medium Drill: server count for an API service
Drill
An API serves 30,000 QPS at peak. Each request uses ~20 ms of CPU. Target 70% utilization on 8-core servers. How many servers?
✅ Worked solution & bounds check
Worked solution
- CPU work = 30,000 × 20 ms = 600,000 ms/s = 600 core-seconds/sec.
- ÷ 0.7 utilization ⇒ ~860 cores.
- ÷ 8 cores/server ⇒ ~108 servers (+ headroom, round to ~120).
Cross-check (Little’s Law): N = 30,000 × 0.02 s = 600 concurrent requests — consistent.
Bounds: if requests are IO-bound (mostly waiting), fewer cores but more threads suffice. Ask: CPU-bound or IO-bound?
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