Diagnosing and Fixing Slow Django Runserver Reloads

Based on your description, you're experiencing slow reload times with Django's development server. This is a common issue with larger projects. Here are some approaches to diagnose and address the problem:

Use Watchman for Better Performance

One of the most effective solutions is to use Watchman, which Django supports for improved reload performance:

1. Install both pywatchman and the Watchman service:

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pip install pywatchman

Then install the Watchman service according to your operating system (available at https://facebook.github.io/watchman/).

With Watchman installed, Django will use kernel signals for auto-reloading instead of polling file timestamps every second. This offers:

• Better performance on large projects
• Reduced response time after code changes
• More robust change detection
• Lower power usage

Django supports pywatchman 1.0.2 and higher.

Adjust Watchman Timeout

If you're already using Watchman and still experiencing delays, you can adjust the timeout:

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export DJANGO_WATCHMAN_TIMEOUT=10  # Increase from default 5 seconds

Identify Slow Initialization Code

To identify what's causing the slow startup:

1. Add timing code to your settings files and key initialization points:

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import time
import logging

logger = logging.getLogger(__name__)

start_time = time.time()
# Your existing imports and code here
logger.info(f"Settings loaded in {time.time() - start_time:.2f} seconds")

2. Add similar timing code to your app's __init__.py, apps.py, and other modules that run during startup.

3. Look for code similar to your long_initialization() example - any code at module level that performs slow operations like:

   • Database queries
   • Network requests
   • File system operations
   • Heavy computations

Common Causes of Slow Startup

1. Too many apps: Each Django app adds overhead during startup
2. Heavy model imports: Complex model relationships that trigger many imports
3. Signal handlers: Especially those that do work at import time
4. AppConfig.ready(): Heavy operations in the ready method
5. Middleware initialization: Complex middleware that does work at startup
6. Template loading: Large number of templates being pre-processed
7. Static files: Large static file collections that need scanning

Optimization Strategies

1. Lazy loading: Move expensive operations out of module level
2. Reduce app count: Consolidate related functionality
3. Optimize imports: Avoid circular imports and use lazy imports where possible
4. Ignore large directories: If using Watchman, configure it to ignore large non-Python directories like node_modules

For Large Projects

For very large projects, you might consider:

1. Split your project: Into smaller services that can be developed independently
2. Use Docker: To isolate development environments
3. Hot module replacement: Consider tools like Django-Livereload-Server for frontend changes

Remember that the development server is designed for convenience, not performance, so some startup time is expected for larger projects.