99ile means the 99th percentile, usually written as p99.
What it means in load testing
If sort all request latencies from fastest to slowest:
- p99 latency is the value where 99% of requests are at or below that latency
- The remaining 1% are slower (the “tail”)
Example
If send 100,000 requests and p99 = 120 ms, that means:
- at least 99,000 requests finished in ≤ 120 ms
- the slowest 1,000 requests took > 120 ms
Why it matters
- p99 captures those “occasionally very slow” experiences (tail latency), which is important for real users.
- Reveal if your system is stable under load or if it has “random slow spikes,”
- Compare to average (mean) can look fine even if a small number of requests are very slow.