NIXL

Mistral AI Engineering Deep Dive: Debugging a Memory Leak in vLLM

Mistral AI's engineering team investigated a memory leak in vLLM that appeared exclusively during disaggregated prefill/decode serving with Mistral Medium 3.1 and graph compilation enabled, causing ~400 MB/min RSS growth. The leak was not visible in heap profilers (Memray, Guppy3, Heaptrack), pointing to off-heap memory allocation tied to NIXL/UCX-based KV cache transfer over InfiniBand. The post is the first in a new Engineering Deep Dive series and documents a methodical descent from Python-level tools to kernel-level tracing to isolate the root cause.