On resource pooling in SITA-like parallel server systems

The routing policy Size Interval Task Assignment (SITA) isolates small arrivals from large arrivals, while choosing intervals to balance the workload of each server. It works well for highly variable arrivals, but the isolation can cause server idle-ness. To improve this, we suggest a scheme to add pooling to these SITA-like systems, which can result in better performance. We propose a routing policy, SITA-JSQ, which chooses a proportion of the dedicated arrivals, originally allocated by the SITA policy with equal loads, as flexible arrivals allocated by a JSQ policy between adjacent servers. Under heavy traffic and Complete Resource Pooling conditions, the asymptotic Brownian Motion limit for the unfinished processing times processes is obtained. Using these limits, we show that SITA-JSQ gives asymptotically better performance with respect to unfinished processing times than SITA. Through simulation, we also demonstrate significant reductions in mean waiting times. Finally, we compare our approach to cycle stealing from idle servers.