Computer Science Department
School of Computer Science, Carnegie Mellon University


Task Assignment with Unknown Duration

Mor Harchol-Balter

August 1999

Keywords: Task assignment, load sharing, load balancing, scheduling, heavy-tailed workloads, high variance, distributed servers, fairness, contrary behavior

We consider a distributed server system and ask which policy should be used for assigning tasks to hosts. In our server, tasks are not preemptible. Also, the task's service demand is not known a priori. We are particularly concerned with the case where the workload is heavy-tailed, as is characteristic of many empirically measured computer workloads. We analyze several natural task assignment policies and propose a new one TAGS (Task Assignment based on Guessing Size). The TAGS algorithm is counterintuitive in many respects, including load unbalancing, non-work-conserving, and fairness. We find that under heavy-tailed workloads, TAGS can outperform all task assignment policies known to us by several orders of magnitude with respect to mean response time and mean slowdown, provided the system load is not too high. We also introduce a new practical performance metric for distributed servers called server expansion. Under the server expansion metric, TAGS significantly outperformns all other task assignment policies, regardless of system load.

37 pages

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