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

pip install modal modal setup # Opens browser for authentication

Hello World with GPU

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

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

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

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

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

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

Full ASGI app

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

@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

Create secret

modal secret create huggingface HF_TOKEN=hf_xxx

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

Scheduling

@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

@app.function( container_idle_timeout=300, # Keep warm 5 min allow_concurrent_inputs=10, # Handle concurrent requests ) def inference(): pass

Model loading best practices

@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

@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

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

Debugging

Test locally

if name == "main": result = my_function.local()

View logs

modal app logs my-app

Common issues

Issue Solution

Cold start latency Increase container_idle_timeout , 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