#
Reduce memory usage

A barrier to using diffusion models is the large amount of memory required. To overcome this challenge, there are several memory-reducing techniques you can use to run even some of the largest models on free-tier or consumer GPUs. Some of these techniques can even be combined to further reduce memory usage.

In many cases, optimizing for memory or speed leads to improved performance in the other, so you should try to optimize for both whenever you can. This guide focuses on minimizing memory usage, but you can also learn more about how to Speed up inference.

The results below are obtained from generating a single 512x512 image from the prompt a photo of an astronaut riding a horse on mars with 50 DDIM steps on a Nvidia Titan RTX, demonstrating the speed-up you can expect as a result of reduced memory consumption.

| latency | speed-up
--- | --- | ---
original | 9.50s | x1
fp16 | 3.61s | x2.63
channels last | 3.30s | x2.88
traced UNet | 3.21s | x2.96
memory-efficient attention | 2.63s | x3.61

##
Sliced VAE

Sliced VAE enables decoding large batches of images with limited VRAM or batches with 32 images or more by decoding the batches of latents one image at a time. You’ll likely want to couple this with enable_xformers_memory_efficient_attention() to reduce memory use further if you have xFormers installed.

To use sliced VAE, call enable_vae_slicing() on your pipeline before inference:

    import torch
    from diffusers import StableDiffusionPipeline

    pipe = StableDiffusionPipeline.from_pretrained(
        "runwayml/stable-diffusion-v1-5",
        torch_dtype=torch.float16,
        use_safetensors=True,
    )
    pipe = pipe.to("cuda")

    prompt = "a photo of an astronaut riding a horse on mars"
    pipe.enable_vae_slicing()
    #pipe.enable_xformers_memory_efficient_attention()
    images = pipe([prompt] * 32).images

You may see a small performance boost in VAE decoding on multi-image batches, and there should be no performance impact on single-image batches.

##
Tiled VAE

Tiled VAE processing also enables working with large images on limited VRAM (for example, generating 4k images on 8GB of VRAM) by splitting the image into overlapping tiles, decoding the tiles, and then blending the outputs together to compose the final image. You should also used tiled VAE with enable_xformers_memory_efficient_attention() to reduce memory use further if you have xFormers installed.

To use tiled VAE processing, call enable_vae_tiling() on your pipeline before inference:

    import torch
    from diffusers import StableDiffusionPipeline, UniPCMultistepScheduler

    pipe = StableDiffusionPipeline.from_pretrained(
        "runwayml/stable-diffusion-v1-5",
        torch_dtype=torch.float16,
        use_safetensors=True,
    )
    pipe.scheduler = UniPCMultistepScheduler.from_config(pipe.scheduler.config)
    pipe = pipe.to("cuda")
    prompt = "a beautiful landscape photograph"
    pipe.enable_vae_tiling()
    #pipe.enable_xformers_memory_efficient_attention()

    image = pipe([prompt], width=3840, height=2224, num_inference_steps=20).images[0]

The output image has some tile-to-tile tone variation because the tiles are decoded separately, but you shouldn’t see any sharp and obvious seams between the tiles. Tiling is turned off for images that are 512x512 or smaller.

##
CPU offloading

Offloading the weights to the CPU and only loading them on the GPU when performing the forward pass can also save memory. Often, this technique can reduce memory consumption to less than 3GB.

To perform CPU offloading, call enable_sequential_cpu_offload():

    import torch
    from diffusers import StableDiffusionPipeline

    pipe = StableDiffusionPipeline.from_pretrained(
        "runwayml/stable-diffusion-v1-5",
        torch_dtype=torch.float16,
        use_safetensors=True,
    )

    prompt = "a photo of an astronaut riding a horse on mars"
    pipe.enable_sequential_cpu_offload()
    image = pipe(prompt).images[0]

CPU offloading works on submodules rather than whole models. This is the best way to minimize memory consumption, but inference is much slower due to the iterative nature of the diffusion process. The UNet component of the pipeline runs several times (as many as `num_inference_steps`); each time, the different UNet submodules are sequentially onloaded and offloaded as needed, resulting in a large number of memory transfers.

Consider using model offloading if you want to optimize for speed because it is much faster. The tradeoff is your memory savings won’t be as large.

When using enable_sequential_cpu_offload(), don’t move the pipeline to CUDA beforehand or else the gain in memory consumption will only be minimal (see this issue for more information).

enable_sequential_cpu_offload() is a stateful operation that installs hooks on the models.

##
Model offloading

Model offloading requires 🤗 Accelerate version 0.17.0 or higher.

Sequential CPU offloading preserves a lot of memory but it makes inference slower because submodules are moved to GPU as needed, and they’re immediately returned to the CPU when a new module runs.

Full-model offloading is an alternative that moves whole models to the GPU, instead of handling each model’s constituent submodules. There is a negligible impact on inference time (compared with moving the pipeline to `cuda`), and it still provides some memory savings.

During model offloading, only one of the main components of the pipeline (typically the text encoder, UNet and VAE) is placed on the GPU while the others wait on the CPU. Components like the UNet that run for multiple iterations stay on the GPU until they’re no longer needed.

Enable model offloading by calling enable_model_cpu_offload() on the pipeline:

    import torch
    from diffusers import StableDiffusionPipeline

    pipe = StableDiffusionPipeline.from_pretrained(
        "runwayml/stable-diffusion-v1-5",
        torch_dtype=torch.float16,
        use_safetensors=True,
    )

    prompt = "a photo of an astronaut riding a horse on mars"
    pipe.enable_model_cpu_offload()
    image = pipe(prompt).images[0]

In order to properly offload models after they’re called, it is required to run the entire pipeline and models are called in the pipeline’s expected order. Exercise caution if models are reused outside the context of the pipeline after hooks have been installed. See Removing Hooks for more information.

enable_model_cpu_offload() is a stateful operation that installs hooks on the models and state on the pipeline.

##
Channels-last memory format

The channels-last memory format is an alternative way of ordering NCHW tensors in memory to preserve dimension ordering. Channels-last tensors are ordered in such a way that the channels become the densest dimension (storing images pixel-per-pixel). Since not all operators currently support the channels-last format, it may result in worst performance but you should still try and see if it works for your model.

For example, to set the pipeline’s UNet to use the channels-last format:

    print(pipe.unet.conv_out.state_dict()["weight"].stride())  # (2880, 9, 3, 1)
    pipe.unet.to(memory_format=torch.channels_last)  # in-place operation
    print(
        pipe.unet.conv_out.state_dict()["weight"].stride()
    )  # (2880, 1, 960, 320) having a stride of 1 for the 2nd dimension proves that it works

##
Tracing

Tracing runs an example input tensor through the model and captures the operations that are performed on it as that input makes its way through the model’s layers. The executable or `ScriptFunction` that is returned is optimized with just-in-time compilation.

To trace a UNet:

    import time
    import torch
    from diffusers import StableDiffusionPipeline
    import functools

    # torch disable grad
    torch.set_grad_enabled(False)

    # set variables
    n_experiments = 2
    unet_runs_per_experiment = 50

    # load inputs
    def generate_inputs():
        sample = torch.randn((2, 4, 64, 64), device="cuda", dtype=torch.float16)
        timestep = torch.rand(1, device="cuda", dtype=torch.float16) * 999
        encoder_hidden_states = torch.randn((2, 77, 768), device="cuda", dtype=torch.float16)
        return sample, timestep, encoder_hidden_states

    pipe = StableDiffusionPipeline.from_pretrained(
        "runwayml/stable-diffusion-v1-5",
        torch_dtype=torch.float16,
        use_safetensors=True,
    ).to("cuda")
    unet = pipe.unet
    unet.eval()
    unet.to(memory_format=torch.channels_last)  # use channels_last memory format
    unet.forward = functools.partial(unet.forward, return_dict=False)  # set return_dict=False as default

    # warmup
    for _ in range(3):
        with torch.inference_mode():
            inputs = generate_inputs()
            orig_output = unet(*inputs)

    # trace
    print("tracing..")
    unet_traced = torch.jit.trace(unet, inputs)
    unet_traced.eval()
    print("done tracing")

    # warmup and optimize graph
    for _ in range(5):
        with torch.inference_mode():
            inputs = generate_inputs()
            orig_output = unet_traced(*inputs)

    # benchmarking
    with torch.inference_mode():
        for _ in range(n_experiments):
            torch.cuda.synchronize()
            start_time = time.time()
            for _ in range(unet_runs_per_experiment):
                orig_output = unet_traced(*inputs)
            torch.cuda.synchronize()
            print(f"unet traced inference took {time.time() - start_time:.2f} seconds")
        for _ in range(n_experiments):
            torch.cuda.synchronize()
            start_time = time.time()
            for _ in range(unet_runs_per_experiment):
                orig_output = unet(*inputs)
            torch.cuda.synchronize()
            print(f"unet inference took {time.time() - start_time:.2f} seconds")

    # save the model
    unet_traced.save("unet_traced.pt")

Replace the `unet` attribute of the pipeline with the traced model:

    from diffusers import StableDiffusionPipeline
    import torch
    from dataclasses import dataclass

    @dataclass
    class UNet2DConditionOutput:
        sample: torch.Tensor

    pipe = StableDiffusionPipeline.from_pretrained(
        "runwayml/stable-diffusion-v1-5",
        torch_dtype=torch.float16,
        use_safetensors=True,
    ).to("cuda")

    # use jitted unet
    unet_traced = torch.jit.load("unet_traced.pt")

    # del pipe.unet
    class TracedUNet(torch.nn.Module):
        def __init__(self):
            super().__init__()
            self.in_channels = pipe.unet.config.in_channels
            self.device = pipe.unet.device

        def forward(self, latent_model_input, t, encoder_hidden_states):
            sample = unet_traced(latent_model_input, t, encoder_hidden_states)[0]
            return UNet2DConditionOutput(sample=sample)

    pipe.unet = TracedUNet()

    with torch.inference_mode():
        image = pipe([prompt] * 1, num_inference_steps=50).images[0]

##
Memory-efficient attention

Recent work on optimizing bandwidth in the attention block has generated huge speed-ups and reductions in GPU memory usage. The most recent type of memory-efficient attention is Flash Attention (you can check out the original code at HazyResearch/flash-attention).

If you have PyTorch >= 2.0 installed, you should not expect a speed-up for inference when enabling `xformers`.

To use Flash Attention, install the following:

- PyTorch > 1.12
- CUDA available
- xFormers

Then call enable_xformers_memory_efficient_attention() on the pipeline:

    from diffusers import DiffusionPipeline
    import torch

    pipe = DiffusionPipeline.from_pretrained(
        "runwayml/stable-diffusion-v1-5",
        torch_dtype=torch.float16,
        use_safetensors=True,
    ).to("cuda")

    pipe.enable_xformers_memory_efficient_attention()

    with torch.inference_mode():
        sample = pipe("a small cat")

    # optional: You can disable it via
    # pipe.disable_xformers_memory_efficient_attention()

The iteration speed when using `xformers` should match the iteration speed of PyTorch 2.0 as described here.

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