The impact of processing order on performance: A taxonomy of semi-FIFO policies

Modern network processors increasingly deal with packets that require heterogeneous processing. We consider a bounded size input queue buffer where each packet requires several rounds of processing before transmission. Usually the transmission order of packets is induced by processing order, but processing order can have significant impact on the performance of buffer management policies even if the transmission order is fixed. We introduce the class of Semi-FIFO policies that decouple processing order from transmission order, restricting the latter to First-In-First-Out (FIFO). 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: greedy, work-conserving, lazy, and push-out policies, and show how these properties affect performance. We generalize our results to additional constraints related to copying cost and conduct a comprehensive simulation study that validates our results.