On-callMediumoc-g033

Subject Gc pausesLevel Mid–Senior~30 minCommon in Reliability & on-call interviewsIndustries Technology, Software development

Question

A Go JSON API service shows latency jitter that started after a deploy: p50 is fine but p99 is sawtoothing and the service occasionally pauses. `runtime` metrics show GC running very frequently (many cycles per second) and `gc_pause_ns` is elevated; allocation rate (`go_memstats_alloc_bytes` rate) roughly tripled after the deploy. CPU usage also rose ~25% but the work done is the same. The deploy 'refactored a hot serialization path.' Heap size is stable — no leak. Triage and mitigate.

What a strong answer looks like

Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.

Learn the concepts

Diagram & narrate the incident

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Run or narrate your approach, then ask the coach.