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# Distributed inference

Distributed inference splits the workload across multiple GPUs. It a useful technique for fitting larger models in memory and can process multiple prompts for higher throughput.

This guide will show you how to use [Accelerate](https://huggingface.co/docs/accelerate/index) and [PyTorch Distributed](https://pytorch.org/tutorials/beginner/dist_overview.html) for distributed inference.

## Accelerate

Accelerate is a library designed to simplify inference and training on multiple accelerators by handling the setup, allowing users to focus on their PyTorch code.

Install Accelerate with the following command.

```bash
uv pip install accelerate
```

Initialize a [`accelerate.PartialState`] class in a Python file to create a distributed environment. The [`accelerate.PartialState`] class manages process management, device control and distribution, and process coordination.

Move the [`DiffusionPipeline`] to [`accelerate.PartialState.device`] to assign a GPU to each process.

```py
import torch
from accelerate import PartialState
from diffusers import DiffusionPipeline

pipeline = DiffusionPipeline.from_pretrained(
    "Qwen/Qwen-Image", torch_dtype=torch.float16
)
distributed_state = PartialState()
pipeline.to(distributed_state.device)
```

Use the [`~accelerate.PartialState.split_between_processes`] utility as a context manager to automatically distribute the prompts between the number of processes.

```py
with distributed_state.split_between_processes(["a dog", "a cat"]) as prompt:
    result = pipeline(prompt).images[0]
    result.save(f"result_{distributed_state.process_index}.png")
```

Call `accelerate launch` to run the script and use the `--num_processes` argument to set the number of GPUs to use.

```bash
accelerate launch run_distributed.py --num_processes=2
```

> [!TIP]
> Refer to this minimal example [script](https://gist.github.com/sayakpaul/cfaebd221820d7b43fae638b4dfa01ba) for running inference across multiple GPUs. To learn more, take a look at the [Distributed Inference with 🤗 Accelerate](https://huggingface.co/docs/accelerate/en/usage_guides/distributed_inference#distributed-inference-with-accelerate) guide.

## PyTorch Distributed

PyTorch [DistributedDataParallel](https://pytorch.org/docs/stable/generated/torch.nn.parallel.DistributedDataParallel.html) enables [data parallelism](https://huggingface.co/spaces/nanotron/ultrascale-playbook?section=data_parallelism), which replicates the same model on each device, to process different batches of data in parallel.

Import `torch.distributed` and `torch.multiprocessing` into a Python file to set up the distributed process group and to spawn the processes for inference on each GPU.

```py
import torch
import torch.distributed as dist
import torch.multiprocessing as mp

from diffusers import DiffusionPipeline

pipeline = DiffusionPipeline.from_pretrained(
    "Qwen/Qwen-Image", torch_dtype=torch.float16,
)
```

Create a function for inference with [init_process_group](https://pytorch.org/docs/stable/distributed.html?highlight=init_process_group#torch.distributed.init_process_group). This method creates a distributed environment with the backend type, the `rank` of the current process, and the `world_size` or number of processes participating (for example, 2 GPUs would be `world_size=2`).

Move the pipeline to `rank` and use `get_rank` to assign a GPU to each process. Each process handles a different prompt.

```py
def run_inference(rank, world_size):
    dist.init_process_group("nccl", rank=rank, world_size=world_size)

    pipeline.to(rank)

    if torch.distributed.get_rank() == 0:
        prompt = "a dog"
    elif torch.distributed.get_rank() == 1:
        prompt = "a cat"

    image = sd(prompt).images[0]
    image.save(f"./{'_'.join(prompt)}.png")
```

Use [mp.spawn](https://pytorch.org/docs/stable/multiprocessing.html#torch.multiprocessing.spawn) to create the number of processes defined in `world_size`.

```py
def main():
    world_size = 2
    mp.spawn(run_inference, args=(world_size,), nprocs=world_size, join=True)


if __name__ == "__main__":
    main()
```

Call `torchrun` to run the inference script and use the `--nproc_per_node` argument to set the number of GPUs to use.

```bash
torchrun run_distributed.py --nproc_per_node=2
```

## device_map

The `device_map` argument enables distributed inference by automatically placing model components on separate GPUs. This is especially useful when a model doesn't fit on a single GPU. You can use `device_map` to selectively load and unload the required model components at a given stage as shown in the example below (assumes two GPUs are available).

Set `device_map="balanced"` to evenly distributes the text encoders on all available GPUs. You can use the `max_memory` argument to allocate a maximum amount of memory for each text encoder. Don't load any other pipeline components to avoid memory usage.

```py
from diffusers import FluxPipeline
import torch

prompt = """
cinematic film still of a cat sipping a margarita in a pool in Palm Springs, California
highly detailed, high budget hollywood movie, cinemascope, moody, epic, gorgeous, film grain
"""

pipeline = FluxPipeline.from_pretrained(
    "black-forest-labs/FLUX.1-dev",
    transformer=None,
    vae=None,
    device_map="balanced",
    max_memory={0: "16GB", 1: "16GB"},
    torch_dtype=torch.bfloat16
)
with torch.no_grad():
    print("Encoding prompts.")
    prompt_embeds, pooled_prompt_embeds, text_ids = pipeline.encode_prompt(
        prompt=prompt, prompt_2=None, max_sequence_length=512
    )
```

After the text embeddings are computed, remove them from the GPU to make space for the diffusion transformer.

```py
import gc 

def flush():
    gc.collect()
    torch.cuda.empty_cache()
    torch.cuda.reset_max_memory_allocated()
    torch.cuda.reset_peak_memory_stats()

del pipeline.text_encoder
del pipeline.text_encoder_2
del pipeline.tokenizer
del pipeline.tokenizer_2
del pipeline

flush()
```

Set `device_map="auto"` to automatically distribute the model on the two GPUs. This strategy places a model on the fastest device first before placing a model on a slower device like a CPU or hard drive if needed. The trade-off of storing model parameters on slower devices is slower inference latency.

```py
from diffusers import AutoModel
import torch 

transformer = AutoModel.from_pretrained(
    "black-forest-labs/FLUX.1-dev", 
    subfolder="transformer",
    device_map="auto",
    torch_dtype=torch.bfloat16
)
```

> [!TIP]
> Run `pipeline.hf_device_map` to see how the various models are distributed across devices. This is useful for tracking model device placement. You can also call `hf_device_map` on the transformer model to see how it is distributed.

Add the transformer model to the pipeline and set the `output_type="latent"` to generate the latents.

```py
pipeline = FluxPipeline.from_pretrained(
    "black-forest-labs/FLUX.1-dev",
    text_encoder=None,
    text_encoder_2=None,
    tokenizer=None,
    tokenizer_2=None,
    vae=None,
    transformer=transformer,
    torch_dtype=torch.bfloat16
)

print("Running denoising.")
height, width = 768, 1360
latents = pipeline(
    prompt_embeds=prompt_embeds,
    pooled_prompt_embeds=pooled_prompt_embeds,
    num_inference_steps=50,
    guidance_scale=3.5,
    height=height,
    width=width,
    output_type="latent",
).images
```

Remove the pipeline and transformer from memory and load a VAE to decode the latents. The VAE is typically small enough to be loaded on a single device.

```py
import torch
from diffusers import AutoencoderKL
from diffusers.image_processor import VaeImageProcessor

vae = AutoencoderKL.from_pretrained(ckpt_id, subfolder="vae", torch_dtype=torch.bfloat16).to("cuda")
vae_scale_factor = 2 ** (len(vae.config.block_out_channels) - 1)
image_processor = VaeImageProcessor(vae_scale_factor=vae_scale_factor)

with torch.no_grad():
    print("Running decoding.")
    latents = FluxPipeline._unpack_latents(latents, height, width, vae_scale_factor)
    latents = (latents / vae.config.scaling_factor) + vae.config.shift_factor

    image = vae.decode(latents, return_dict=False)[0]
    image = image_processor.postprocess(image, output_type="pil")
    image[0].save("split_transformer.png")
```

By selectively loading and unloading the models you need at a given stage and sharding the largest models across multiple GPUs, it is possible to run inference with large models on consumer GPUs.

## Context parallelism

[Context parallelism](https://huggingface.co/spaces/nanotron/ultrascale-playbook?section=context_parallelism) splits input sequences across multiple GPUs to reduce memory usage. Each GPU processes its own slice of the sequence.

Use [`~ModelMixin.set_attention_backend`] to switch to a more optimized attention backend. Refer to this [table](../optimization/attention_backends#available-backends) for a complete list of available backends.

Most attention backends are compatible with context parallelism. Open an [issue](https://github.com/huggingface/diffusers/issues/new) if a backend is not compatible.

### Ring Attention

Key (K) and value (V) representations communicate between devices using [Ring Attention](https://huggingface.co/papers/2310.01889). This ensures each split sees every other token's K/V. Each GPU computes attention for its local K/V and passes it to the next GPU in the ring. No single GPU holds the full sequence, which reduces communication latency.

Pass a [`ContextParallelConfig`] to the `parallel_config` argument of the transformer model. The config supports the `ring_degree` argument that determines how many devices to use for Ring Attention.

```py
import torch
from torch import distributed as dist
from diffusers import DiffusionPipeline, ContextParallelConfig

def setup_distributed():
    if not dist.is_initialized():
        dist.init_process_group(backend="nccl")
    rank = dist.get_rank()
    device = torch.device(f"cuda:{rank}")
    torch.cuda.set_device(device)
    return device

def main():
    device = setup_distributed()
    world_size = dist.get_world_size()

    pipeline = DiffusionPipeline.from_pretrained(
        "black-forest-labs/FLUX.1-dev", torch_dtype=torch.bfloat16
    ).to(device)
    pipeline.transformer.set_attention_backend("_native_cudnn")

    cp_config = ContextParallelConfig(ring_degree=world_size)
    pipeline.transformer.enable_parallelism(config=cp_config)

    prompt = """
    cinematic film still of a cat sipping a margarita in a pool in Palm Springs, California
    highly detailed, high budget hollywood movie, cinemascope, moody, epic, gorgeous, film grain
    """

    # Must specify generator so all ranks start with same latents (or pass your own)
    generator = torch.Generator().manual_seed(42)
    image = pipeline(
        prompt,
        guidance_scale=3.5,
        num_inference_steps=50,
        generator=generator,
    ).images[0]

    if dist.get_rank() == 0:
        image.save(f"output.png")

    if dist.is_initialized():
        dist.destroy_process_group()


if __name__ == "__main__":
    main()
```

The script above needs to be run with a distributed launcher, such as [torchrun](https://docs.pytorch.org/docs/stable/elastic/run.html), that is compatible with PyTorch. `--nproc-per-node` is set to the number of GPUs available.

```shell
torchrun --nproc-per-node 2 above_script.py
```

### Ulysses Attention

[Ulysses Attention](https://huggingface.co/papers/2309.14509) splits a sequence across GPUs and performs an *all-to-all* communication (every device sends/receives data to every other device). Each GPU ends up with all tokens for only a subset of attention heads. Each GPU computes attention locally on all tokens for its head, then performs another all-to-all to regroup results by tokens for the next layer.

[`ContextParallelConfig`] supports Ulysses Attention through the `ulysses_degree` argument. This determines how many devices to use for Ulysses Attention.

Pass the [`ContextParallelConfig`] to [`~ModelMixin.enable_parallelism`].

```py
# Depending on the number of GPUs available.
pipeline.transformer.enable_parallelism(config=ContextParallelConfig(ulysses_degree=2))
```

### Unified Attention

[Unified Sequence Parallelism](https://huggingface.co/papers/2405.07719) combines Ring Attention and Ulysses Attention into a single approach for efficient long-sequence processing. It applies Ulysses's *all-to-all* communication first to redistribute heads and sequence tokens, then uses Ring Attention to process the redistributed data, and finally reverses the *all-to-all* to restore the original layout.

This hybrid approach leverages the strengths of both methods:
- **Ulysses Attention** efficiently parallelizes across attention heads
- **Ring Attention** handles very long sequences with minimal memory overhead
- Together, they enable 2D parallelization across both heads and sequence dimensions

[`ContextParallelConfig`] supports Unified Attention by specifying both `ulysses_degree` and `ring_degree`. The total number of devices used is `ulysses_degree * ring_degree`, arranged in a 2D grid where Ulysses and Ring groups are orthogonal (non-overlapping).
Pass the [`ContextParallelConfig`] with both `ulysses_degree` and `ring_degree` set to bigger than 1 to [`~ModelMixin.enable_parallelism`].

```py
pipeline.transformer.enable_parallelism(config=ContextParallelConfig(ulysses_degree=2, ring_degree=2))
```

> [!TIP]
> Unified Attention is to be used when there are enough devices to arrange in a 2D grid (at least 4 devices).

We ran a benchmark with Ulysess, Ring, and Unified Attention with [this script](https://github.com/huggingface/diffusers/pull/12693#issuecomment-3694727532) on a node of 4 H100 GPUs. The results are summarized as follows:

| CP Backend         | Time / Iter (ms) | Steps / Sec | Peak Memory (GB) |
|--------------------|------------------|-------------|------------------|
| ulysses            | 6670.789         | 7.50        | 33.85            |
| ring               | 13076.492        | 3.82        | 56.02            |
| unified_balanced   | 11068.705        | 4.52        | 33.85            |

From the above table, it's clear that Ulysses provides better throughput, but the number of devices it can use remains limited to the number of attention heads, a limitation that is solved by unified attention.

### parallel_config

Pass `parallel_config` during model initialization to enable context parallelism.

```py
CKPT_ID = "black-forest-labs/FLUX.1-dev"

cp_config = ContextParallelConfig(ring_degree=2)
transformer = AutoModel.from_pretrained(
    CKPT_ID, 
    subfolder="transformer", 
    torch_dtype=torch.bfloat16, 
    parallel_config=cp_config
)

pipeline = DiffusionPipeline.from_pretrained(
    CKPT_ID, transformer=transformer, torch_dtype=torch.bfloat16,
).to(device)
```
