p95, p99 or p99.9? Choosing latency percentiles for SLOs, benchmarks and dashboards
Every team argues about whether to track p95 or p99. The answer depends on traffic volume, fan-out and what the number is for. A practical guide to choosing percentiles for SLOs, alerts, benchmarks and capacity planning, with the sample-size maths, coordinated omission and how to report results honestly.