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Workload models are frequently used by Performance Testing Engineers, APM Engineers, Capacity Planners / Capacity Managers including Performance Engineers as an input their respective processes. Workload models help define the relevant business critical transactions, batch jobs, messages, workflows, etc. that are responsible for consumption of resources and have an impact on the overall system utilization including the SLA and End User Experience. Let’s see the definitions of a Workload model before we look at how Workload models are put together and used by Performance Testing Engineers, APM Engineers or Capacity Managers.
Definition: Workload model is a combination of different Business Processes performed by Concurrent Users with appropriate Think Times, Service Times and Pacing (Time between two consecutive transactions). Workload models stipulate the nature of the business process, the type of the user executing the process along with the rate at which the user is executing the business process.
For details on Little’s Law please visit the Little’s Law tab within the Analytical Models section within Documentation.
A Workload model used for Performance Testing for example, leverages the concept of Little’s Law (Part of Operational Theory) to design a Workload (set of business process or activities) that will be injected by the Performance Testing tool into the SUT (System Under Test). Application of Little’s Law to Workload models ensures that the Workload model are mathematically sound and are in harmony with the basic fundamental laws of performance.
Performance Testing Engineers use Workload Models to design their Performance Testing scenarios within the Performance Testing tool. The Workload model in essence provides the Performance Testing Engineer with the following set of input:
- Business process name
- Rate at which business processes are to be executed (Non Functional Requirement)
- Expected Response time (assumed)
- Expected Think Time (assumed)
- Expected Throughput (Non Functional Requirement)
- Pacing Time
- Ramp up time
- Ramp down time
Capacity Managers tend to use Performance Models as an input into their modelling process.
Workload modelling makes use of the following notations:
- N – Number of users present within the system
- C – Number of completions
- X – Throughput or Rate of Departure
- A – Number of Arrivals
- λ – Rate of Arrival
- Rt – Time spent by Customers within the system
Workload modelling using Little’ to establish a relationship between the rates at which transactions should be executed along with the think times, the expected throughput and finally the transaction response times.
Little’s Law basically states that the long-term average number of customers in a stable system N is equal to the long-term average effective arrival rate, λ, multiplied by the average time a customer spends in the system, W or Rt, or expressed algebraically:
- N = λ * Rt ……………….. [ N = Number of Users in the System, Rt = Response Time, λ = Arrival Rate ]
Little’s Law can also be stated as:
- N = Rt * X ……………….. [ N = Number of Users in the System, Rt = Response Time, X = Throughput ]
For a system where Zt (Think Time is Non Zero) Little’s Law can be stated as:
- N = [Rt + Zt] * X ……………….. [ N = Number of Users in the System, Rt = Response Time, Zt = Think Time, X = Throughput ]
For more details please refer to – http://www1.practicalperformanceanalyst.com/resources/important-formulae/what-is-littles-law/
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