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# DDP

[DistributedDataParallel (DDP)](https://docs.pytorch.org/tutorials/beginner/ddp_series_theory.html) maintains a full copy of a model on each GPU. Each GPU processes a non-overlapping shard of data with a forward and backward pass. Before the optimizer step, an all-reduce averages gradients across all GPUs so every model copy stays identical. Use DDP when your model fits on a single GPU.

```text
                         ┌─────────────────┐
                         │  training data  │
                         └────────┬────────┘
               ┌──────────────────┼──────────────────┐
               │ shard 0          │ shard 1          │ shard 2
               ▼                  ▼                  ▼
        ┌─────────────┐    ┌─────────────┐    ┌─────────────┐
        │   model     │    │   model     │    │   model     │
        │  (copy 0)   │    │  (copy 1)   │    │  (copy 2)   │
        │   GPU 0     │    │   GPU 1     │    │   GPU 2     │
        └──────┬──────┘    └──────┬──────┘    └──────┬──────┘
               │ grads            │ grads            │ grads
               └──────────────────┼──────────────────┘
                               all-reduce
                          (average gradients)
               ┌──────────────────┼──────────────────┐
               ▼                  ▼                  ▼
        ┌─────────────┐    ┌─────────────┐    ┌─────────────┐
        │  optimizer  │    │  optimizer  │    │  optimizer  │
        │    step     │    │    step     │    │    step     │
        └─────────────┘    └─────────────┘    └─────────────┘
          (identical)        (identical)        (identical)
```

DDP activates automatically when you launch with a multi-process launcher like [Accelerate](./accelerate).

```cli
# 4 GPUs on one machine
accelerate launch --num_processes 4 train.py
```

## Configure DDP

Pass these [`TrainingArguments`] to control DDP behavior.

- [`~TrainingArguments.gradient_accumulation_steps`] determines when to perform the all-reduce. [`Trainer`] skips the all-reduce on intermediate accumulation steps and runs it only on the final micro-batch. For example, with `gradient_accumulation_steps=4`, the all-reduce runs every 4 backward passes.
- [`~TrainingArguments.ddp_find_unused_parameters`] traverses the autograd graph at the end of the forward pass for parameters that won't receive a gradient and marks them as ready so they don't block the all-reduce. Don't use with [`~TrainingArguments.gradient_checkpointing`] because gradient checkpointing discards intermediate activations and recomputes them on the fly.
- [`~TrainingArguments.ddp_bucket_cap_mb`] is the bucket size for batching gradients into a single all-reduce during the backward pass. A larger bucket means fewer all-reduce calls and less launch overhead.
- [`~TrainingArguments.ddp_broadcast_buffers`] synchronizes model buffers (such as BatchNorm running statistics) from rank 0 to all other ranks at the start of every forward pass. Disable if your model only uses LayerNorm. Don't use with [`~TrainingArguments.gradient_checkpointing`].
- [`~TrainingArguments.ddp_backend`] sets the communication backend. Use `"nccl"` for NVIDIA GPUs (default and fastest), `"gloo"` for CPU training or debugging, and `"xccl"`, `"hccl"`, or `"cncl"` for other hardware.
- [`~TrainingArguments.ddp_timeout`] sets the time limit for all processes and operations (all-reduce, broadcast) to complete. If a process hangs, like when loading a large model slowly, the timeout raises an error instead of blocking indefinitely.

```py
from transformers import TrainingArguments

args = TrainingArguments(
    ...,
    gradient_accumulation_steps=4,
    ddp_backend="nccl",
    ddp_find_unused_parameters=False,
    ddp_bucket_cap_mb=25,
    ddp_broadcast_buffers=True,
    ddp_timeout=1800,
)
```

## Next steps

- See [FSDP](./fsdp) for training models too large to fit on a single GPU.
- See [DeepSpeed](./deepspeed) for ZeRO optimization and offloading.
- Read the [Data Parallelism](https://nanotron-ultrascale-playbook.static.hf.space/index.html#data_parallelism) chapter from The Ultra-Scale Playbook for more information about how DDP works.
