Mutex contention plateaus throughput
A Go in-memory metrics-aggregation service can't keep up at 16 cores: throughput plateaus around the same number whether you give it 4 cores or 16, and per-request latency rises with load. A CPU profile shows a large fraction of time in `sync.(*Mutex).Lock`/runtime futex; the mutex protects a single global `map[string]*counter` that every request reads-modify-writes under one lock. `GOMAXPROCS` is set correctly and the box isn't CPU-starved overall. Adding cores didn't help throughput. Triage and explain why more cores don't help.
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.
0:00 of about 40 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.