Adaptive Allocation of Central Processing Unit Quanta

Abstract

The allocation of the central processing unit (CPU) of a computer system in quanta of fixed length in round-robin fashion favors jobs with shorter total CPU processing time by reducing the time they spend waiting in queue below what it would be if all the lobs were served in first-come-first-served order This effect can be accentuated by the use of short quanta. The main disadvantage of this allocation policy is the resulting time the CPU spends in overhead activities when switching from one task to the other, this too will increase with smaller quanta. Thus, it appears useful to consider adaptive CPU allocation policies to reduce the overhead during high traffic conditions when saturation of this resource is more likely while keeping a small quantum during periods of low arrival traffic. In this paper we analyse such a policy, it is assumed that each time at least r (a threshold) arrivals occur during a quantum, the job currently using the CPU is allocated an additional quantum (if It is needed). Thus, the number of job arrivals during a quantum is used as a sensor of the intensity of arrival traffic. This policy, which can be easily implemented in hardware, is analysed using a mathematical model yielding the average response time for jobs as a function of mean total CPU time, the quantum size, r, and a fixed overhead for switching tasks, with a Poisson arrival process. Numerical results to illustrate the effect of this policy are presented.