Distributed PyTorch Lightning Training on Ray

This library adds new PyTorch Lightning plugins for distributed training using the Ray distributed computing framework.

These PyTorch Lightning Plugins on Ray enable quick and easy parallel training while still leveraging all the benefits of PyTorch Lightning and using your desired training protocol, either PyTorch Distributed Data Parallel or Horovod.

Once you add your plugin to the PyTorch Lightning Trainer, you can parallelize training to all the cores in your laptop, or across a massive multi-node, multi-GPU cluster with no additional code changes.

This library also comes with an integration with Ray Tune for distributed hyperparameter tuning experiments.

Installation

You can install Ray Lightning via pip:

pip install ray_lightning

Or to install master:

pip install git+https://github.com/ray-project/ray_lightning#ray_lightning

PyTorch Distributed Data Parallel Plugin on Ray

The RayPlugin provides Distributed Data Parallel training on a Ray cluster. PyTorch DDP is used as the distributed training protocol, and Ray is used to launch and manage the training worker processes.

Here is a simplified example:

import pytorch_lightning as pl
from ray_lightning import RayPlugin

# Create your PyTorch Lightning model here.
ptl_model = MNISTClassifier(...)
plugin = RayPlugin(num_workers=4, num_cpus_per_worker=1, use_gpu=True)

# Don't set ``gpus`` in the ``Trainer``.
# The actual number of GPUs is determined by ``num_workers``.
trainer = pl.Trainer(..., plugins=[plugin])
trainer.fit(ptl_model)

Because Ray is used to launch processes, instead of the same script being called multiple times, you CAN use this plugin even in cases when you cannot use the standard DDPPlugin such as

  • Jupyter Notebooks, Google Colab, Kaggle

  • Calling fit or test multiple times in the same script

Multi-node Distributed Training

Using the same examples above, you can run distributed training on a multi-node cluster with just 2 simple steps. 1) Use Ray’s cluster launcher to start a Ray cluster- ray up my_cluster_config.yaml. 2) Execute your Python script on the Ray cluster- ray submit my_cluster_config.yaml train.py. This will rsync your training script to the head node, and execute it on the Ray cluster.

You no longer have to set environment variables or configurations and run your training script on every single node.

Multi-node Training from your Laptop

Ray provides capabilities to run multi-node and GPU training all from your laptop through Ray Client

You can follow the instructions here to setup the cluster. Then, add this line to the beginning of your script to connect to the cluster:

# replace with the appropriate host and port
ray.init("ray://<head_node_host>:10001")

Now you can run your training script on the laptop, but have it execute as if your laptop has all the resources of the cluster essentially providing you with an infinite laptop.

Note: When using with Ray Client, you must disable checkpointing and logging for your Trainer by setting checkpoint_callback and logger to False.

Horovod Plugin on Ray

Or if you prefer to use Horovod as the distributed training protocol, use the HorovodRayPlugin instead.

import pytorch_lightning as pl
from ray_lightning import HorovodRayPlugin

# Create your PyTorch Lightning model here.
ptl_model = MNISTClassifier(...)

# 2 nodes, 4 workers per node, each using 1 CPU and 1 GPU.
plugin = HorovodRayPlugin(num_hosts=2, num_slots=4, use_gpu=True)

# Don't set ``gpus`` in the ``Trainer``.
# The actual number of GPUs is determined by ``num_slots``.
trainer = pl.Trainer(..., plugins=[plugin])
trainer.fit(ptl_model)

Model Parallel Sharded Training on Ray

The RayShardedPlugin integrates with FairScale to provide sharded DDP training on a Ray cluster. With sharded training, leverage the scalability of data parallel training while drastically reducing memory usage when training large models.

import pytorch_lightning as pl
from ray_lightning import RayShardedPlugin

# Create your PyTorch Lightning model here.
ptl_model = MNISTClassifier(...)
plugin = RayShardedPlugin(num_workers=4, num_cpus_per_worker=1, use_gpu=True)

# Don't set ``gpus`` in the ``Trainer``.
# The actual number of GPUs is determined by ``num_workers``.
trainer = pl.Trainer(..., plugins=[plugin])
trainer.fit(ptl_model)

See the Pytorch Lightning docs for more information on sharded training.

Hyperparameter Tuning with Ray Tune

ray_lightning also integrates with Ray Tune to provide distributed hyperparameter tuning for your distributed model training. You can run multiple PyTorch Lightning training runs in parallel, each with a different hyperparameter configuration, and each training run parallelized by itself. All you have to do is move your training code to a function, pass the function to tune.run, and make sure to add the appropriate callback (Either TuneReportCallback or TuneReportCheckpointCallback) to your PyTorch Lightning Trainer.

Example using ray_lightning with Tune:

from ray import tune

from ray_lightning import RayPlugin
from ray_lightning.tune import TuneReportCallback, get_tune_ddp_resources

def train_mnist(config):

    # Create your PTL model.
    model = MNISTClassifier(config)

    # Create the Tune Reporting Callback
    metrics = {"loss": "ptl/val_loss", "acc": "ptl/val_accuracy"}
    callbacks = [TuneReportCallback(metrics, on="validation_end")]

    trainer = pl.Trainer(
        max_epochs=4,
        callbacks=callbacks,
        plugins=[RayPlugin(num_workers=4, use_gpu=False)])
    trainer.fit(model)

config = {
    "layer_1": tune.choice([32, 64, 128]),
    "layer_2": tune.choice([64, 128, 256]),
    "lr": tune.loguniform(1e-4, 1e-1),
    "batch_size": tune.choice([32, 64, 128]),
}

# Make sure to pass in ``resources_per_trial`` using the ``get_tune_ddp_resources`` utility.
analysis = tune.run(
        train_mnist,
        metric="loss",
        mode="min",
        config=config,
        num_samples=num_samples,
        resources_per_trial=get_tune_ddp_resources(num_workers=4),
        name="tune_mnist")

print("Best hyperparameters found were: ", analysis.best_config)

FAQ

RaySGD already has a Pytorch Lightning integration. What’s the difference between this integration and that?

The key difference is which Trainer you’ll be interacting with. In this library, you will still be using Pytorch Lightning’s Trainer. You’ll be able to leverage all the features of Pytorch Lightning, and Ray is used just as a backend to handle distributed training.

With RaySGD’s integration, you’ll be converting your LightningModule to be RaySGD compatible, and will be interacting with RaySGD’s TorchTrainer. RaySGD’s TorchTrainer is not as feature rich nor as easy to use as Pytorch Lightning’s Trainer (no built in support for logging, early stopping, etc.). However, it does have built in support for fault-tolerant and elastic training. If these are hard requirements for you, then RaySGD’s integration with PTL might be a better option.

I see that RayPlugin is based off of Pytorch Lightning’s DDPSpawnPlugin. However, doesn’t the PTL team discourage the use of spawn?

As discussed here, using a spawn approach instead of launch is not all that detrimental. The original factors for discouraging spawn were:

  1. not being able to use ‘spawn’ in a Jupyter or Colab notebook, and

  2. not being able to use multiple workers for data loading.

Neither of these should be an issue with the RayPlugin due to Ray’s serialization mechanisms. The only thing to keep in mind is that when using this plugin, your model does have to be serializable/pickleable.

API Reference

class ray_lightning.RayPlugin(num_workers: int = 1, num_cpus_per_worker: int = 1, use_gpu: bool = False, init_hook: Callable = None, **ddp_kwargs: Union[Any, Dict[str, Any]])[source]

Pytorch Lightning plugin for DDP training on a Ray cluster.

This plugin is used to manage distributed training using DDP and Ray for process launching. Internally, the specified number of Ray actors are launched in the cluster and are registered as part of a Pytorch DDP process group. The Pytorch Lightning trainer is instantiated on the driver and sent to each of these training workers where training is executed. The distributed training protocol is handled by Pytorch DDP.

Each training worker is configured to reserve num_cpus_per_worker CPUS and 1 GPU if use_gpu is set to True.

If using this plugin, you should run your code like a normal Python script: python train.py, and only on the head node if running in a distributed Ray cluster. There is no need to run this script on every single node.

Parameters
  • num_workers (int) – Number of training workers to use.

  • num_cpus_per_worker (int) – Number of CPUs per worker.

  • use_gpu (bool) – Whether to use GPU for allocation. For GPU to be used, you must also set the gpus arg in your Pytorch Lightning Trainer to a value > 0.

  • init_hook (Callable) – A function to run on each worker upon instantiation.

  • **ddp_kwargs – Additional arguments to pass into DistributedDataParallel initialization

Example

import pytorch_lightning as ptl
from ray_lightning import RayAccelerator

ptl_model = MNISTClassifier(...)
plugin = RayPlugin(num_workers=4, cpus_per_worker=1,
    use_gpu=True)

# If using GPUs, set the ``gpus`` arg to a value > 0.
# The actual number of GPUs is determined by ``num_workers``.
trainer = pl.Trainer(..., gpus=1, plugins=[plugin])
trainer.fit(ptl_model)

PublicAPI (beta): This API is in beta and may change before becoming stable.

class ray_lightning.HorovodRayPlugin(num_hosts: int = 1, num_slots: int = 1, use_gpu: bool = False)[source]

Pytorch Lightning Plugin for Horovod training on a Ray cluster.

This plugin is used to manage distributed training on a Ray cluster via the Horovod training framework. Internally, the specified number of Ray actors are launched in the cluster and are configured as part of the Horovod ring. The Pytorch Lightning trainer is instantiated on the driver and sent to each of these training workers where training is executed. The distributed training protocol is handled by Horovod.

Each training worker is configured to reserve 1 CPU and if 1 GPU if use_gpu is set to True.

If using this plugin, you should run your code like a normal Python script: python train.py, and not with horovodrun.

Parameters
  • num_hosts (int) – The number of nodes/machines to execute the job on.

  • num_slots (int) – Number of workers to be placed on each machine.

  • use_gpu (bool) – Whether to use GPU for allocation. For GPU to be used, you must also set the gpus arg in your Pytorch Lightning Trainer to a value > 0.

Example

import pytorch_lightning as ptl
from ray_lightning import HorovodRayPlugin

ptl_model = MNISTClassifier(...)
# 2 nodes, 4 workers per node, each using 1 CPU and 1 GPU.
plugin = HorovodRayPlugin(num_hosts=2, num_slots=4,
    use_gpu=True)

# If using GPUs, set the ``gpus`` arg to a value > 0.
# The actual number of GPUs is determined by ``num_slots``.
trainer = pl.Trainer(..., gpus=1, plugins=[plugin])
trainer.fit(ptl_model)

PublicAPI (beta): This API is in beta and may change before becoming stable.

class ray_lightning.RayShardedPlugin(num_workers: int = 1, num_cpus_per_worker: int = 1, use_gpu: bool = False, init_hook: Callable = None, **ddp_kwargs: Union[Any, Dict[str, Any]])[source]

PublicAPI (beta): This API is in beta and may change before becoming stable.

Tune Integration

class ray_lightning.tune.TuneReportCallback(metrics: Union[None, str, List[str], Dict[str, str]] = None, on: Union[str, List[str]] = 'validation_end')[source]

Distributed PyTorch Lightning to Ray Tune reporting callback

Reports metrics to Ray Tune, specifically when training is done remotely with Ray via the accelerators in this library.

Args:
metrics (str|list|dict): Metrics to report to Tune.

If this is a list, each item describes the metric key reported to PyTorch Lightning, and it will reported under the same name to Tune. If this is a dict, each key will be the name reported to Tune and the respective value will be the metric key reported to PyTorch Lightning.

on (str|list): When to trigger checkpoint creations.

Must be one of the PyTorch Lightning event hooks (less the on_), e.g. “batch_start”, or “train_end”. Defaults to “validation_end”.

Example:

import pytorch_lightning as pl
from ray_lightning import RayPlugin
from ray_lightning.tune import TuneReportCallback

# Create plugin.
ray_plugin = RayPlugin(num_workers=4, use_gpu=True)

# Report loss and accuracy to Tune after each validation epoch:
trainer = pl.Trainer(plugins=[ray_plugin], callbacks=[
    TuneReportCallback(["val_loss", "val_acc"],
        on="validation_end")])

# Same as above, but report as `loss` and `mean_accuracy`:
trainer = pl.Trainer(plugins=[ray_plugin], callbacks=[
    TuneReportCallback(
        {"loss": "val_loss", "mean_accuracy": "val_acc"},
        on="validation_end")])

PublicAPI (beta): This API is in beta and may change before becoming stable.

class ray_lightning.tune.TuneReportCheckpointCallback(metrics: Union[None, str, List[str], Dict[str, str]] = None, filename: str = 'checkpoint', on: Union[str, List[str]] = 'validation_end')[source]

PyTorch Lightning to Tune reporting and checkpointing callback.

Saves checkpoints after each validation step. Also reports metrics to Tune, which is needed for checkpoint registration. To be used specifically with the plugins in this library.

Args:
metrics (str|list|dict): Metrics to report to Tune.

If this is a list, each item describes the metric key reported to PyTorch Lightning, and it will reported under the same name to Tune. If this is a dict, each key will be the name reported to Tune and the respective value will be the metric key reported to PyTorch Lightning.

filename (str): Filename of the checkpoint within the

checkpoint directory. Defaults to “checkpoint”.

on (str|list): When to trigger checkpoint creations. Must be

one of the PyTorch Lightning event hooks (less the on_), e.g. “batch_start”, or “train_end”. Defaults to “validation_end”.

Example:

import pytorch_lightning as pl
from ray_lightning import RayPlugin
from ray_lightning.tune import TuneReportCheckpointCallback.

# Create the Ray plugin.
ray_plugin = RayPlugin()

# Save checkpoint after each training batch and after each
# validation epoch.
trainer = pl.Trainer(plugins=[ray_plugin], callbacks=[
    TuneReportCheckpointCallback(
        metrics={"loss": "val_loss",
                "mean_accuracy": "val_acc"},
        filename="trainer.ckpt", on="validation_end")])

PublicAPI (beta): This API is in beta and may change before becoming stable.

ray_lightning.tune.get_tune_ddp_resources(num_workers: int = 1, cpus_per_worker: int = 1, use_gpu: bool = False) → Dict[str, int][source]

Returns the PlacementGroupFactory to use for Ray Tune. PublicAPI (beta): This API is in beta and may change before becoming stable.