FastAPIframework~8 mins

Uvicorn server basics in FastAPI - Performance & Optimization

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Performance: Uvicorn server basics

MEDIUM IMPACT

Uvicorn affects how fast your FastAPI app starts and handles requests, impacting server response time and throughput.

Running a FastAPI app with a single worker vs multiple workers

FastAPI

uvicorn main:app --host 0.0.0.0 --port 8000 --workers 4

Multiple workers allow parallel request handling, improving throughput and responsiveness.

📈 Performance GainHandles multiple requests concurrently, reducing wait time and improving INP.

Running a FastAPI app with a single worker vs multiple workers

FastAPI

uvicorn main:app --host 0.0.0.0 --port 8000

Single worker can only handle one request at a time, causing delays under load.

📉 Performance CostBlocks handling multiple requests, increasing response time under concurrency.

Performance Comparison

Pattern	Concurrency	CPU Usage	Response Latency	Verdict
Single worker	1 request at a time	Low CPU usage	High under load	[X] Bad
Multiple workers	Multiple requests concurrently	Higher CPU usage	Lower latency	[OK] Good

Rendering Pipeline

Uvicorn runs the ASGI server that receives HTTP requests, passes them to FastAPI, and sends back responses. It manages event loops and worker processes.

→Request Handling

→Concurrency Management

→Response Sending

⚠️ BottleneckSingle worker limits concurrency, causing request queuing and slower response times.

Core Web Vital Affected

INP

Uvicorn affects how fast your FastAPI app starts and handles requests, impacting server response time and throughput.

Optimization Tips

1Use multiple workers in Uvicorn to improve request concurrency.

2Monitor server response times to detect bottlenecks.

3Adjust worker count based on CPU cores and expected load.

Performance Quiz - 3 Questions

Test your performance knowledge

What is the main performance benefit of running Uvicorn with multiple workers?

AIt decreases the memory usage per request.

BIt reduces the size of the server binary.

CIt allows handling multiple requests at the same time.

DIt improves the visual layout of the webpage.

DevTools: Network

How to check: Open browser DevTools, go to Network tab, make multiple requests to your API, and observe response times.

What to look for: Look for increased response times or queued requests indicating server bottlenecks.