fb-survey - The Foreground-Background queue a survey Misja...

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The Foreground-Background queue: a survey Misja Nuyens * Adam Wierman September 12, 2007 Abstract Computer systems researchers have begun to apply the Foreground-Background ( FB ) schedul- ing discipline to a variety of applications, and as a result, there has been a resurgence in theo- retical research studying FB . In this paper, we bring together results from both of these research streams to provide a survey of state-of-the-art theoretical results characterizing the performance of FB . Our emphasis throughout is on the impact of these results on computer systems. Keywords: scheduling policies, FB , FBPS , LAS , LAST , SET , SEPT , M/G/1 queue 1 Introduction Scheduling is a common mechanism for improving computer-system performance without purchas- ing additional resources. Simple policies such as First-Come-First-Served ( FCFS ) and Processor- Sharing ( PS ), which shares the service capacity equally among all jobs in the system, are most commonly used in computer systems. However, many recent system designs use policies that give priority to jobs with small service demands in order to reduce the mean response time (sojourn time, waiting time) and mean queue length, see, e.g., [27, 47]. The emergence of policies that prioritize small jobs is motivated by the Shortest-Remaining- Processing-Time ( SRPT ) policy, which always serves the job in the system that needs the least amount of service in order to complete: SRPT is known to be optimal with respect to mean response time and mean queue length [58, 59]. The improvement of SRPT over FCFS and PS with respect to mean response time is quite dramatic as soon as there is even moderate variability in the service distribution. This is particularly true for heavy-tailed service distributions, which appear frequently as models for service-demand distributions in computer systems, see, e.g., [17, 62]. * Department of Mathematics, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, [email protected] Computer Science Department, California Institute of Technology, 1200 E. California Boulevard, MC 256-80, Pasadena, CA 91125, USA, [email protected] . 1
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Though SRPT is optimal with respect to mean response time, it is often not possible to use SRPT in computer systems, because in many cases, the scheduler is blind to the service demands of jobs. For instance, an operating system does not know how long a process will need to run, and a router does not know the length of a flow in the network. However, even when the scheduling policy is blind, i.e., it cannot use job-size information to prioritize, the scheduler can use other statistics in order to prioritize jobs with small service demands. One such statistic is the age or attained service of a job, which is defined as the amount of service already given to the job.
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