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Python Memory Leak Debugging and Mitigation

1 post · Dec 2025

What this trail covers

Lyft engineers encountered and resolved a memory leak during a Python version upgrade from 3.8 to 3.10. The issue manifested as increased latency and frequent container restarts. The investigation involved using a custom memory profiling tool based on tracemalloc and identifying an incompatibility between gevent's monkey patching and urllib3's connection pooling, specifically related to weakref.finalize. Downgrading urllib3 to 1.26.15 resolved the issue. The post also details challenges with gunicorn's preload setting when using signal handlers.

All Lyft trails
  1. From Python3.8 to Python3.10: Our Journey Through a Memory LeakThis post details the debugging process for a Python memory leak that occurred after upgrading from Python 3.8 to 3.10. It highlights the use of a custom tracemalloc-based memory profiling tool and the identification of an incompatibility between gevent's monkey patching and urllib3's connection pooling (specifically weakref.finalize) as the root cause. The solution involved downgrading urllib3 to 1.26.15. Additionally, it describes a challenge encountered with gunicorn's 'preload' setting when attempting to use signal handlers for profiling.