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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 and PyTorch Distributed 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.

    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.

    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 split_between_processes utility as a context manager to automatically distribute the prompts between the number of processes.

    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.

    accelerate launch run_distributed.py --num_processes=2

> Refer to this minimal example script for running inference across multiple GPUs. To learn more, take a look at the Distributed Inference with 🤗 Accelerate guide.

##
PyTorch Distributed

PyTorch DistributedDataParallel enables 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.

    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. 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.

    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 to create the number of processes defined in `world_size`.

    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.

    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.

    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.

    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.

    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
    )

> 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.

    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.

    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 splits input sequences across multiple GPUs to reduce memory usage. Each GPU processes its own slice of the sequence.

Use set_attention_backend() to switch to a more optimized attention backend. Refer to this table for a complete list of available backends.

###
Ring Attention

Key (K) and value (V) representations communicate between devices using Ring Attention. 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.

    import torch
    from diffusers import AutoModel, QwenImagePipeline, ContextParallelConfig

    try:
        torch.distributed.init_process_group("nccl")
        rank = torch.distributed.get_rank()
        device = torch.device("cuda", rank % torch.cuda.device_count())
        torch.cuda.set_device(device)

        transformer = AutoModel.from_pretrained("Qwen/Qwen-Image", subfolder="transformer", torch_dtype=torch.bfloat16, parallel_config=ContextParallelConfig(ring_degree=2))
        pipeline = QwenImagePipeline.from_pretrained("Qwen/Qwen-Image", transformer=transformer, torch_dtype=torch.bfloat16, device_map="cuda")
        pipeline.transformer.set_attention_backend("flash")

        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, num_inference_steps=50, generator=generator).images[0]

        if rank == 0:
            image.save("output.png")

    except Exception as e:
        print(f"An error occurred: {e}")
        torch.distributed.breakpoint()
        raise

    finally:
        if torch.distributed.is_initialized():
            torch.distributed.destroy_process_group()

###
Ulysses Attention

Ulysses Attention 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 `enable_parallelism()`.

    pipeline.transformer.enable_parallelism(config=ContextParallelConfig(ulysses_degree=2))

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