Performance Analysis
When evaluating web service response times, why is the 99th percentile (the boundary of the slowest 1%) often examined in addition to the mean?
- AThe mean can look good while some requests are very slow, and when a task waits on many components, those slow responses set overall latency
- BThe 99th percentile is always smaller than the mean
- CThe mean cannot be calculated
Show answer and explanation
Answer: A. The mean can look good while some requests are very slow, and when a task waits on many components, those slow responses set overall latency
Response-time distributions tend to be mostly fast with a small, very slow tail. Looking only at the mean hides that tail. Moreover, when rendering a page waits on 100 components, any one slow response makes the whole slow, so the slowest 1% affects most users. Systems are therefore managed against targets for the slow end (tail latency).
- A. Correct. This is called tail latency.
- B. Being on the slow side, it is usually larger than the mean.
- C. The mean can be computed but is insufficient alone.