p99 Latency - CI/CD Glossary Definition
p99 latency (the 99th percentile) is the value below which 99 percent of measured latencies fall: only the slowest 1 percent are worse. It is the standard way to quantify tail latency, the rare-but-painful slow cases that medians hide.
Why it matters
In a CI fleet, p50 runner-pickup time might be 5 seconds while p99 is 90 seconds because of occasional cold starts or queue contention. Users remember the 90-second waits. Tracking p99 (and p99.9) surfaces problems, such as queueing or cold starts, that averages and the median completely miss.
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