Lightning AI is excited to announce the release of Lightning 2.1 ⚡ It's the culmination of work from 79 contributors who have worked on features, bug-fixes, and documentation for a total of over 750+ commits since v2.0.

The theme of 2.1 is "bigger, better, faster": Bigger because training large multi-billion parameter models has gotten even more efficient thanks to FSDP, efficient initialization and sharded checkpointing improvements, better because it's easier than ever to scale models without making substantial code changes or installing third-party packages and faster because it leverages the latest hardware features to speed up training in low-bit precision thanks to new precision plugins like bitsandbytes and transformer engine.
And of course, as the name implies, this release fully leverages the latest features in PyTorch 2.1 🎉

• Highlights
  • Improvements To Large-Scale Training With FSDP
  • True Half-Precision
  • Bitsandbytes Quantization
  • Transformer Engine
  • Lightning on TPU Goes Brrr
  • Granular Control Over Checkpoints in Fabric
• Backward Incompatible Changes
  • PyTorch Lightning
  • Lightning Fabric
• Full Changelog
  • PyTorch Lightning
  • Lightning Fabric
  • Lightning App
• Contributors

Highlights

Improvements To Large-Scale Training With FSDP

The FSDP strategy for training large billion-parameter models gets substantial improvements and new features in Lightning 2.1, both in Trainer and Fabric (in case you didn't know, Fabric is the latest addition to the Lightning family of tools to scale models without the boilerplate code).
FSDP is now more user-friendly to configure, has memory management and speed improvements, and we have a brand new end-to-end user guide with best practices (Trainer, Fabric).

Efficient Saving and Loading of Large Checkpoints

When training large billion-parameter models with FSDP, saving and resuming training, or even just loading model parameters for finetuning can be challenging, as users are are often plagued by out-of-memory errors and speed bottlenecks.

In 2.1, we made several improvements. Starting with saving checkpoints, we added support for distributed/sharded checkpoints, enabled through the setting state_dict_type in the strategy (#18364, #18358):

Trainer:

import lightning as L
from lightning.pytorch.strategies import FSDPStrategy

# Default used by the strategy
strategy = FSDPStrategy(state_dict_type="full")

# Enable saving distributed checkpoints
strategy = FSDPStrategy(state_dict_type="sharded")

trainer = L.Trainer(strategy=strategy, ...)

Fabric:

import lightning as L
from lightning.fabric.strategies import FSDPStrategy

# Saving distributed checkpoints is the default
strategy = FSDPStrategy(state_dict_type="sharded")

# Save consolidated (single file) checkpoints
strategy = FSDPStrategy(state_dict_type="full")

fabric = L.Fabric(strategy=strategy, ...)

Distributed checkpoints are the fastest and most memory efficient way to save the state of very large models.
The distributed checkpoint format also makes it efficient to load these checkpoints back for resuming training in parallel, and it reduces the impact on CPU memory usage significantly. Furthermore, we've also introduced lazy-loading for non-distributed checkpoints (#18150, #18379), which greatly reduces the impact on CPU memory usage when loading a consolidated (single-file) checkpoint (e.g. for finetuning). Learn more about these features in our FSDP guides (Trainer, Fabric).

Fast and Memory-Optimized Initialization

A major challenge that users face when working with large models such as LLMs is dealing with the extreme memory requirements. Even something as simple as instantiating a model becomes non-trivial if the model is so large it won't fit in a single GPU or even a single machine. In Lightning 2.1, we are introducing empty-weights initialization through the Fabric.init_module() (#17462, #17627) and Trainer.init_module()/LightningModule.configure_model() (#18004, #18004, #18385) methods:

Trainer:

import lightning as L

class MyModel(L.LightningModule):
    def __init__(self):
        super().__init__()
        # Delay initialization of model to `configure_model()`

    def configure_model(self):
        # Model initialized in correct precision and weights on meta-device
        self.model = ...

    ...

trainer = L.Trainer(strategy="fsdp", ...)
trainer.fit(model)

Fabric:

import lightning as L

fabric = L.Fabric(strategy="fsdp", ...)

# Model initialized in correct precision and weights on meta-device
with fabric.init_module(empty_weights=True):
    model = ...
    

# You can also initialize buffers and tensors directly on device and dtype
with fabric.init_tensor():
    model.mask.create()
    model.kv_cache.create()
    x = torch.randn(4, 128)

# Materialization and sharding of model happens inside here
model = fabric.setup(model)

Read more about this new feature and its other benefits in our docs (Trainer, Fabric).

User-Friendly Configuration

We made it super easy to configure the sharding- and activation-checkpointing policy when you want to auto-wrap particular layers of your model for advanced control (#18045, #18084).

  import lightning as L
  from lightning.pytorch.strategies import FSDPStrategy
- from torch.distributed.fsdp.wrap import ModuleWrapPolicy

- strategy = FSDPStrategy(auto_wrap_policy=ModuleWrapPolicy({MyTransformerBlock}))
+ strategy = FSDPStrategy(auto_wrap_policy={MyTransformerBlock})
  trainer = L.Trainer(strategy=strategy, ...)

Furthermore, the sharding strategy can now be conveniently set with a string value (#18087):

  import lightning as L
  from lightning.pytorch.strategies import FSDPStrategy
- from torch.distributed.fsdp.fully_sharded_data_parallel import ShardingStrategy

- strategy = FSDPStrategy(sharding_strategy=ShardingStrategy.SHARD_GRAD_OP)
+ strategy = FSDPStrategy(sharding_strategy="SHARD_GRAD_OP")
  trainer = L.Trainer(strategy=strategy, ...)

You no longer need to remember the long PyTorch imports! Fabric also supports all these improvements shown above.

True Half-Precision

Lightning now supports true half-precision for training and inference with all built-in strategies (#18193, #18217, #18213, #18219). With this setting, the memory required to store the model weights is only half of what is normally needed when running with float32. In addition, you get the same speed benefits as mixed precision training (precision="16-mixed") has:

import lightning as L

# default
trainer = L.Trainer(precision="32-true")

# train with model weights in `torch.float16`
trainer = L.Trainer(precision="16-true")

# train with model weights in `torch.bfloat16`
# (if hardware supports it)
trainer = L.Trainer(precision="bf16-true")

The same settings are also available in Fabric! We recommend to try bfloat16 training (precision="bf16-true") as it is often more numerically stable than regular 16-bit precision (precision="16-true").

Bitsandbytes Quantization

With the new Bitsandbytes precision plugin #18655, you can now quantize your model for significant memory savings during training, finetuning, or inference with a selection of several state-of-the-art quantization algorithms (int8, fp4, nf4 and more). For the first time, Trainer and Fabric make bitsandbytes easy to use for general models.

Trainer:

import lightning as L
from lightning.pytorch.plugins import BitsandbytesPrecisionPlugin

# this will pick out the compute dtype automatically, by default `bfloat16`
precision = BitsandbytesPrecisionPlugin("nf4-dq")
trainer = L.Trainer(plugins=precision)

Fabric:

import lightning as L
from lightning.fabric.plugins import BitsandbytesPrecision

# this will pick out the compute dtype automatically, by default `bfloat16`
precision = BitsandbytesPrecision("nf4-dq")
trainer = L.Fabric(plugins=precision)

Learn more!

Transformer Engine

The Transformer Engine by NVIDIA is a library for accelerating transformer layers on the new Hopper (H100) generation of GPUs. With the integration in Lightning Trainer and Fabric (#17597, #18459), you have easy access to the 8-bit mixed precision for significant speed ups:

Trainer:

import lightning as L

# Select 8-bit mixed precision via TransformerEngine, with model weights in float16
trainer = L.Trainer(precision="transformer-engine-float16")

Fabric:

import lightning as L

# Select 8-bit mixed precision via TransformerEngine, with model weights in float16
fabric = L.Fabric(precision="transformer-engine-float16")

More configuration options are available through the respective plugins in Trainer and Fabric.

Lightning on TPU Goes Brrr

Lightning 2.1 runs on the latest generation of TPU hardware on Google Cloud! TPU-v4 and TPU-v5 (#17227) are now fully supported both in Fabric and Trainer and run using the new PjRT runtime by default (#17352). PjRT is the runtime used by Jax and has shown an average improvement of 35% on benchmarks.

Trainer:

import lightning as L

trainer = L.Trainer(accelerator="tpu", devices=8)
model = MyModel()
trainer.fit(model)  # uses PjRT if available

Fabric:

import lightning as L


def train(fabric):
    ...

fabric = L.Fabric(accelerator="tpu")
fabric.launch(train)  # uses PjRT if available

And what's even more exciting, you can now scale massive multi-billion parameter models on TPUs using FSDP (#17421).

import lightning as L
from lightning.fabric.strategies import XLAFSDPStrategy

strategy = XLAFSDPStrategy(
    # Most arguments from the PyTorch native FSDP strategy are also available here!
    auto_wrap_policy={Block},
    activation_checkpointing_policy={Block},
    state_dict_type="full",
    sequential_save=True,
)
    
fabric = L.Fabric(devices=8, strategy=strategy)
fabric.launch(finetune)

You can find a full end-to-end finetuning example script in our Lit-GPT repository. The new XLA-FSDP strategy is experimental and currently only available in Fabric. Support in the Trainer will follow in the future.

Granular Control Over Checkpoints in Fabric

Several improvements for checkpoint saving and loading have landed in Fabric, enabling more fine-grained control over what is saved/loaded while reducing boilerplate code:

1. There is a new Fabric.load_raw() method with which you can load model- or optimizer state-dicts saved externally by a non-Fabric application (e.g., raw PyTorch) (#18049)

   import lightning as L
   
   fabric = L.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"))

2. A new parameter Fabric.load(..., strict=True|False) to disable strict loading (#17645)

   import lightning as L
   
   fabric = L.Fabric()
   model = MyModel()
   state = {"model": model}
   
   # strict loading is the default
   fabric.load("path/to/checkpoint.ckpt", state, strict=True)
   
   # disable strict loading
   fabric.load("path/to/checkpoint.ckpt", state, strict=False)

3. A new parameter Fabric.save(..., filter=...) that enables you to exclude certain parameters of your model without writing boilerplate code for it (#17845)

   import lightning as L
   
   fabric = L.Fabric()
   model, optimizer = ...
   
   state = {"model": model, "optimizer": optimizer, "foo": 123}
   
   # save only the weights that match a pattern
   filter = {"model": lambda k, v: "weight" in k}
   fabric.save("path/to/checkpoint.ckpt", state, filter=filter)

You can read more about the new options in our checkpoint guide.

Backward Incompatible Changes

The release of PyTorch Lightning 2.0 was a big step into a new chapter: It brought a more polished API and removed a lot of legacy code and outdated as well as experimental features, at the cost of a long list of breaking changes resulting in more work needed than usual to upgrade from 1.9 to 2.0. Moving forward, we promised to maintain full backward compatibility of our public core APIs to guarantee a smooth upgrade experience for everyone, and with 2.1 we are happy to deliver on this promise. A few exceptions were made in places where the change was justified if it significantly improves the user experience, improves performance, or fixes the correctness of a feature. These changes will likely not impact most users.

PyTorch Lightning

TPU/XLA Changes

When selecting device indices via devices=[i], the Trainer now selects the i-th TPU core (0-based, previously it was 1-based) (#17227)

Before:

# Selects the first TPU core (1-based index)
trainer = Trainer(accelerator="tpu", devices=[1])

Now:

# Selects the second TPU core (0-based index)
trainer = Trainer(accelerator="tpu", devices=[1])

Multi-GPU in Jupyter Notebooks

Due to lack of reliability, Trainer now only runs on one GPU instead of all GPUs in a Jupyter notebook if devices="auto" (default) (#18291)

Before:

import lightning as L

# In Jupyter notebooks, this would select all available GPUs (DDP)
trainer = L.Trainer(accelerator="cuda", devices="auto")

Now:

# In Jupyter notebooks, this now selects only one GPU (the first)
trainer = L.Trainer(accelerator="cuda", devices="auto")

# You can still explicitly select multiple
trainer = L.Trainer(accelerator="cuda", devices=8)

Device Access in Setup Hook

• During LightningModule.setup(), the self.device now returns the device the module will be placed on instead of cpu (#18021)

Before:

def setup(self, stage):
    # CPU regardless of the accelerator used
    print(self.device)

Now:

def setup(self, stage):
    # CPU/CUDA/MPS/XLA depending on accelerator
    print(self.device)

Miscellaneous Changes

• self.loged tensors are now kept in the original device to reduce unnecessary host-to-device synchronizations (#17334)
• The FSDPStrategy now loads checkpoints after the configure_model/configure_sharded_model hook (#18358)
• The FSDPStrategy.load_optimizer_state_dict and FSDPStrategy.load_model_state_dict are a no-op now (#18358)
• Removed experimental support for torchdistx due to a lack of project maintenance (#17995)
• Dropped support for PyTorch 1.11 (#18691)

Lightning Fabric

We thank the community for the amazing feedback we got for Fabric so far - keep it coming. The list of breaking changes is short and won't affect the vast majority of users.

Sharding Context Manager in Fabric.run()

We removed automatic sharding support with Fabric.run or using fabric.launch(fn). This only impacts FSDP and DeepSpeed strategy users who use this way of launching. Please note that Fabric.run is a legacy construct from the LightningLite days, and is not recommended today. Please instantiate your large FSDP or DeepSpeed model under the newly added fabric.init_module context manager (#17832).

Before:

import lightning as L

def train(fabric):
    # FSDP's `enable_wrap` context or `deepspeed.zero.Init()`
    # were applied automaticaly here
    model = LargeModel()
    ...
        
fabric = L.Fabric()
fabric.launch(train)

Now:

def train(fabric):
    # Use `init_module` explicitly to apply these context managers
    with fabric.init_module():
        model = LargeModel()
    ...

Multi-GPU in Jupyter Notebooks

Due to lack of reliability, Fabric now only runs on one GPU instead of all GPUs in a Jupyter notebook if devices="auto" (default) (#18291)

Before:

import lightning as L

# In Jupyter notebooks, this would select all available GPUs (DDP)
fabric = L.Fabric(accelerator="cuda", devices="auto")

Now:

# In Jupyter notebooks, this now selects only one GPU (the first)
fabric = L.Fabric(accelerator="cuda", devices="auto")

# You can still explicitly select multiple
fabric = L.Fabric(accelerator="cuda", devices=8)

CHANGELOG

PyTorch Lightning

Lightning Fabric

Lightning App

Full commit list: 2.0.0...2.1.0

Contributors

Veteran

@adamjstewart @akreuzer @ethanwharris @dmitsf @lantiga @nicolai86 @pl-ghost @carmocca @awaelchli @justusschock @edenlightning @belerico @lightningforever @nisheethlahoti @tchaton @yurijmikhalevich @mauvilsa @rlizzo @rusmux @yhl48 @Liyang90 @jerome-habana @JustinGoheen @Borda @speediedan @SkafteNicki @dcfidalgo

New

@saryazdi @parambharat @kshitij12345 @woqidaideshi @colehawkins @md-121 @gkroiz @idc9 @BoringDonut @OmerShubi @ishandutta0098 @ryan597 @leng-yue @alicanb @One-sixth @santurini @SpirinEgor @KogaiIrina @shanmugamr1992 @janeyx99 @asmith26 @dingusagar @AleksanderWWW @strawberrypie @solyaH @kaczmarj @voidful @water-vapor @bkiat1123 @rhiga2 @baskrahmer @felipewhitaker @mukhery @Quasar-Kim @robieta @one-matrix @jere357 @schmidt-ai @schuhschuh @anio @rjarun8 @callumhay @minhlong94 @klieret @giorgioskij @shihaoyin @JonathanRayner @NripeshN @marcimarc1 @bilelomrani1 @NikolasWolke @0x404 @quintenroets @Borodin @amorehead @SebastianGer @ioangatop @Tribhuvan0 @f0k @sameertantry @kwsp @nik777 @matsumotosan

Did you know?

When Chuck Norris trains a neural network, it not only learns, but it also gains the ability to defend itself from adversarial attacks by roundhouse kicking them into submission.