Modal Serverless GPU

Comprehensive guide to running ML workloads on Modal's serverless GPU cloud platform.

When to use Modal

Use Modal when:

• Running GPU-intensive ML workloads without managing infrastructure
• Deploying ML models as auto-scaling APIs
• Running batch processing jobs (training, inference, data processing)
• Need pay-per-second GPU pricing without idle costs
• Prototyping ML applications quickly
• Running scheduled jobs (cron-like workloads)

Key features:

• Serverless GPUs: T4, L4, A10G, L40S, A100, H100, H200, B200 on-demand
• Python-native: Define infrastructure in Python code, no YAML
• Auto-scaling: Scale to zero, scale to 100+ GPUs instantly
• Sub-second cold starts: Rust-based infrastructure for fast container launches
• Container caching: Image layers cached for rapid iteration
• Web endpoints: Deploy functions as REST APIs with zero-downtime updates

Use alternatives instead:

• RunPod: For longer-running pods with persistent state
• Lambda Labs: For reserved GPU instances
• SkyPilot: For multi-cloud orchestration and cost optimization
• Kubernetes: For complex multi-service architectures

Quick start

Installation

bash
pip install modal
modal setup  # Opens browser for authentication

Hello World with GPU

python
import modal

app = modal.App("hello-gpu")

@app.function(gpu="T4")
def gpu_info():
    import subprocess
    return subprocess.run(["nvidia-smi"], capture_output=True, text=True).stdout

@app.local_entrypoint()
def main():
    print(gpu_info.remote())

Run: modal run hello_gpu.py

Basic inference endpoint

python
import modal

app = modal.App("text-generation")
image = modal.Image.debian_slim().pip_install("transformers", "torch", "accelerate")

@app.cls(gpu="A10G", image=image)
class TextGenerator:
    @modal.enter()
    def load_model(self):
        from transformers import pipeline
        self.pipe = pipeline("text-generation", model="gpt2", device=0)

    @modal.method()
    def generate(self, prompt: str) -> str:
        return self.pipe(prompt, max_length=100)[0]["generated_text"]

@app.local_entrypoint()
def main():
    print(TextGenerator().generate.remote("Hello, world"))

Core concepts

Key components

| Component | Purpose | |-----------|---------| | App | Container for functions and resources | | Function | Serverless function with compute specs | | Cls | Class-based functions with lifecycle hooks | | Image | Container image definition | | Volume | Persistent storage for models/data | | Secret | Secure credential storage |

Execution modes

| Command | Description | |---------|-------------| | modal run script.py | Execute and exit | | modal serve script.py | Development with live reload | | modal deploy script.py | Persistent cloud deployment |

GPU configuration

Available GPUs

| GPU | VRAM | Best For | |-----|------|----------| | T4 | 16GB | Budget inference, small models | | L4 | 24GB | Inference, Ada Lovelace arch | | A10G | 24GB | Training/inference, 3.3x faster than T4 | | L40S | 48GB | Recommended for inference (best cost/perf) | | A100-40GB | 40GB | Large model training | | A100-80GB | 80GB | Very large models | | H100 | 80GB | Fastest, FP8 + Transformer Engine | | H200 | 141GB | Auto-upgrade from H100, 4.8TB/s bandwidth | | B200 | Latest | Blackwell architecture |

GPU specification patterns

python
# Single GPU
@app.function(gpu="A100")

# Specific memory variant
@app.function(gpu="A100-80GB")

# Multiple GPUs (up to 8)
@app.function(gpu="H100:4")

# GPU with fallbacks
@app.function(gpu=["H100", "A100", "L40S"])

# Any available GPU
@app.function(gpu="any")

Container images

python
# Basic image with pip
image = modal.Image.debian_slim(python_version="3.11").pip_install(
    "torch==2.1.0", "transformers==4.36.0", "accelerate"
)

# From CUDA base
image = modal.Image.from_registry(
    "nvidia/cuda:12.1.0-cudnn8-devel-ubuntu22.04",
    add_python="3.11"
).pip_install("torch", "transformers")

# With system packages
image = modal.Image.debian_slim().apt_install("git", "ffmpeg").pip_install("whisper")

Persistent storage

python
volume = modal.Volume.from_name("model-cache", create_if_missing=True)

@app.function(gpu="A10G", volumes={"/models": volume})
def load_model():
    import os
    model_path = "/models/llama-7b"
    if not os.path.exists(model_path):
        model = download_model()
        model.save_pretrained(model_path)
        volume.commit()  # Persist changes
    return load_from_path(model_path)

Web endpoints

FastAPI endpoint decorator

python
@app.function()
@modal.fastapi_endpoint(method="POST")
def predict(text: str) -> dict:
    return {"result": model.predict(text)}

Full ASGI app

python
from fastapi import FastAPI
web_app = FastAPI()

@web_app.post("/predict")
async def predict(text: str):
    return {"result": await model.predict.remote.aio(text)}

@app.function()
@modal.asgi_app()
def fastapi_app():
    return web_app

Web endpoint types

| Decorator | Use Case | |-----------|----------| | @modal.fastapi_endpoint() | Simple function → API | | @modal.asgi_app() | Full FastAPI/Starlette apps | | @modal.wsgi_app() | Django/Flask apps | | @modal.web_server(port) | Arbitrary HTTP servers |

Dynamic batching

python
@app.function()
@modal.batched(max_batch_size=32, wait_ms=100)
async def batch_predict(inputs: list[str]) -> list[dict]:
    # Inputs automatically batched
    return model.batch_predict(inputs)

Secrets management

bash
# Create secret
modal secret create huggingface HF_TOKEN=hf_xxx

python
@app.function(secrets=[modal.Secret.from_name("huggingface")])
def download_model():
    import os
    token = os.environ["HF_TOKEN"]

Scheduling

python
@app.function(schedule=modal.Cron("0 0 * * *"))  # Daily midnight
def daily_job():
    pass

@app.function(schedule=modal.Period(hours=1))
def hourly_job():
    pass

Performance optimization

Cold start mitigation

python
# Modal 1.0 autoscaler params: scaledown_window (was container_idle_timeout).
# Input concurrency moved to the @modal.concurrent decorator.
@app.function(scaledown_window=300)  # Keep warm 5 min
@modal.concurrent(max_inputs=10)     # Handle concurrent requests per container
def inference():
    pass

Model loading best practices

python
@app.cls(gpu="A100")
class Model:
    @modal.enter()  # Run once at container start
    def load(self):
        self.model = load_model()  # Load during warm-up

    @modal.method()
    def predict(self, x):
        return self.model(x)

Parallel processing

python
@app.function()
def process_item(item):
    return expensive_computation(item)

@app.function()
def run_parallel():
    items = list(range(1000))
    # Fan out to parallel containers
    results = list(process_item.map(items))
    return results

Common configuration

python
@app.function(
    gpu="A100",
    memory=32768,              # 32GB RAM
    cpu=4,                     # 4 CPU cores
    timeout=3600,              # 1 hour max
    scaledown_window=120,      # Keep warm 2 min (was container_idle_timeout)
    retries=3,                 # Retry on failure
    max_containers=10,         # Max concurrent containers (was concurrency_limit)
    min_containers=1,          # Keep N containers warm (was keep_warm)
)
def my_function():
    pass

> Modal 1.0 autoscaler renames (see the migration guide): > - container_idle_timeout → scaledown_window > - concurrency_limit → max_containers > - keep_warm → min_containers > - allow_concurrent_inputs=N → the @modal.concurrent(max_inputs=N) decorator

Debugging

python
# Test locally
if __name__ == "__main__":
    result = my_function.local()

# View logs
# modal app logs my-app

Common issues

| Issue | Solution | |-------|----------| | Cold start latency | Increase scaledown_window, use @modal.enter() | | GPU OOM | Use larger GPU (A100-80GB), enable gradient checkpointing | | Image build fails | Pin dependency versions, check CUDA compatibility | | Timeout errors | Increase timeout, add checkpointing |

References

• Advanced Usage - Multi-GPU, distributed training, cost optimization
• Troubleshooting - Common issues and solutions

Resources

• Documentation: https://modal.com/docs
• Examples: https://github.com/modal-labs/modal-examples
• Pricing: https://modal.com/pricing
• Discord: https://discord.gg/modal