P90

P90 is a statistical term commonly used in observability to understand the distribution of data. For example, if we measure the response times of a service and calculate the P90, it tells us that 90% of the response times are below this value. This is useful for understanding the worst-case performance scenario in a system.

If the P90 of the request latencies in your application is 300ms, it means that 90% of requests completed with a latency below 300ms, and 10% were above. With large datasets, it is recommended to measure worse case scenarios using P90 or other percentiles, as relying on the maximum latency can be misleading due to outliers.