCHEDULERS.register_module()
class ReduceOnPlateauLR(LRSchedulerMixin, ReduceOnPlateauParamScheduler):
"""Reduce the learning rate of each parameter group when a metric has
stopped improving. Models often benefit from reducing the learning rate by
a factor of 2-10 once learning stagnates. This scheduler reads a metrics
quantity and if no improvement is seen for a ``patience`` number of epochs,
the learning rate is reduced.
Args:
optimizer (Optimizer or OptimWrapper): optimizer or Wrapped
optimizer.
monitor (str): Key name of the value to monitor in metrics dict.
rule (str): One of `less`, `greater`. In `less` rule, learning rate
will be reduced when the quantity monitored has stopped
decreasing; in `greater` rule it will be reduced when the
quantity monitored has stopped increasing. Defaults to 'less'.
The ``rule`` is the renaming of ``mode`` in pytorch.
factor (float): Factor by which the learning rate will be
reduced. new_param = param * factor. Defaults to 0.1.
patience (int): Number of epochs with no improvement after
which learning rate will be reduced. For example, if
``patience = 2``, then we will ignore the first 2 epochs
with no improvement, and will only decrease the learning rate after
the 3rd epoch if the monitor value still hasn't improved then.
Defaults to 10.
threshold (float): Threshold for measuring the new optimum,
to only focus on significant changes. Defaults to 1e-4.
threshold_rule (str): One of `rel`, `abs`. In `rel` rule,
dynamic_threshold = best * ( 1 + threshold ) in 'greater'
rule or best * ( 1 - threshold ) in `less` rule.
In `abs` rule, dynamic_threshold = best + threshold in
`greater` rule or best - threshold in `less` rule.
Defaults to 'rel'.
cooldown (int): Number of epochs to wait before resuming
normal operation after learning rate has been reduced.
Defaults to 0.
min_value (float or list[float]): A scalar or a sequence of scalars.
A lower bound on the learning rate of each parameter group
respectively. Defaults to 0. .
eps (float): Minimal decay applied to learning rate. If the difference
between new and old learning rate is smaller than eps, the update
is ignored. Defaults to 1e-8.
begin (int): Step at which to start triggering the scheduler
to monitor in val within the interval calculated
according to epoch of training. Defaults to 0.
end (int): Step at which to stop triggering the scheduler
to monitor in val within the interval calculated
according to epoch of training. Defaults to INF.
last_step (int): The index of last step. Used for resume without
state dict. Defaults to -1.
by_epoch (bool): Whether the scheduled parameters are updated by
epochs. Defaults to True.
verbose (bool): Whether to print the value for each update.
Defaults to False.
"""
```