An Evaluation of CPU Efficiency Under Dynamic Quantum Allocation
U. Narayan Bhat, Richard E. Nance · Journal of the ACM · 1979
A model for a t,me-shanng operating system is developed in order to assess the effects of dynamic quantum allocation and overhead vanabdity on central processing unit (CPU) efficiency CPU efficiency is determmed by the proportion of time devoted to user-oriented (problem state) tasks within a busy period Computational results indicate that a dynamic quantum allocation strategy produces significant differences in CPU efficiency compared to a constant quantum The differences are affected significantly by the variability among allocated quantum values and the demand on the system Overhead variability also has a pronounced effect.A function that depicts overhead as decreasing with demand produces more stable values of CPU efficiency The interaction between demand and the amount of overhead is observed to be significant KEY WORDS AND PHRASES dynamic quantum allocation, overhead vanabihty, semi-Markov model, time-sharing system, CPU efficiency CR CATEGORIES 4 35 1. Introductzon DESIGN CONSIDERATIONS IN TIME-SHARING OPERATING SYSTEMS.With a timesharing operating system, a single program is given control of the central processing unit (CPU) until execution is completed or a maximum ume limit is reached.This time slice, called a quantum, is followed by a period during which the CPU is controlled by the operating system while control of the CPU is removed from one program and assigned to another.This duration is called the overhead.A quite natural view is that the quantum represents a period of effective processing while the overhead is a necessary but nonproductive time requirement (see Mullery and Driscoll [18]).As Wichman [13, p. 194] states, "What one wants is some cheap method, in terms of processor time, of controlling what's going on."However, in a relauvely heterogeneous environment, characterized by several program prlonty levels, the scheduling Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the pubhcat,on and its date appear, and notice is given that copying is by permission of the AsSoclaUon for Computing Machinery To copy otherwise, or to republish, requires a fee and/or specific permission This report is a major revision containing corrections to and extensions of results given m "Dynamic Quantum Allocation and Swap-Time Vanabihty in Time-Sharing Operating Systems," VPI & SU Technical Report CS73001-R and SMU Technical Report CP-73009