Latency (engineering)

The elapsed delay between a request or event and the corresponding response. A latency model must specify whether setup, waiting, transfer and processing overhead are included.

At constant transfer rate $v$ bytes per second, moving $B$ bytes takes $B/v$ seconds. The size and rate must use compatible units. This term alone does not include seek time, rotational latency or controller setup costs.

The expected value of a latency random variable. If a cache hit takes total time $t_h$, a cache miss takes total time $t_m$, and the cache hit rate is $h$, conditioning gives $\mathbb{E}[T]=h{t}_h+(1-h)t_m$. An overhead already included in either total should not be charged again.