Source code for ignite.handlers.ema_handler

from copy import deepcopy
from typing import Optional, Union

import torch.nn as nn

from ignite.engine import CallableEventWithFilter, Engine, Events, EventsList

__all__ = ["EMAHandler"]

[docs]class EMAHandler: r"""Exponential moving average (EMA) handler can be used to compute a smoothed version of model. The EMA model is updated as follows: .. math:: \theta_{\text{EMA}, t+1} = (1 - \lambda) \cdot \theta_{\text{EMA}, t} + \lambda \cdot \theta_{t} where :math:`\theta_{\text{EMA}, t}` and :math:`\theta_{t}` are the EMA weights and online model weights at :math:`t`-th iteration, respectively; :math:`\lambda` is the update momentum. The handler allows for linearly warming up the momentum in the beginning when training process is not stable. Current momentum can be retrieved from ``Engine.state.ema_momentum``. Args: model: the online model for which an EMA model will be computed. If ``model`` is ``DataParallel`` or ``DistributedDataParallel``, the EMA smoothing will be applied to ``model.module`` . momentum: the update momentum after warmup phase, should be float in range :math:`\left(0, 1 \right)`. momentum_warmup: the initial update momentum during warmup phase, the value should be smaller than ``momentum``. Momentum will linearly increase from this value to ``momentum`` in ``warmup_iters`` iterations. If ``None``, no warmup will be performed. warmup_iters: iterations of warmup. If ``None``, no warmup will be performed. Attributes: ema_model: the exponential moving averaged model. model: the online model that is tracked by EMAHandler. It is ``model.module`` if ``model`` in the initialization method is an instance of ``DistributedDataParallel``. momentum: the update momentum after warmup phase. momentum_warmup: the initial update momentum. warmup_iters: number of warmup iterations. Note: The EMA model is already in ``eval`` mode. If model in the arguments is an ``nn.Module`` or ``DistributedDataParallel``, the EMA model is an ``nn.Module`` and it is on the same device as the online model. If the model is an ``nn.DataParallel``, then the EMA model is an ``nn.DataParallel``. Note: It is recommended to initialize and use an EMA handler in following steps: 1. Initialize ``model`` (``nn.Module`` or ``DistributedDataParallel``) and ``ema_handler`` (``EMAHandler``). 2. Build ``trainer`` (``ignite.engine.Engine``). 3. Resume from checkpoint for ``model`` and ``ema_handler.ema_model``. 4. Attach ``ema_handler`` to ``trainer``. Examples: .. code-block:: python device = torch.device("cuda:0") model = nn.Linear(2, 1).to(device) # update the ema every 5 iterations ema_handler = EMAHandler( model, momentum=0.0002, momentum_warmup=0.0001, warmup_iters=10000) # get the ema model, which is an instance of nn.Module ema_model = ema_handler.ema_model trainer = Engine(train_step_fn) to_load = {"model": model, "ema_model", ema_model, "trainer", trainer} if resume_from is not None: Checkpoint.load_objects(to_load, checkpoint=resume_from) # update the EMA model every 5 iterations ema_handler.attach(trainer, name="ema_momentum", event=Events.ITERATION_COMPLETED(every=5)) # add other handlers to_save = to_load ckpt_handler = Checkpoint(to_save, DiskSaver(...), ...) trainer.add_event_handler(Events.EPOCH_COMPLETED, ckpt_handler) # current momentum can be retrieved from engine.state, # the attribute name is the `name` parameter used in the attach function @trainer.on(Events.ITERATION_COMPLETED): def print_ema_momentum(engine): print(f"current momentum: {engine.state.ema_momentum}" # use ema model for validation val_step_fn = get_val_step_fn(ema_model) evaluator = Engine(val_step_fn) @trainer.on(Events.EPOCH_COMPLETED) def run_validation(engine): The following example shows how to attach two handlers to the same trainer: .. code-block:: python generator = build_generator(...) discriminator = build_discriminator(...) gen_handler = EMAHandler(generator) disc_handler = EMAHandler(discriminator) step_fn = get_step_fn(...) engine = Engine(step_fn) # update EMA model of generator every 1 iteration gen_handler.attach(engine, "gen_ema_momentum", event=Events.ITERATION_COMPLETED) # update EMA model of discriminator every 2 iteration disc_handler.attach(engine, "dis_ema_momentum", event=Events.ITERATION_COMPLETED(every=2)) @engine.on(Events.ITERATION_COMPLETED) def print_ema_momentum(engine): print(f"current momentum for generator: {engine.state.gen_ema_momentum}") print(f"current momentum for discriminator: {engine.state.disc_ema_momentum}") .. versionadded:: 0.4.6 """ def __init__( self, model: nn.Module, momentum: float = 0.0002, momentum_warmup: Optional[float] = None, warmup_iters: Optional[int] = None, ) -> None: if momentum_warmup is not None and not 0 < momentum_warmup < 1: raise ValueError(f"Invalid momentum_warmup: {momentum_warmup}") if not 0 < momentum < 1: raise ValueError(f"Invalid momentum: {momentum}") if momentum_warmup is not None and not momentum_warmup <= momentum: raise ValueError( f"momentum_warmup should be less than or equal to momentum, but got " f"momentum_warmup: {momentum_warmup} and momentum: {momentum}" ) if warmup_iters is not None and not (isinstance(warmup_iters, int) and warmup_iters > 0): raise ValueError(f"Invalid warmup_iters: {warmup_iters}") if not isinstance(model, nn.Module): raise ValueError( f"model should be an instance of nn.Module or its subclasses, but got" f"model: {model.__class__.__name__}" ) self.momentum_warmup = momentum_warmup self.momentum = momentum self.warmup_iters = warmup_iters if isinstance(model, nn.parallel.DistributedDataParallel): model = model.module self.model = model self.ema_model = deepcopy(self.model) for param in self.ema_model.parameters(): param.detach_() self.ema_model.eval() def _get_momentum(self, curr_iter: int) -> float: """Get current momentum, `curr_iter` should be 1-based. When `curr_iter = 1`, `momentum = self.momentum_warmup`; when `curr_iter >= self.warmup_iters`, `momentum = self.momentum`""" # TODO: use ignite's parameter scheduling, see also GitHub issue #2090 if curr_iter < 1: raise ValueError(f"curr_iter should be at least 1, but got {curr_iter}.") # no warmup if self.momentum_warmup is None or self.warmup_iters is None: return self.momentum denominator = max(1, self.warmup_iters - 1) momentum = self.momentum_warmup + (self.momentum - self.momentum_warmup) * (curr_iter - 1) / denominator return min(self.momentum, momentum) def _update_ema_model(self, engine: Engine, name: str) -> None: """Update weights of ema model""" momentum = getattr(engine.state, name) for ema_p, model_p in zip(self.ema_model.parameters(), self.model.parameters()): ema_p.mul_(1.0 - momentum).add_(, alpha=momentum) # assign the buffers for ema_b, model_b in zip(self.ema_model.buffers(), self.model.buffers()): = def _update_ema_momentum(self, engine: Engine, name: str) -> None: """Update momentum in engine.state""" curr_iter = engine.state.iteration momentum = self._get_momentum(curr_iter) setattr(engine.state, name, momentum)
[docs] def attach( self, engine: Engine, name: str = "ema_momentum", event: Union[str, Events, CallableEventWithFilter, EventsList] = Events.ITERATION_COMPLETED, ) -> None: """Attach the handler to engine. After the handler is attached, the ``Engine.state`` will add an new attribute with name ``name``. Then, current momentum can be retrieved by from ``Engine.state`` when the engine runs. Args: engine: trainer to which the handler will be attached. name: attribute name for retrieving EMA momentum from ``Engine.state``. It should be a unique name since a trainer can have multiple EMA handlers. event: event when the EMA momentum and EMA model are updated. """ if hasattr(engine.state, name): raise ValueError( f"Attribute: '{name}' is already in Engine.state. Thus it might be " f"overridden by other EMA handlers. Please select another name." ) setattr(engine.state, name, 0.0) # first update momentum, then update ema model engine.add_event_handler(event, self._update_ema_momentum, name) engine.add_event_handler(event, self._update_ema_model, name)

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