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P99 latency.

also: P99 · p99 · tail latency · 99th percentile · percentile latency

The latency 99% of requests beat: percentiles describe the slow tail that averages hide.

the two-minute refresher

950 requests at 50 ms and one at 5 s: the average looks fine, the tail doesn't.
P50 is the median; P95 and P99 show what your slowest users feel.
The tail comes from GC pauses, cold caches, network congestion and slow dependencies.
Set timeouts and SLOs from percentiles, never from the average.

Where the course teaches it

W02 · Day 1Timeout Helllearn it →W02 · Day 1Timeout Hellrevisit →

Related concepts

Timeout budget
Hedged request
SLI