1. A computer-implemented method of generating a queuing model, comprising: defining a workload comprising a plurality of service requests, each service request corresponding to a class of a plurality of classes; applying the workload to a computer system that receives and processes service requests; measuring a response time of the computer system for each request of the workload; estimating a mean service demand for each class based on the response times and a base queuing model that represents the computer system; and generating the queuing model based on the mean service demands and characteristics of the workload.