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Training Tricks

Lightning implements various tricks to help during training


Accumulate gradients

Accumulated gradients runs K small batches of size N before doing a backwards pass. The effect is a large effective batch size of size KxN.

See also

Trainer

# DEFAULT (ie: no accumulated grads)
trainer = Trainer(accumulate_grad_batches=1)

Gradient Clipping

Gradient clipping may be enabled to avoid exploding gradients. By default, this will clip the gradient norm by calling torch.nn.utils.clip_grad_norm_() computed over all model parameters together. If the Trainer’s gradient_clip_algorithm is set to 'value' ('norm' by default), this will use instead torch.nn.utils.clip_grad_norm_() for each parameter instead.

Note

If using mixed precision, the gradient_clip_val does not need to be changed as the gradients are unscaled before applying the clipping function.

See also

Trainer

# DEFAULT (ie: don't clip)
trainer = Trainer(gradient_clip_val=0)

# clip gradients' global norm to <=0.5
trainer = Trainer(gradient_clip_val=0.5)  # gradient_clip_algorithm='norm' by default

# clip gradients' maximum magnitude to <=0.5
trainer = Trainer(gradient_clip_val=0.5, gradient_clip_algorithm="value")

Stochastic Weight Averaging

Stochastic Weight Averaging (SWA) can make your models generalize better at virtually no additional cost. This can be used with both non-trained and trained models. The SWA procedure smooths the loss landscape thus making it harder to end up in a local minimum during optimization.

For a more detailed explanation of SWA and how it works, read this post by the PyTorch team.

See also

StochasticWeightAveraging (Callback)

# Enable Stochastic Weight Averaging
trainer = Trainer(stochastic_weight_avg=True)

Auto scaling of batch size

Auto scaling of batch size may be enabled to find the largest batch size that fits into memory. Larger batch size often yields better estimates of gradients, but may also result in longer training time. Inspired by https://github.com/BlackHC/toma.

See also

Trainer

# DEFAULT (ie: don't scale batch size automatically)
trainer = Trainer(auto_scale_batch_size=None)

# Autoscale batch size
trainer = Trainer(auto_scale_batch_size=None | "power" | "binsearch")

# find the batch size
trainer.tune(model)

Currently, this feature supports two modes ‘power’ scaling and ‘binsearch’ scaling. In ‘power’ scaling, starting from a batch size of 1 keeps doubling the batch size until an out-of-memory (OOM) error is encountered. Setting the argument to ‘binsearch’ will initially also try doubling the batch size until it encounters an OOM, after which it will do a binary search that will finetune the batch size. Additionally, it should be noted that the batch size scaler cannot search for batch sizes larger than the size of the training dataset.

Note

This feature expects that a batch_size field is either located as a model attribute i.e. model.batch_size or as a field in your hparams i.e. model.hparams.batch_size. The field should exist and will be overridden by the results of this algorithm. Additionally, your train_dataloader() method should depend on this field for this feature to work i.e.

def train_dataloader(self):
    return DataLoader(train_dataset, batch_size=self.batch_size | self.hparams.batch_size)

Warning

Due to these constraints, this features does NOT work when passing dataloaders directly to .fit().

The scaling algorithm has a number of parameters that the user can control by invoking the scale_batch_size() method:

# Use default in trainer construction
trainer = Trainer()
tuner = Tuner(trainer)

# Invoke method
new_batch_size = tuner.scale_batch_size(model, *extra_parameters_here)

# Override old batch size (this is done automatically)
model.hparams.batch_size = new_batch_size

# Fit as normal
trainer.fit(model)
The algorithm in short works by:
  1. Dumping the current state of the model and trainer

  2. Iteratively until convergence or maximum number of tries max_trials (default 25) has been reached:
    • Call fit() method of trainer. This evaluates steps_per_trial (default 3) number of training steps. Each training step can trigger an OOM error if the tensors (training batch, weights, gradients, etc.) allocated during the steps have a too large memory footprint.

    • If an OOM error is encountered, decrease batch size else increase it. How much the batch size is increased/decreased is determined by the chosen strategy.

  3. The found batch size is saved to either model.batch_size or model.hparams.batch_size

  4. Restore the initial state of model and trainer

Warning

Batch size finder is not supported for DDP yet, it is coming soon.

Advanced GPU Optimizations

When training on single or multiple GPU machines, Lightning offers a host of advanced optimizations to improve throughput, memory efficiency, and model scaling. Refer to Advanced GPU Optimized Training for more details.

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