🤗 Accelerate’s internal mechanisms

Internally, 🤗 Accelerate works by first analyzing the environment in which the script is launched to determine which kind of distributed setup is used, how many different processes there are and which one the current script is in. All that information is stored in the ~AcceleratorState.

This class is initialized the first time you instantiate an ~Accelerator as well as performing any specific initialization your distributed setup needs. Its state is then uniquely shared through all instances of AcceleratorState. (The same can also be done with the PartialState, a more barebones version it inherits)

Then, when calling prepare(), the library:

• wraps your model(s) in the container adapted for the distributed setup,
• wraps your optimizer(s) in an AcceleratedOptimizer,
• wraps your scheduler(s) in an AcceleratedScheduler
• creates a new version of your dataloader(s) in a DataLoaderShard or DataLoaderDispatcher

While the model(s), optimizer(s), and scheduler(s) are just put in simple wrappers, the dataloader(s) are re-created. This is mostly because PyTorch does not let the user change the batch_sampler of a dataloader once it’s been created and the library handles the sharding of your data between processes by changing that batch_sampler to yield every other num_processes batches (if enabled).

The DataLoaderShard subclasses DataLoader to add the following functionality:

• it synchronizes the appropriate random number generator of all processes at each new iteration, to ensure any randomization (like shuffling) is done the exact same way across processes.
• it puts the batches on the proper device before yielding them (unless you have opted out of device_placement=True).

The DataLoaderDispatcher subclasses differs from the DataLoaderShard in that when iterating through the DataLoader, the data is all starting from process 0 and then split and sent off to each process rather than it happening at the dataset level.

The random number generator synchronization will by default synchronize:

• the generator attribute of a given sampler (like the PyTorch RandomSampler) for PyTorch >= 1.6
• the main random number generator in PyTorch <=1.5.1

You can choose which random number generator(s) to synchronize with the rng_types argument of the main Accelerator. In PyTorch >= 1.6, it is recommended to rely on a local generator to avoid setting the same seed in the main random number generator in all processes.

For more details about the internals, see the Internals page.

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