# Using Local SGD with Accelerate

Local SGD is a technique for distributed training where gradients are not synchronized every step. Thus, each process updates its own version of the model weights and after a given number of steps these weights are synchronized by averaging across all processes. This improves communication efficiency and can lead to substantial training speed up especially when a computer lacks a faster interconnect such as NVLink.
Unlike gradient accumulation (where improving communication efficiency requires increasing the effective batch size), Local SGD does not require changing a batch size or a learning rate / schedule. However, if necessary, Local SGD can be combined with gradient accumulation as well.

In this tutorial you will see how to quickly setup  Local SGD Accelerate. Compared to a standard Accelerate setup, this requires only two extra lines of code.

This example will use a very simplistic PyTorch training loop that performs gradient accumulation every two batches:

```python
device = "cuda"
model.to(device)

gradient_accumulation_steps = 2

for index, batch in enumerate(training_dataloader):
    inputs, targets = batch
    inputs = inputs.to(device)
    targets = targets.to(device)
    outputs = model(inputs)
    loss = loss_function(outputs, targets)
    loss = loss / gradient_accumulation_steps
    loss.backward()
    if (index + 1) % gradient_accumulation_steps == 0:
        optimizer.step()
        scheduler.step()
        optimizer.zero_grad()
```

## Converting it to Accelerate

First the code shown earlier will be converted to use Accelerate  with neither a LocalSGD or a gradient accumulation helper:

```diff
+ from accelerate import Accelerator
+ accelerator = Accelerator()

+ model, optimizer, training_dataloader, scheduler = accelerator.prepare(
+     model, optimizer, training_dataloader, scheduler
+ )

  for index, batch in enumerate(training_dataloader):
      inputs, targets = batch
-     inputs = inputs.to(device)
-     targets = targets.to(device)
      outputs = model(inputs)
      loss = loss_function(outputs, targets)
      loss = loss / gradient_accumulation_steps
+     accelerator.backward(loss)
      if (index+1) % gradient_accumulation_steps == 0:
          optimizer.step()
          scheduler.step()
```

## Letting Accelerate handle model synchronization 

All that is left now is to let Accelerate handle model parameter synchronization **and** the gradient accumulation for us. For simplicity let us assume we need to synchronize every 8 steps. This is
achieved by adding one `with LocalSGD` statement and one call `local_sgd.step()` after every optimizer step:

```diff
+local_sgd_steps=8

+with LocalSGD(accelerator=accelerator, model=model, local_sgd_steps=8, enabled=True) as local_sgd:
    for batch in training_dataloader:
        with accelerator.accumulate(model):
            inputs, targets = batch
            outputs = model(inputs)
            loss = loss_function(outputs, targets)
            accelerator.backward(loss)
            optimizer.step()
            scheduler.step()
            optimizer.zero_grad()
+           local_sgd.step()
```

Under the hood, the Local SGD code **disables** automatic gradient synchronization (but accumulation still works as expected!). Instead it averages model parameters every `local_sgd_steps` steps (as well as at the end of the training loop).

## Limitations

The current implementation works only with basic multi-GPU (or multi-CPU) training without, e.g., [DeepSpeed.](https://github.com/deepspeedai/DeepSpeed).

## References

    Although we are not aware of the true origins of this simple approach, the idea of local SGD is quite old and goes
    back to at least:

    Zhang, J., De Sa, C., Mitliagkas, I., & Ré, C. (2016). [Parallel SGD: When does averaging help?. arXiv preprint
    arXiv:1606.07365.](https://arxiv.org/abs/1606.07365)

    We credit the term Local SGD to the following paper (but there might be earlier references we are not aware of).

    Stich, Sebastian Urban. ["Local SGD Converges Fast and Communicates Little." ICLR 2019-International Conference on
    Learning Representations. No. CONF. 2019.](https://arxiv.org/abs/1805.09767)


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