Fabric Methods¶

launch¶

With launch() you can conveniently launch your script or a function into multiple processes for distributed training on a single machine.

# Launch the script on 2 devices and init distributed backend
fabric = Fabric(devices=2)
fabric.launch()

The same can be done with code inside a function:

def run(fabric):
    # Your distributed code here
    ...


# Launch a function on 2 devices and init distributed backend
fabric = Fabric(devices=2)
fabric.launch(run)

For example, you can use the latter for multi-GPU training inside a Jupyter notebook. For launching distributed training with the CLI, multi-node cluster, or cloud, see Launch distributed training.

setup¶

Set up a model and corresponding optimizer(s). If you need to set up multiple models, call setup() on each of them. Moves the model and optimizer to the correct device automatically.

model = nn.Linear(32, 64)
optimizer = torch.optim.SGD(model.parameters(), lr=0.001)

# Set up model and optimizer for accelerated training
model, optimizer = fabric.setup(model, optimizer)

# If you don't want Fabric to set the device
model, optimizer = fabric.setup(model, optimizer, move_to_device=False)

The setup method also prepares the model for the selected precision choice so that operations during forward() get cast automatically.

setup_dataloaders¶

Set up one or multiple data loaders for accelerated operation. If you run a distributed strategy (e.g., DDP), Fabric automatically replaces the sampler. In addition, the data loader will be configured to move the returned data tensors to the correct device automatically.

train_data = torch.utils.DataLoader(train_dataset, ...)
test_data = torch.utils.DataLoader(test_dataset, ...)

train_data, test_data = fabric.setup_dataloaders(train_data, test_data)

# If you don't want Fabric to move the data to the device
train_data, test_data = fabric.setup_dataloaders(train_data, test_data, move_to_device=False)

# If you don't want Fabric to replace the sampler in the context of distributed training
train_data, test_data = fabric.setup_dataloaders(train_data, test_data, use_distributed_sampler=False)

backward¶

This replaces any occurrences of loss.backward() and makes your code accelerator and precision agnostic.

output = model(input)
loss = loss_fn(output, target)

# loss.backward()
fabric.backward(loss)

clip_gradients¶

Clip the gradients of the model to a given max value or max norm. This is useful if your model experiences exploding gradients during training.

# Clip gradients to a max value of +/- 0.5
fabric.clip_gradients(model, optimizer, clip_val=0.5)

# Clip gradients such that their total norm is no bigger than 2.0
fabric.clip_gradients(model, optimizer, max_norm=2.0)

# By default, clipping by norm uses the 2-norm
fabric.clip_gradients(model, optimizer, max_norm=2.0, norm_type=2)

# You can also choose the infinity-norm, which clips the largest
# element among all
fabric.clip_gradients(model, optimizer, max_norm=2.0, norm_type="inf")

The clip_gradients() method is agnostic to the precision and strategy being used.

to_device¶

Use to_device() to move models, tensors, or collections of tensors to the current device. By default setup() and setup_dataloaders() already move the model and data to the correct device, so calling this method is only necessary for manual operation when needed.

data = torch.load("dataset.pt")
data = fabric.to_device(data)

seed_everything¶

Make your code reproducible by calling this method at the beginning of your run.

# Instead of `torch.manual_seed(...)`, call:
fabric.seed_everything(1234)

This covers PyTorch, NumPy, and Python random number generators. In addition, Fabric takes care of properly initializing the seed of data loader worker processes (can be turned off by passing workers=False).

init_module¶

Instantiating a nn.Module in PyTorch creates all parameters on CPU in float32 precision by default. To speed up initialization, you can force PyTorch to create the model directly on the target device and with the desired precision without changing your model code.

fabric = Fabric(accelerator="cuda", precision="16-true")

with fabric.init_module():
    # models created here will be on GPU and in float16
    model = MyModel()

This eliminates the waiting time to transfer the model parameters from the CPU to the device. For strategies that handle large sharded models (FSDP, DeepSpeed), the init_module() method will allocate the model parameters on the meta device first before sharding. This makes it possible to work with models that are larger than the memory of a single device.

autocast¶

Let the precision backend autocast the block of code under this context manager. This is optional and already done by Fabric for the model’s forward method (once the model was setup()). You need this only if you wish to autocast more operations outside the ones in model forward:

model, optimizer = fabric.setup(model, optimizer)

# Fabric handles precision automatically for the model
output = model(inputs)

with fabric.autocast():  # optional
    loss = loss_function(output, target)

fabric.backward(loss)
...

print¶

Print to the console via the built-in print function, but only on the main process. This avoids excessive printing and logs when running on multiple devices/nodes.

# Print only on the main process
fabric.print(f"{epoch}/{num_epochs}| Train Epoch Loss: {loss}")

save¶

Save the state of objects to a checkpoint file. Replaces all occurrences of torch.save(...) in your code. Fabric will handle the saving part correctly, whether running a single device, multi-devices, or multi-nodes.

# Define the state of your program/loop
state = {
    "model1": model1,
    "model2": model2,
    "optimizer": optimizer,
    "iteration": iteration,
}

# Instead of `torch.save(...)`
fabric.save("path/to/checkpoint.ckpt", state)

You should pass the model and optimizer objects directly into the dictionary so Fabric can unwrap them and automatically retrieve their state-dict.

load¶

Load checkpoint contents from a file and restore the state of objects in your program. Replaces all occurrences of torch.load(...) in your code. Fabric will handle the loading part correctly, whether running a single device, multi-device, or multi-node.

# Define the state of your program/loop
state = {
    "model1": model1,
    "model2": model2,
    "optimizer": optimizer,
    "iteration": iteration,
}

# Restore the state of objects (in-place)
fabric.load("path/to/checkpoint.ckpt", state)

# Or load everything and restore your objects manually
checkpoint = fabric.load("./checkpoints/version_2/checkpoint.ckpt")
model.load_state_dict(checkpoint["model"])
...

To load the state of your model or optimizer from a raw PyTorch checkpoint (not saved with Fabric), use load_raw() instead. See also: Checkpoints

load_raw¶

Load the state-dict of a model or optimizer from a raw PyTorch checkpoint not saved by Fabric.

model = MyModel()

# A model weights file saved by your friend who doesn't use Fabric
fabric.load_raw("path/to/model.pt", model)

# Equivalent to this:
# model.load_state_dict(torch.load("path/to/model.pt"))

barrier¶

Call this if you want all processes to wait and synchronize. Once all processes have entered this call, execution continues. Useful for example, when you want to download data on one process and make all others wait until the data is written to disk.

if fabric.global_rank == 0:
    print("Downloading dataset. This can take a while ...")
    download_dataset("http://...")

# All other processes wait here until rank 0 is done with downloading:
fabric.barrier()

# After everyone reached the barrier, they can access the downloaded files:
load_dataset()

all_gather, all_reduce, broadcast¶

You can send tensors and other data between processes using collective operations. The three most common ones, broadcast(), all_gather() and all_reduce() are available directly on the Fabric object for convenience:

broadcast(): Send a tensor from one process to all others.
all_gather(): Gather tensors from every process and stack them.
all_reduce(): Apply a reduction function on tensors across processes (sum, mean, etc.).

# Send the value of a tensor from rank 0 to all others
result = fabric.broadcast(tensor, src=0)

# Every process gets the stack of tensors from everybody else
all_tensors = fabric.all_gather(tensor)

# Sum a tensor across processes (everyone gets the result)
reduced_tensor = fabric.all_reduce(tensor, reduce_op="sum")

# Also works with a collection of tensors (dict, list, tuple):
collection = {"loss": torch.tensor(...), "data": ...}
gathered_collection = fabric.all_gather(collection, ...)
reduced_collection = fabric.all_reduce(collection, ...)

Important

Every process needs to enter the collective calls, and tensors need to have the same shape across all processes. Otherwise, the program will hang!

Learn more about distributed communication.

no_backward_sync¶

Use this context manager when performing gradient accumulation and using a distributed strategy (e.g., DDP). It will speed up your training loop by cutting redundant communication between processes during the accumulation phase.

# Accumulate gradient 8 batches at a time
is_accumulating = batch_idx % 8 != 0

with fabric.no_backward_sync(model, enabled=is_accumulating):
    output = model(input)
    loss = ...
    fabric.backward(loss)
    ...

# Step the optimizer every 8 batches
if not is_accumulating:
    optimizer.step()
    optimizer.zero_grad()

Both the model’s .forward() and the fabric.backward() call need to run under this context as shown in the example above. For single-device strategies, it is a no-op. Some strategies don’t support this:

deepspeed
dp
xla

For these, the context manager falls back to a no-op and emits a warning.

call¶

Use this to run all registered callback hooks with a given name and inputs. It is useful when building a Trainer that allows the user to run arbitrary code at fixed points in the training loop.

class MyCallback:
    def on_train_start(self):
        ...

    def on_train_epoch_end(self, model, results):
        ...


fabric = Fabric(callbacks=[MyCallback()])

# Call any hook by name
fabric.call("on_train_start")

# Pass in additional arguments that the hook requires
fabric.call("on_train_epoch_end", model=..., results={...})

# Only the callbacks that have this method defined will be executed
fabric.call("undefined")

log and log_dict¶

These methods allow you to send scalar metrics to a logger registered in Fabric.

# Set the logger in Fabric
fabric = Fabric(loggers=TensorBoardLogger(...))

# Anywhere in your training loop or model:
fabric.log("loss", loss)

# Or send multiple metrics at once:
fabric.log_dict({"loss": loss, "accuracy": acc})

If no loggers are given to Fabric (default), log and log_dict won’t do anything. Here is what’s happening under the hood (pseudo code) when you call .log() or log_dict:

# When you call .log() or .log_dict(), we do this:
for logger in fabric.loggers:
    logger.log_metrics(metrics=metrics, step=step)