Engineer Debugs vLLM KV Cache Corruption That Caused Live Outage at Peak Traffic

An on-call engineer faced a production outage after KV cache corruption errors surfaced in a vLLM PagedAttention model-serving setup during peak traffic of over 14,000 requests per second. The incident triggered repeated failures when querying the KV store, with stack traces pointing to cache corruption deep in the paged attention pipeline. Three initial remediation attempts — adjusting NCCL timeouts, restarting Kubernetes pods, and checking GPU health — each failed and consumed roughly an hour of debugging time. The root cause turned out to be a single-line fix in the Dockerfile, which the engineer described as a frustrating but instructive outcome. Following the incident, the team added Grafana and PagerDuty alerts to catch NCCL communication failures and high all-reduce latency before users are impacted in the future.