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Accelerator

The Accelerator is the main class provided by 🤗 Accelerate. It serves at the main entrypoint for the API.

Quick adaptation of your code

To quickly adapt your script to work on any kind of setup with 🤗 Accelerate just:

  1. Initialize an Accelerator object (that we will call accelerator throughout this page) as early as possible in your script.
  2. Pass your dataloader(s), model(s), optimizer(s), and scheduler(s) to the prepare() method.
  3. Remove all the .cuda() or .to(device) from your code and let the accelerator handle the device placement for you.

Step three is optional, but considered a best practice.

  1. Replace loss.backward() in your code with accelerator.backward(loss)
  2. Gather your predictions and labels before storing them or using them for metric computation using gather()

Step five is mandatory when using distributed evaluation

In most cases this is all that is needed. The next section lists a few more advanced use cases and nice features you should search for and replace by the corresponding methods of your accelerator:

Advanced recommendations

Printing

print statements should be replaced by print() to be printed once per process

- print("My thing I want to print!")
+ accelerator.print("My thing I want to print!")

Executing processes

Once on a single server

For statements that should be executed once per server, use is_local_main_process:

if accelerator.is_local_main_process:
    do_thing_once_per_server()

A function can be wrapped using the on_local_main_process() function to achieve the same behavior on a function’s execution:

@accelerator.on_local_main_process
def do_my_thing():
    "Something done once per server"
    do_thing_once_per_server()

Only ever once across all servers

For statements that should only ever be executed once, use is_main_process:

if accelerator.is_main_process:
    do_thing_once()

A function can be wrapped using the on_main_process() function to achieve the same behavior on a function’s execution:

@accelerator.on_main_process
def do_my_thing():
    "Something done once per server"
    do_thing_once()

On specific processes

If a function should be ran on a specific overall or local process index, there are similar decorators to achieve this:

@accelerator.on_local_process(local_process_idx=0)
def do_my_thing():
    "Something done on process index 0 on each server"
    do_thing_on_index_zero_on_each_server()
@accelerator.on_process(process_index=0)
def do_my_thing():
    "Something done on process index 0"
    do_thing_on_index_zero()

Synchronicity control

Use wait_for_everyone() to make sure all processes join that point before continuing. (Useful before a model save for instance)

Saving and loading

Use unwrap_model() before saving to remove all special model wrappers added during the distributed process.

model = MyModel()
model = accelerator.prepare(model)
# Unwrap
model = accelerator.unwrap_model(model)

Use save() instead of torch.save:

  state_dict = model.state_dict()
- torch.save(state_dict, "my_state.pkl")
+ accelerator.save(state_dict, "my_state.pkl")

Operations

Use clipgrad_norm() instead of torch.nn.utils.clip_grad_norm_ and clipgrad_value() instead of torch.nn.utils.clip_grad_value

Gradient Accumulation

To perform gradient accumulation use accumulate() and specify a gradient_accumulation_steps. This will also automatically ensure the gradients are synced or unsynced when on multi-device training, check if the step should actually be performed, and auto-scale the loss:

- accelerator = Accelerator()
+ accelerator = Accelerator(gradient_accumulation_steps=2)

  for (input, label) in training_dataloader:
+     with accelerator.accumulate(model):
          predictions = model(input)
          loss = loss_function(predictions, labels)
          accelerator.backward(loss)
          optimizer.step()
          scheduler.step()
          optimizer.zero_grad()

Overall API documentation:

class accelerate.Accelerator

< >

( device_placement: bool = True split_batches: bool = False fp16: bool = None mixed_precision: typing.Union[accelerate.utils.dataclasses.PrecisionType, str] = None gradient_accumulation_steps: int = 1 cpu: bool = False deepspeed_plugin: DeepSpeedPlugin = None fsdp_plugin: FullyShardedDataParallelPlugin = None rng_types: typing.Union[typing.List[typing.Union[str, accelerate.utils.dataclasses.RNGType]], NoneType] = None log_with: typing.Union[typing.List[typing.Union[str, accelerate.utils.dataclasses.LoggerType, accelerate.tracking.GeneralTracker]], NoneType] = None logging_dir: typing.Union[str, os.PathLike, NoneType] = None dispatch_batches: typing.Optional[bool] = None step_scheduler_with_optimizer: bool = True kwargs_handlers: typing.Optional[typing.List[accelerate.utils.dataclasses.KwargsHandler]] = None )

Parameters

  • device_placement (bool, optional, defaults to True) — Whether or not the accelerator should put objects on device (tensors yielded by the dataloader, model, etc…).
  • split_batches (bool, optional, defaults to False) — Whether or not the accelerator should split the batches yielded by the dataloaders across the devices. If True the actual batch size used will be the same on any kind of distributed processes, but it must be a round multiple of the num_processes you are using. If False, actual batch size used will be the one set in your script multiplied by the number of processes.
  • mixed_precision (str, optional) — Whether or not to use mixed precision training (fp16 or bfloat16). Choose from ‘no’,‘fp16’,‘bf16’. Will default to the value in the environment variable MIXED_PRECISION, which will use the default value in the accelerate config of the current system or the flag passed with the accelerate.launch command. ‘fp16’ requires pytorch 1.6 or higher. ‘bf16’ requires pytorch 1.10 or higher.
  • gradient_accumulation_steps (int, optional, default to 1) — The number of steps that should pass before gradients are accumulated. A number > 1 should be combined with Accelerator.accumulate.
  • cpu (bool, optional) — Whether or not to force the script to execute on CPU. Will ignore GPU available if set to True and force the execution on one process only.
  • deepspeed_plugin (DeepSpeedPlugin, optional) — Tweak your DeepSpeed related args using this argument. This argument is optional and can be configured directly using accelerate config
  • fsdp_plugin (FullyShardedDataParallelPlugin, optional) — Tweak your FSDP related args using this argument. This argument is optional and can be configured directly using accelerate config
  • rng_types (list of str or RNGType) — The list of random number generators to synchronize at the beginning of each iteration in your prepared dataloaders. Should be one or several of:

    • "torch": the base torch random number generator
    • "cuda": the CUDA random number generator (GPU only)
    • "xla": the XLA random number generator (TPU only)
    • "generator": the torch.Generator of the sampler (or batch sampler if there is no sampler in your dataloader) or of the iterable dataset (if it exists) if the underlying dataset is of that type.

    Will default to ["torch"] for PyTorch versions <=1.5.1 and ["generator"] for PyTorch versions >= 1.6.

  • log_with (list of str, LoggerType or GeneralTracker, optional) — A list of loggers to be setup for experiment tracking. Should be one or several of:

    • "all"
    • "tensorboard"
    • "wandb"
    • "comet_ml" If "all” is selected, will pick up all available trackers in the environment and intialize them. Can also accept implementations of GeneralTracker for custom trackers, and can be combined with "all".
  • logging_dir (str, os.PathLike, optional) — A path to a directory for storing logs of locally-compatible loggers.
  • dispatch_batches (bool, optional) — If set to True, the dataloader prepared by the Accelerator is only iterated through on the main process and then the batches are split and broadcast to each process. Will default to True for DataLoader whose underlying dataset is an IterableDataset, False otherwise.
  • step_scheduler_with_optimizer (bool, *optional, defaults to True) -- Set Trueif the learning rate scheduler is stepped at the same time as the optimizer,False` if only done under certain circumstances (at the end of each epoch, for instance).
  • kwargs_handlers (List[KwargHandler], optional) — A list of KwargHandler to customize how the objects related to distributed training or mixed precision are created. See kwargs for more information.

Creates an instance of an accelerator for distributed training (on multi-GPU, TPU) or mixed precision training.

Attributes:

  • device (torch.device) — The device to use.
  • distributed_type (DistributedType) — The distributed training configuration.
  • local_process_index (int) — The process index on the current machine.
  • mixed_precision (str) — The configured mixed precision mode.
  • num_processes (int) — The total number of processes used for training.
  • optimizer_step_was_skipped (bool) — Whether or not the optimizer update was skipped (because of gradient overflow in mixed precision), in which case the learning rate should not be changed.
  • process_index (int) — The overall index of the current process among all processes.
  • state (AcceleratorState) — The distributed setup state.
  • sync_gradients (bool) — Whether the gradients are currently being synced across all processes.
  • use_distributed (bool) — Whether the current configuration is for distributed training.

accumulate

< >

( model )

Parameters

  • model (torch.nn.Module) — PyTorch Module that was prepared with Accelerator.prepare

A context manager that will lightly wrap around and perform gradient accumulation automatically

autocast

< >

( )

Will apply automatic mixed-precision inside the block inside this context manager, if it is enabled. Nothing different will happen otherwise.

backward

< >

( loss **kwargs )

Use accelerator.backward(loss) in lieu of loss.backward().

clear

< >

( )

Alias for Accelerate.free_memory, releases all references to the internal objects stored and call the garbage collector. You should call this method between two trainings with different models/optimizers.

clip_grad_norm_

< >

( parameters max_norm norm_type = 2 )

Should be used in place of torch.nn.utils.clip_grad_norm_.

clip_grad_value_

< >

( parameters clip_value )

Should be used in place of torch.nn.utils.clip_grad_value_.

end_training

< >

( )

Runs any special end training behaviors, such as stopping trackers on the main process only.

free_memory

< >

( )

Will release all references to the internal objects stored and call the garbage collector. You should call this method between two trainings with different models/optimizers.

gather

< >

( tensor ) → torch.Tensor, or a nested tuple/list/dictionary of torch.Tensor

Parameters

  • tensor (torch.Tensor, or a nested tuple/list/dictionary of torch.Tensor) — The tensors to gather across all processes.

Returns

torch.Tensor, or a nested tuple/list/dictionary of torch.Tensor

The gathered tensor(s). Note that the first dimension of the result is num_processes multiplied by the first dimension of the input tensors.

Gather the values in tensor across all processes and concatenate them on the first dimension. Useful to regroup the predictions from all processes when doing evaluation.

Note: This gather happens in all processes.

gather_for_metrics

< >

( tensor )

Parameters

  • tensor (torch.Tensor, or a nested tuple/list/dictionary of torch.Tensor) — The tensors for calculating metrics across all processes.

Gathers tensor and potentially drops duplicates in the last batch if on a distributed system. Should be used for gathering the inputs and targets for metric calculation.

get_tracker

< >

( name: str )

Parameters

  • name (str) — The name of a tracker, corresponding to the .name property.

Returns a tracker from self.trackers based on name on the main process only.

init_trackers

< >

( project_name: str config: typing.Optional[dict] = None init_kwargs: typing.Optional[dict] = {} )

Parameters

  • project_name (str) — The name of the project. All trackers will save their data based on this
  • config (dict, optional) — Optional starting configuration to be logged.
  • init_kwargs (dict, optional) — A nested dictionary of kwargs to be passed to a specific tracker’s __init__ function. Should be formatted like this:

Initializes a run for all trackers stored in self.log_with, potentially with starting configurations

load_state

< >

( input_dir: str )

Parameters

  • input_dir (str or os.PathLike) — The name of the folder all relevant weights and states were saved in.

Loads the current states of the model, optimizer, scaler, RNG generators, and registered objects.

local_main_process_first

< >

( )

Lets the local main process go inside a with block.

The other processes will enter the with block after the main process exits.

log

< >

( values: dict step: typing.Optional[int] = None log_kwargs: typing.Optional[dict] = {} )

Parameters

  • values (dict) — Values should be a dictionary-like object containing only types int, float, or str.
  • step (int, optional) — The run step. If included, the log will be affiliated with this step.
  • log_kwargs (dict, optional) — A nested dictionary of kwargs to be passed to a specific tracker’s log function. Should be formatted like this:

Logs values to all stored trackers in self.trackers on the main process only.

main_process_first

< >

( )

Lets the main process go first inside a with block.

The other processes will enter the with block after the main process exits.

no_sync

< >

( model )

Parameters

  • model (torch.nn.Module) — PyTorch Module that was prepared with Accelerator.prepare

A context manager to disable gradient synchronizations across DDP processes by calling torch.nn.parallel.DistributedDataParallel.no_sync.

If model is not in DDP, this context manager does nothing

on_local_main_process

< >

( func )

A decorator that will run the decorated function on the local main process only.

on_local_process

< >

( local_process_idx )

A decorator that will run the decorated function on a given local process index only.

on_main_process

< >

( func )

A decorator that will run the decorated function on the main process only.

on_process

< >

( process_idx )

A decorator that will run the decorated function on a given process index only.

pad_across_processes

< >

( tensor dim = 0 pad_index = 0 pad_first = False )

Parameters

  • tensor (nested list/tuple/dictionary of torch.Tensor) — The data to gather.
  • dim (int, optional, defaults to 0) — The dimension on which to pad.
  • pad_index (int, optional, defaults to 0) — The value with which to pad.
  • pad_first (bool, optional, defaults to False) — Whether to pad at the beginning or the end.

Recursively pad the tensors in a nested list/tuple/dictionary of tensors from all devices to the same size so they can safely be gathered.

prepare

< >

( *args )

Prepare all objects passed in args for distributed training and mixed precision, then return them in the same order.

Accepts the following type of objects:

  • torch.utils.data.DataLoader: PyTorch Dataloader
  • torch.nn.Module: PyTorch Module
  • torch.optim.Optimizer: PyTorch Optimizer

print

< >

( *args **kwargs )

Use in replacement of print() to only print once per server.

reduce

< >

( tensor reduction = 'sum' ) → torch.Tensor, or a nested tuple/list/dictionary of torch.Tensor

Parameters

  • tensor (torch.Tensor, or a nested tuple/list/dictionary of torch.Tensor) — The tensors to reduce across all processes.
  • reduction (str, optional, defaults to “sum”) — A reduction type, can be one of ‘sum’, ‘mean’, or ‘none’. If ‘none’, will not perform any operation.

Returns

torch.Tensor, or a nested tuple/list/dictionary of torch.Tensor

The reduced tensor(s).

Reduce the values in tensor across all processes based on reduction.

Note: All processes get the reduced value.

register_for_checkpointing

< >

( *objects )

Makes note of objects and will save or load them in during save_state or load_state.

These should be utilized when the state is being loaded or saved in the same script. It is not designed to be used in different scripts

Every object must have a load_state_dict and state_dict function to be stored.

save

< >

( obj f )

Parameters

  • f (str or os.PathLike) — Where to save the content of obj.

Save the object passed to disk once per machine. Use in place of torch.save.

save_state

< >

( output_dir: str )

Parameters

  • output_dir (str or os.PathLike) — The name of the folder to save all relevant weights and states.

Saves the current states of the model, optimizer, scaler, RNG generators, and registered objects.

unscale_gradients

< >

( optimizer = None )

Parameters

  • optimizer (torch.optim.Optimizer or List[torch.optim.Optimizer], optional) — The optimizer(s) for which to unscale gradients. If not set, will unscale gradients on all optimizers that were passed to prepare().

Unscale the gradients in mixed precision training with AMP. This is a noop in all other settings.

unwrap_model

< >

( model )

Parameters

  • model (torch.nn.Module) — The model to unwrap.

Unwraps the model from the additional layer possible added by prepare(). Useful before saving the model.

wait_for_everyone

< >

( )

Will stop the execution of the current process until every other process has reached that point (so this does nothing when the script is only run in one process). Useful to do before saving a model.