Stable Diffusion CLI

This example shows Stable Ddiffusion 1.5 with a number of optimizations that makes it run faster on Modal. The example takes about 10s to cold start and about 1.0s per image generated.

For instance, here are 9 images produced by the prompt An 1600s oil painting of the New York City skyline

stable diffusion slackbot

There is also a Stable Diffusion Slack bot example which does not have all the optimizations, but shows how you can set up a Slack command to trigger Stable Diffusion.

Optimizations used in this example

As mentioned, we use a few optimizations to run this faster:

  • Use run_function to download the model while building the container image
  • Use a container lifecycle method to initialize the model on container startup
  • Use A10G GPUs
  • Use 16 bit floating point math

Basic setup

from __future__ import annotations

import io
import os
import time
from pathlib import Path

import modal

All Modal programs need a Stub — an object that acts as a recipe for the application. Let’s give it a friendly name.

stub = modal.Stub("stable-diffusion-cli")

We will be using typer to create our CLI interface.

import typer

app = typer.Typer()

Model dependencies

Your model will be running remotely inside a container. We will be installing all the model dependencies in the next step. We will also be “baking the model” into the image by running a Python function as a part of building the image. This lets us start containers much faster, since all the data that’s needed is already inside the image.

model_id = "runwayml/stable-diffusion-v1-5"
cache_path = "/vol/cache"

def download_models():
    import diffusers
    import torch

    hugging_face_token = os.environ["HUGGINGFACE_TOKEN"]

    # Download scheduler configuration. Experiment with different schedulers
    # to identify one that works best for your use-case.
    scheduler = diffusers.DPMSolverMultistepScheduler.from_pretrained(
    scheduler.save_pretrained(cache_path, safe_serialization=True)

    # Downloads all other models.
    pipe = diffusers.StableDiffusionPipeline.from_pretrained(
    pipe.save_pretrained(cache_path, safe_serialization=True)

image = (
    .pip_install("xformers", pre=True)
stub.image = image

Using container lifecycle methods

Modal lets you implement code that runs every time a container starts. This can be a huge optimization when you’re calling a function multiple times, since Modal reuses the same containers when possible.

The way to implement this is to turn the Modal function into a method on a class that also implement the Python context manager interface, meaning it has the __enter__ method (the __exit__ method is optional).

We have also have applied a few model optimizations to make the model run faster. On an A10G, the model takes about 6.5s to load into memory, and then 1.6s per generation on average. On a T4, it takes 13s to load and 3.7s per generation. Other optimizations are also available here.

This is our Modal function. The function runs through the StableDiffusionPipeline pipeline. It sends the PIL image back to our CLI where we save the resulting image in a local file.

class StableDiffusion:
    def __enter__(self):
        import diffusers
        import torch

        torch.backends.cuda.matmul.allow_tf32 = True

        scheduler = diffusers.DPMSolverMultistepScheduler.from_pretrained(
            denoise_final=True,  # important if steps are <= 10
        self.pipe = diffusers.StableDiffusionPipeline.from_pretrained(
            cache_path, scheduler=scheduler

    def run_inference(
        self, prompt: str, steps: int = 20, batch_size: int = 4
    ) -> list[bytes]:
        import torch

        with torch.inference_mode():
            with torch.autocast("cuda"):
                images = self.pipe(
                    [prompt] * batch_size,

        # Convert to PNG bytes
        image_output = []
        for image in images:
            with io.BytesIO() as buf:
      , format="PNG")
        return image_output

This is the command we’ll use to generate images. It takes a prompt, samples (the number of images you want to generate), steps which configures the number of inference steps the model will make, and batch_size which determines how many images to generate for a given prompt.

def entrypoint(
    prompt: str, samples: int = 5, steps: int = 10, batch_size: int = 1
        f"prompt => {prompt}, steps => {steps}, samples => {samples}, batch_size => {batch_size}"

    dir = Path("/tmp/stable-diffusion")
    if not dir.exists():
        dir.mkdir(exist_ok=True, parents=True)

    sd = StableDiffusion()
    for i in range(samples):
        t0 = time.time()
        images =, steps, batch_size)
        total_time = time.time() - t0
            f"Sample {i} took {total_time:.3f}s ({(total_time)/len(images):.3f}s / image)."
        for j, image_bytes in enumerate(images):
            output_path = dir / f"output_{j}_{i}.png"
            print(f"Saving it to {output_path}")
            with open(output_path, "wb") as f:

And this is our entrypoint; where the CLI is invoked. Explore CLI options with: modal run --help


This example can generate pictures in about a second, with startup time of about 10s for the first picture.

See distribution of latencies below. This data was gathered by running 500 requests in sequence (meaning only the first request incurs a cold start). As you can see, the 90th percentile is 1.2s and the 99th percentile is 2.30s.


Try this on Modal!

You can run this on Modal with 60 seconds of work!
Creating an account is free and no credit card is required. After creating an account, install the Modal Python package and create an API token:
pip install modal-client
modal token new
git clone
cd modal-examples
modal run 06_gpu_and_ml/