torch.utils.checkpoint

torch.utils.checkpoint.checkpoint(function, *args, **kwargs)

Checkpoint a model or part of the model

Checkpointing works by trading compute for memory. Rather than storing all intermediate activations of the entire computation graph for computing backward, the checkpointed part does not save intermediate activations, and instead recomputes them in backward pass. It can be applied on any part of a model.

Specifically, in the forward pass, function will run in torch.no_grad() manner, i.e., not storing the intermediate activations. Instead, the forward pass saves the inputs tuple and the function parameter. In the backwards pass, the saved inputs and function is retrieved, and the forward pass is computed on function again, now tracking the intermediate activations, and then the gradients are calculated using these activation values.

Warning

Checkpointing doesn’t work with torch.autograd.grad(), but only with torch.autograd.backward().

Warning

If function invocation during backward does anything different than the one during forward, e.g., due to some global variable, the checkpointed version won’t be equivalent, and unfortunately it can’t be detected.

Warning

If checkpointed segment contains tensors detached from the computational graph by detach() or torch.no_grad(), the backward pass will raise an error. This is because checkpoint makes all the outputs require gradients which causes issues when a tensor is defined to have no gradient in the model. To circumvent this, detach the tensors outside of the checkpoint function.

Parameters

• function – describes what to run in the forward pass of the model or part of the model. It should also know how to handle the inputs passed as the tuple. For example, in LSTM, if user passes (activation, hidden), function should correctly use the first input as activation and the second input as hidden

• preserve_rng_state (bool, optional, default=True) – Omit stashing and restoring the RNG state during each checkpoint.

• args – tuple containing inputs to the function

Returns

Output of running function on *args

torch.utils.checkpoint.checkpoint_sequential(functions, segments, input, **kwargs)

A helper function for checkpointing sequential models.

Sequential models execute a list of modules/functions in order (sequentially). Therefore, we can divide such a model in various segments and checkpoint each segment. All segments except the last will run in torch.no_grad() manner, i.e., not storing the intermediate activations. The inputs of each checkpointed segment will be saved for re-running the segment in the backward pass.

See checkpoint() on how checkpointing works.

Warning

Checkpointing doesn’t work with torch.autograd.grad(), but only with torch.autograd.backward().

Parameters

• functions – A torch.nn.Sequential or the list of modules or functions (comprising the model) to run sequentially.

• segments – Number of chunks to create in the model

• input – A Tensor that is input to functions

• preserve_rng_state (bool, optional, default=True) – Omit stashing and restoring the RNG state during each checkpoint.

Returns

Output of running functions sequentially on *inputs

Example

>>> model = nn.Sequential(...)
>>> input_var = checkpoint_sequential(model, chunks, input_var)