On choosing a task assignment policy for a distributed server system
We consider a distributed server system in which each host processes tasks in First-Come-First-Served order and each task's service demand is known immediately upon task arrival. We consider four task assignment policies commonly proposed for such distributed server systems: Round Robin: Random; Size-Based. in which all tasks within a given size range are assigned to a particular host: and Dynamic-Least-Work-Remaining. in which a task is assigned to the host with the least outstanding work. Using analysis and simulation, we explore the influence of task size variability on which task assignment policy is best. Surprisingly, we find that not one of the above task assignment policies is best. In particular, we find that when the task sizes are nut highly variable. the Dynamic policy is preferable. However, when task sizes show the degree of variability more characteristic of empirically measured workloads, the size-based policy is the best choice. We use the resulting observations to argue in favor of a specific size-based policy, SITA-E, that shows very good performance for realistic task size distributions. (C) 1999 Academic Press.
task assignment distributed servers scheduling heavy-tailed workloads high valiance queueing theory