Tail Latency
Tail Latency — Latency at the slow end of a distribution, commonly summarized with percentiles such as p95, p99 or p99.9.
Why the Tail Matters
An average can remain low while a small fraction of requests take much longer. In fan-out systems, the probability that at least one dependency is slow increases as more parallel calls are required.
Tail behavior therefore becomes a reliability concern, not merely a statistical detail.
Common Sources
Queueing near saturation, GC, storage stalls, lock contention, retries and heterogeneous work sizes can all widen the tail.
Percentiles need a stated window and measurement boundary. Averaging percentiles from independent instances is generally not equivalent to merging their underlying distributions.