distributed-training-accelerate

HuggingFace Accelerate - Unified Distributed Training

Quick start

Accelerate simplifies distributed training to 4 lines of code.

Installation:

pip install accelerate

Convert PyTorch script (4 lines):

import torch
+ from accelerate import Accelerator

+ accelerator = Accelerator()

  model = torch.nn.Transformer()
  optimizer = torch.optim.Adam(model.parameters())
  dataloader = torch.utils.data.DataLoader(dataset)

+ model, optimizer, dataloader = accelerator.prepare(model, optimizer, dataloader)

  for batch in dataloader:
      optimizer.zero_grad()
      loss = model(batch)
-     loss.backward()
+     accelerator.backward(loss)
      optimizer.step()

Run (single command):

accelerate launch train.py

Common workflows

Workflow 1: From single GPU to multi-GPU

Original script:

# train.py
import torch

model = torch.nn.Linear(10, 2).to('cuda')
optimizer = torch.optim.Adam(model.parameters())
dataloader = torch.utils.data.DataLoader(dataset, batch_size=32)

for epoch in range(10):
    for batch in dataloader:
        batch = batch.to('cuda')
        optimizer.zero_grad()
        loss = model(batch).mean()
        loss.backward()
        optimizer.step()

With Accelerate (4 lines added):

# train.py
import torch
from accelerate import Accelerator  # +1

accelerator = Accelerator()  # +2

model = torch.nn.Linear(10, 2)
optimizer = torch.optim.Adam(model.parameters())
dataloader = torch.utils.data.DataLoader(dataset, batch_size=32)

model, optimizer, dataloader = accelerator.prepare(model, optimizer, dataloader)  # +3

for epoch in range(10):
    for batch in dataloader:
        # No .to('cuda') needed - automatic!
        optimizer.zero_grad()
        loss = model(batch).mean()
        accelerator.backward(loss)  # +4
        optimizer.step()

Configure (interactive):

accelerate config

Questions:

• Which machine? (single/multi GPU/TPU/CPU)
• How many machines? (1)
• Mixed precision? (no/fp16/bf16/fp8)
• DeepSpeed? (no/yes)

Launch (works on any setup):

# Single GPU
accelerate launch train.py

# Multi-GPU (8 GPUs)
accelerate launch --multi_gpu --num_processes 8 train.py

# Multi-node
accelerate launch --multi_gpu --num_processes 16 \
  --num_machines 2 --machine_rank 0 \
  --main_process_ip $MASTER_ADDR \
  train.py

Workflow 2: Mixed precision training

Enable FP16/BF16:

from accelerate import Accelerator

# FP16 (with gradient scaling)
accelerator = Accelerator(mixed_precision='fp16')

# BF16 (no scaling, more stable)
accelerator = Accelerator(mixed_precision='bf16')

# FP8 (H100+)
accelerator = Accelerator(mixed_precision='fp8')

model, optimizer, dataloader = accelerator.prepare(model, optimizer, dataloader)

# Everything else is automatic!
for batch in dataloader:
    with accelerator.autocast():  # Optional, done automatically
        loss = model(batch)
    accelerator.backward(loss)

Workflow 3: DeepSpeed ZeRO integration

Enable DeepSpeed ZeRO-2:

from accelerate import Accelerator

accelerator = Accelerator(
    mixed_precision='bf16',
    deepspeed_plugin={
        "zero_stage": 2,  # ZeRO-2
        "offload_optimizer": False,
        "gradient_accumulation_steps": 4
    }
)

# Same code as before!
model, optimizer, dataloader = accelerator.prepare(model, optimizer, dataloader)

Or via config:

accelerate config
# Select: DeepSpeed → ZeRO-2

deepspeed_config.json:

{
    "fp16": {"enabled": false},
    "bf16": {"enabled": true},
    "zero_optimization": {
        "stage": 2,
        "offload_optimizer": {"device": "cpu"},
        "allgather_bucket_size": 5e8,
        "reduce_bucket_size": 5e8
    }
}

Launch:

accelerate launch --config_file deepspeed_config.json train.py

Workflow 4: FSDP (Fully Sharded Data Parallel)

Enable FSDP:

from accelerate import Accelerator, FullyShardedDataParallelPlugin

fsdp_plugin = FullyShardedDataParallelPlugin(
    sharding_strategy="FULL_SHARD",  # ZeRO-3 equivalent
    auto_wrap_policy="TRANSFORMER_AUTO_WRAP",
    cpu_offload=False
)

accelerator = Accelerator(
    mixed_precision='bf16',
    fsdp_plugin=fsdp_plugin
)

model, optimizer, dataloader = accelerator.prepare(model, optimizer, dataloader)

Or via config:

accelerate config
# Select: FSDP → Full Shard → No CPU Offload

Workflow 5: Gradient accumulation

Accumulate gradients:

from accelerate import Accelerator

accelerator = Accelerator(gradient_accumulation_steps=4)

model, optimizer, dataloader = accelerator.prepare(model, optimizer, dataloader)

for batch in dataloader:
    with accelerator.accumulate(model):  # Handles accumulation
        optimizer.zero_grad()
        loss = model(batch)
        accelerator.backward(loss)
        optimizer.step()

Effective batch size: batch_size * num_gpus * gradient_accumulation_steps

When to use vs alternatives

Use Accelerate when:

• Want simplest distributed training
• Need single script for any hardware
• Use HuggingFace ecosystem
• Want flexibility (DDP/DeepSpeed/FSDP/Megatron)
• Need quick prototyping

Key advantages:

• 4 lines: Minimal code changes
• Unified API: Same code for DDP, DeepSpeed, FSDP, Megatron
• Automatic: Device placement, mixed precision, sharding
• Interactive config: No manual launcher setup
• Single launch: Works everywhere

Use alternatives instead:

• PyTorch Lightning: Need callbacks, high-level abstractions
• Ray Train: Multi-node orchestration, hyperparameter tuning
• DeepSpeed: Direct API control, advanced features
• Raw DDP: Maximum control, minimal abstraction

Common issues

Issue: Wrong device placement

Don't manually move to device:

# WRONG
batch = batch.to('cuda')

# CORRECT
# Accelerate handles it automatically after prepare()

Issue: Gradient accumulation not working

Use context manager:

# CORRECT
with accelerator.accumulate(model):
    optimizer.zero_grad()
    accelerator.backward(loss)
    optimizer.step()

Issue: Checkpointing in distributed

Use accelerator methods:

# Save only on main process
if accelerator.is_main_process:
    accelerator.save_state('checkpoint/')

# Load on all processes
accelerator.load_state('checkpoint/')

Issue: Different results with FSDP

Ensure same random seed:

from accelerate.utils import set_seed
set_seed(42)

Advanced topics

Megatron integration: See references/megatron-integration.md for tensor parallelism, pipeline parallelism, and sequence parallelism setup.

Custom plugins: See references/custom-plugins.md for creating custom distributed plugins and advanced configuration.

Performance tuning: See references/performance.md for profiling, memory optimization, and best practices.

Hardware requirements

• CPU: Works (slow)
• Single GPU: Works
• Multi-GPU: DDP (default), DeepSpeed, or FSDP
• Multi-node: DDP, DeepSpeed, FSDP, Megatron
• TPU: Supported
• Apple MPS: Supported

Launcher requirements:

• DDP: torch.distributed.run (built-in)
• DeepSpeed: deepspeed (pip install deepspeed)
• FSDP: PyTorch 1.12+ (built-in)
• Megatron: Custom setup

Resources

• Docs: https://huggingface.co/docs/accelerate
• GitHub: https://github.com/huggingface/accelerate
• Version: 1.11.0+
• Tutorial: "Accelerate your scripts"
• Examples: https://github.com/huggingface/accelerate/tree/main/examples
• Used by: HuggingFace Transformers, TRL, PEFT, all HF libraries