# Knowledge Base & Community Wiki

## Reference Formulae

Here’s a list of some of the most commonly used Fundamental Formulae from a Performance modelling standpoint.

- X = C / T ………[ X = Throughput, C = Number of Completions, T = Total Elapsed Time]

- D
_{i}= V_{i}* S_{i}……… [D = Service Demand, V = Visit Count, S = Service Time]

- U
_{avg}= X * S_{avg}……… [ U_{avg}= Utilization, X = Throughput, S_{avg}= Service Time]

- X
_{max}< or = 1 / S_{max}……… [ X_{max}= Max Throughput, S_{max}= Max Service Time]

- S
_{avg}= B / C ……… [ S_{avg}= Service Time, C = Completions]

- R
_{avg}= Qt + S_{avg}……… [ R = Response Time, Q = Queuing Time, S_{avg}= Service Time]

- Q
_{avg}= X * Q_{t}……… [ Q = Queue Length, Q_{t}= Queuing Time, X = Throughput]

- N = (R+Z) * X
**:**Little’s Law ……… [ N = Number in System, R = Response Time, X = Throughput, Z = Think Time]

Little’s Law provides a set of equations to understand the relationship between User Concurrency in a System, the Response time (Response = Wait time or Queue Time + Service Time) and the Transactional throughput.

Little’s Law was given to us by Prof. John Little (http://en.wikipedia.org/wiki/Little%27s_law).

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