performance testing laws

Here are some of important law the drives performance testing activity.

1. Little’s Law
In performance testing environment, the Total Number of Users in the System is the Product of the Throughput and the total time taken for processing the requests. This is one of the most frequently used law while doing performance testing
N = X * (Z + R) where,
N – Number of Users
X – Throughput
Z – Think Time and
R – Response Time
Example: If the total time at a particular request ( Z + R) is 15 Sec's, and the Throughput is 100 Requests/Sec, then the Total Number of Users is 15*100 = 1500 Users

2. Bounding Analysis Law 
The Maximum Throughput of a System is equal to the inverse of the Service Demand.
Zmax = 1/SD
Throughput – The number of requests processed by a system in unit time
SD – Time taken by the system to process a particular request
Example: if the Service Demand of a System is 0.1, the maximum Throughput of the system is 1/0.1 which is 10 Requests/Sec.

3. Forced Flow Law
The Request Arrival Rate is equal to the throughput, when the system is not 100% utilized. In other words, whatever flows into the system, flows out of the system, till the system is fully utilized
Request Arrival = Throughput, when U<100%
Example: when the Utilization is 50% and Arrival rate is 5 Requests/Second, the Throughput is also 5 Requests/Second.

4. Utilization Law 
The Utilization Law is just an extension of the Bounding Analysis Law. The Utilization of a System is the Product of Throughput and Service Demand
U = Z * SD
Example: If the Throughput is 5 Requests/Sec and the Service Demand is 0.1, then the Utilization is 5 * 0.1, which is 50%
At Peak Utilization, U =1, so,
Z = 1/SD, which is the maximum Throughput of the System (which is the bounding Analysis Law)

5. Amdahl's law
http://en.wikipedia.org/wiki/Amdahl%27s_law