The Impact of Processing Order on Performance: a Taxonomy of Semi-FIFO Policies
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Modern network processors increasingly deal with packets that require heterogeneous processing. We consider the problem of managing a bounded size input queue buffer where each packet requires several rounds of processing before it can be transmitted out. This to maximize the total number of successfully transmitted packets. Usually the transmission order of the packets is induced by the processing order. However, processing order can have a significant impact on the performance of buffer management policies even if the order of transmission is fixed. For this reason we decouple processing order from transmission order and restrict our transmission order to First-In-First-Out (FIFO) but allow for different orders of packet processing, introducing the class of such policies as Semi-FIFO. In this work, we build a taxonomy of Semi-FIFO policies and provide worst case guarantees for different processing orders. We consider various special cases and properties of Semi-FIFO policies, e.g., greedy, work-conserving, lazy, and push-out policies, and show how these properties affect performance. We generalize our results to additional constraints on the push-out mechanism designed to deal with copying cost. Further, we conduct a comprehensive simulation study that validates our results.