Importance of Little's Law

Little's law is quite simple and intuitively appealing.

The law states that the average number of customers in a system (over some time interval), N, is equal to their average arrival rate, X, multiplied by their average time in the system, R.

N = X . R (or) for easy remembrance use L = A . W

This law is very important to check whether the load testing tool is not a bottleneck.
For Example, in a shop , if there are always 2 customers available at the counter queue , wherein the customers are entering the shop at the rate of 4 per second , then time taken for the customers to leave from the shop can be calculated as

N = X. R

R = 2/4 = 0.5 seconds


A Simple example of how to use this law to know how many virtual users licenses are required:

Web system that has peak user load per hour = 2000 users
Expected Response time per transaction = 4 seconds
The peak page hits/sec = 80 hits/sec

For carrying out Performance tests for the above web system, we need to calculate how many number of Virtual user licenses we need to purchase.

N = X . R
N = 80 . 4 = 320

Therefore 320 virtual user licenses are enough to carry out the Load Test.

Fundamental laws of Performance Testing

Sometime back , i could realize how I was struggling to isolate simple Performance bottlenecks.

Though i managed to identify performance bottlenecks,I know that i was missing something..but really not sure what was it. Pea (Performance Engineering Associates) helped me to identify those missing pieces. Thanks to Pea.

Yes..Its the basics about Queuing Theory.

PEA was formed by professionals who have had the opportunity to lead assignments that brought about several radical changes by bridging the gap between theory and practice in PE.

Fundamental Laws
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1. Utilization law
2. Little's law
3. Response Time law
4. Forced Flow law
5. Service Demand law

Check out the free online book on Queueing Theory available in pea's web site.

http://www.pea-online.com/resources.htm

Nice article from the book "Performance by Design - Computer Capacity Planning by Example" by Virgilio A.F. Almeida, Lawernce W.Dowdy, Daniel A.Menasce.

http://www.informit.com/articles/article.asp?p=170721&seqNum=2&rl=1

Watch out for more information about Performance Engineering Vedas shortly.