ray-train

# Ray Train - Distributed Training Orchestration

## Quick start

Ray Train scales machine learning training from single GPU to multi-node clusters with minimal code changes.

**Installation**:
```bash
pip install -U "ray[train]"
```

**Basic PyTorch training** (single node):

```python
import ray
from ray import train
from ray.train import ScalingConfig
from ray.train.torch import TorchTrainer
import torch
import torch.nn as nn

# Define training function
def train_func(config):
    # Your normal PyTorch code
    model = nn.Linear(10, 1)
    optimizer = torch.optim.SGD(model.parameters(), lr=0.01)

    # Prepare for distributed (Ray handles device placement)
    model = train.torch.prepare_model(model)

    for epoch in range(10):
        # Your training loop
        output = model(torch.randn(32, 10))
        loss = output.sum()
        loss.backward()
        optimizer.step()
        optimizer.zero_grad()

        # Report metrics (logged automatically)
        train.report({"loss": loss.item(), "epoch": epoch})

# Run distributed training
trainer = TorchTrainer(
    train_func,
    scaling_config=ScalingConfig(
        num_workers=4,  # 4 GPUs/workers
        use_gpu=True
    )
)

result = trainer.fit()
print(f"Final loss: {result.metrics['loss']}")
```

**That's it!** Ray handles:
- Distributed coordination
- GPU allocation
- Fault tolerance
- Checkpointing
- Metric aggregation

## Common workflows

### Workflow 1: Scale existing PyTorch code

**Original single-GPU code**:
```python
model = MyModel().cuda()
optimizer = torch.optim.Adam(model.parameters())

for epoch in range(epochs):
    for batch in dataloader:
        loss = model(batch)
        loss.backward()
        optimizer.step()
```

**Ray Train version** (scales to multi-GPU/multi-node):
```python
from ray.train.torch import TorchTrainer
from ray import train

def train_func(config):
    model = MyModel()
    optimizer = torch.optim.Adam(model.parameters())

    # Prepare for distributed (automatic device placement)
    model = train.torch.prepare_model(model)
    dataloader = train.torch.prepare_data_loader(dataloader)

    for epoch in range(epochs):
        for batch in dataloader:
            loss = model(batch)
            loss.backward()
            optimizer.step()

            # Report metrics
            train.report({"loss": loss.item()})

# Scale to 8 GPUs
trainer = TorchTrainer(
    train_func,
    scaling_config=ScalingConfig(num_workers=8, use_gpu=True)
)
trainer.fit()
```

**Benefits**: Same code runs on 1 GPU or 1000 GPUs

### Workflow 2: HuggingFace Transformers integration

```python
from ray.train.huggingface import TransformersTrainer
from transformers import AutoModelForCausalLM, AutoTokenizer, TrainingArguments

def train_func(config):
    # Load model and tokenizer
    model = AutoModelForCausalLM.from_pretrained("gpt2")
    tokenizer = AutoTokenizer.from_pretrained("gpt2")

    # Training arguments (HuggingFace API)
    training_args = TrainingArguments(
        output_dir="./output",
        num_train_epochs=3,
        per_device_train_batch_size=8,
        learning_rate=2e-5,
    )

    # Ray automatically handles distributed training
    from transformers import Trainer
    trainer = Trainer(
        model=model,
        args=training_args,
        train_dataset=train_dataset,
    )

    trainer.train()

# Scale to multi-node (2 nodes × 8 GPUs = 16 workers)
trainer = TransformersTrainer(
    train_func,
    scaling_config=ScalingConfig(
        num_workers=16,
        use_gpu=True,
        resources_per_worker={"GPU": 1}
    )
)

result = trainer.fit()
```

### Workflow 3: Hyperparameter tuning with Ray Tune

```python
from ray import tune
from ray.train.torch import TorchTrainer
from ray.tune.schedulers import ASHAScheduler

def train_func(config):
    # Use hyperparameters from config
    lr = config["lr"]
    batch_size = config["batch_size"]

    model = MyModel()
    optimizer = torch.optim.Adam(model.parameters(), lr=lr)

    model = train.torch.prepare_model(model)

    for epoch in range(10):
        # Training loop
        loss = train_epoch(model, optimizer, batch_size)
        train.report({"loss": loss, "epoch": epoch})

# Define search space
param_space = {
    "lr": tune.loguniform(1e-5, 1e-2),
    "batch_size": tune.choice([16, 32, 64, 128])
}

# Run 20 trials with early stopping
tuner = tune.Tuner(
    TorchTrainer(
        train_func,
        scaling_config=ScalingConfig(num_workers=4, use_gpu=True)
    ),
    param_space=param_space,
    tune_config=tune.TuneConfig(
        num_samples=20,
        scheduler=ASHAScheduler(metric="loss", mode="min")
    )
)

results = tuner.fit()
best = results.get_best_result(metric="loss", mode="min")
print(f"Best hyperparameters: {best.config}")
```

**Result**: Distributed hyperparameter search across cluster

### Workflow 4: Checkpointing and fault tolerance

```python
from ray import train
from ray.train import Checkpoint

def train_func(config):
    model = MyModel()
    optimizer = torch.optim.Adam(model.parameters())

    # Try to resume from checkpoint
    checkpoint = train.get_checkpoint()
    if checkpoint:
        with checkpoint.as_directory() as checkpoint_dir:
            state = torch.load(f"{checkpoint_dir}/model.pt")
            model.load_state_dict(state["model"])
            optimizer.load_state_dict(state["optimizer"])
            start_epoch = state["epoch"]
    else:
        start_epoch = 0

    model = train.torch.prepare_model(model)

    for epoch in range(start_epoch, 100):
        loss = train_epoch(model, optimizer)

        # Save checkpoint every 10 epochs
        if epoch % 10 == 0:
            checkpoint = Checkpoint.from_directory(
                train.get_context().get_trial_dir()
            )
            torch.save({
                "model": model.state_dict(),
                "optimizer": optimizer.state_dict(),
                "epoch": epoch
            }, checkpoint.path / "model.pt")

            train