torch.utils.checkpoint

Note

Checkpointing is implemented by rerunning a forward-pass segment for each checkpointed segment during backward. This can cause persistent states like the RNG state to be advanced than they would without checkpointing. By default, checkpointing includes logic to juggle the RNG state such that checkpointed passes making use of RNG (through dropout for example) have deterministic output as compared to non-checkpointed passes. The logic to stash and restore RNG states can incur a moderate performance hit depending on the runtime of checkpointed operations. If deterministic output compared to non-checkpointed passes is not required, supply preserve_rng_state=False to checkpoint or checkpoint_sequential to omit stashing and restoring the RNG state during each checkpoint.

The stashing logic saves and restores the RNG state for the current device and the device of all cuda Tensor arguments to the run_fn. However, the logic has no way to anticipate if the user will move Tensors to a new device within the run_fn itself. Therefore, if you move Tensors to a new device (“new” meaning not belonging to the set of [current device + devices of Tensor arguments]) within run_fn, deterministic output compared to non-checkpointed passes is never guaranteed.

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.

The output of function can contain non-Tensor values and gradient recording is only performed for the Tensor values. Note that if the output consists of nested structures (ex: custom objects, lists, dicts etc.) consisting of Tensors, these Tensors nested in custom structures will not be considered as part of autograd.

Warning

Checkpointing currently only supports torch.autograd.backward() and only if its inputs argument is not passed. torch.autograd.grad() is not supported.

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.

Warning

At least one of the inputs needs to have requires_grad=True if grads are needed for model inputs, otherwise the checkpointed part of the model won’t have gradients. At least one of the outputs needs to have requires_grad=True as well.

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 currently only supports torch.autograd.backward() and only if its inputs argument is not passed. torch.autograd.grad() is not supported.

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)