Shadow Servers and Priority Scheduling

Queuing network models have been applied to the analysis of computer system performance since the early 1970s. Shortly after their introduction, a number of major extensions to the underlying mathematical theory were developed by a worldwide community of researchers. Among the most important of these new extensions was the ability to represent priority scheduling at CPUs. Sevcik's shadow server approximation, which appeared in 1977, provided the first computationally viable technique for analyzing the effect of CPU priority scheduling within a queuing network model. The shadow server approximation is based on a powerful and intuitively appealing heuristic. It provided a solution to an important open problem and contributed substantially to the practical success of commercial modeling tools based on the theory of queuing networks.