#
Distributed inference

On distributed setups, you can run inference across multiple GPUs with 🤗 Accelerate or PyTorch Distributed, which is useful for generating with multiple prompts in parallel.

This guide will show you how to use 🤗 Accelerate and PyTorch Distributed for distributed inference.

##
🤗 Accelerate

🤗 Accelerate is a library designed to make it easy to train or run inference across distributed setups. It simplifies the process of setting up the distributed environment, allowing you to focus on your PyTorch code.

To begin, create a Python file and initialize an accelerate.PartialState to create a distributed environment; your setup is automatically detected so you don’t need to explicitly define the `rank` or `world_size`. Move the DiffusionPipeline to `distributed_state.device` to assign a GPU to each process.

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

    import torch
    from accelerate import PartialState
    from diffusers import DiffusionPipeline

    pipeline = DiffusionPipeline.from_pretrained(
        "stable-diffusion-v1-5/stable-diffusion-v1-5", torch_dtype=torch.float16, use_safetensors=True
    )
    distributed_state = PartialState()
    pipeline.to(distributed_state.device)

    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")

Use the `--num_processes` argument to specify the number of GPUs to use, and call `accelerate launch` to run the script:

    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 supports `DistributedDataParallel` which enables data parallelism.

To start, create a Python file and import `torch.distributed` and `torch.multiprocessing` to set up the distributed process group and to spawn the processes for inference on each GPU. You should also initialize a DiffusionPipeline:

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

    from diffusers import DiffusionPipeline

    sd = DiffusionPipeline.from_pretrained(
        "stable-diffusion-v1-5/stable-diffusion-v1-5", torch_dtype=torch.float16, use_safetensors=True
    )

You’ll want to create a function to run inference; `init_process_group` handles creating a distributed environment with the type of backend to use, the `rank` of the current process, and the `world_size` or the number of processes participating. If you’re running inference in parallel over 2 GPUs, then the `world_size` is 2.

Move the DiffusionPipeline to `rank` and use `get_rank` to assign a GPU to each process, where each process handles a different prompt:

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

        sd.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")

To run the distributed inference, call `mp.spawn` to run the `run_inference` function on the number of GPUs 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()

Once you’ve completed the inference script, use the `--nproc_per_node` argument to specify the number of GPUs to use and call `torchrun` to run the script:

    torchrun run_distributed.py --nproc_per_node=2

You can use `device_map` within a DiffusionPipeline to distribute its model-level components on multiple devices. Refer to the Device placement guide to learn more.

##
Model sharding

Modern diffusion systems such as Flux are very large and have multiple models. For example, Flux.1-Dev is made up of two text encoders - T5-XXL and CLIP-L - a diffusion transformer, and a VAE. With a model this size, it can be challenging to run inference on consumer GPUs.

Model sharding is a technique that distributes models across GPUs when the models don’t fit on a single GPU. The example below assumes two 16GB GPUs are available for inference.

Start by computing the text embeddings with the text encoders. Keep the text encoders on two GPUs by setting `device_map="balanced"`. The `balanced` strategy evenly distributes the model on all available GPUs. Use the `max_memory` parameter to allocate the maximum amount of memory for each text encoder on each GPU.

Only load the text encoders for this step! The diffusion transformer and VAE are loaded in a later step to preserve memory.

    from diffusers import FluxPipeline
    import torch

    prompt = "a photo of a dog with cat-like look"

    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
        )

Once 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()

Load the diffusion transformer next which has 12.5B parameters. This time, set `device_map="auto"` to automatically distribute the model across two 16GB GPUs. The `auto` strategy is backed by Accelerate and available as a part of the Big Model Inference feature. It starts by distributing a model across the fastest device first (GPU) before moving to slower devices like the CPU and hard drive if needed. The trade-off of storing model parameters on slower devices is slower inference latency.

    from diffusers import FluxTransformer2DModel
    import torch

    transformer = FluxTransformer2DModel.from_pretrained(
        "black-forest-labs/FLUX.1-dev",
        subfolder="transformer",
        device_map="auto",
        torch_dtype=torch.bfloat16
    )

At any point, you can try `print(pipeline.hf_device_map)` to see how the various models are distributed across devices. This is useful for tracking the device placement of the models. You can also try `print(transformer.hf_device_map)` to see how the transformer model is sharded across devices.

Add the transformer model to the pipeline for denoising, but set the other model-level components like the text encoders and VAE to `None` because you don’t need them yet.

    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 as they’re no longer needed.

    del pipeline.transformer
    del pipeline

    flush()

Finally, decode the latents with the VAE into an image. The VAE is typically small enough to be loaded on a single GPU.

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

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

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