Programmable Packet Scheduling With SP-PIFO: Theory, Algorithms and Evaluation

Push-In First-Out (PIFO) is a theoretical hardware model for programmable packet scheduling, enabling scheduling policies to be comprehensibly and dynamically reconfigured, and SP-PIFO is a practical emulation for PIFO that can be readily implemented with stock P4 switches. The efficiency of SP-PIFO hinges on a simple heuristic, Push-Up/Push-Down (PUPD), which is responsible for dynamically adapting the mapping of input packets to a fixed set of strict priority queues so as to minimize the rate of scheduling errors with respect to an ideal PIFO. In this paper, we present the first formal analysis of the PUPD algorithm. Our competitive analysis yields that the capacity of PUPD to emulate an optimal PIFO model is getting linearly worse as we keep on adding priority queues to the system. Motivated by this finding, we present an optimal offline scheme, which, given a stochastic model of the input, outputs the optimal SP-PIFO configuration in polynomial time, and we introduce an online heuristic that aims to approximate the offline optimum without requiring a stochastic input model. Our simulations show that the online algorithm can improve the performance of SP-PIFO by a factor of 2x in certain configurations.