SP-PIFO: Approximating Push-In First-Out Behaviors using Strict-Priority Queues

Push-In First-Out (PIFO) queues are hardware primitives which enable programmable packet scheduling by providing the abstraction of a priority queue at line rate. However, implementing them at scale is not easy: just hardware designs (not implementations) exist, which support only about 1k flows. In this paper, we introduce SP-PIFO, a programmable packet scheduler which closely approximates the behavior of PIFO queues using strict-priority queues—at line rate, at scale, and on existing devices. The key insight behind SPPIFO is to dynamically adapt the mapping between packet ranks and available strict-priority queues to minimize the scheduling errors with respect to an ideal PIFO. We present a mathematical formulation of the problem and derive an adaptation technique which closely approximates the optimal queue mapping without any traffic knowledge. We fully implement SP-PIFO in P4 and evaluate it on real workloads. We show that SP-PIFO: (i) closely matches PIFO, with as little as 8 priority queues; (ii) scales to large amount of flows and ranks; and (iii) quickly adapts to traffic variations. We also show that SP-PIFO runs at line rate on existing hardware (Barefoot Tofino), with a negligible memory footprint.