Sifter: An Inversion-Free and Large-Capacity Programmable Packet Scheduler

Packet schedulers play a crucial role in determining the order in which packets are served. They achieve this by assigning a rank to each packet and sorting them based on these ranks. However, when dealing with a large number of flows at high packet rates, sorting functions can become extremely complex and time-consuming. To address this issue, fast-approximating packet schedulers have been proposed, but they come with the risk of producing scheduling errors, or packet inversions, which can lead to undesirable consequences. We present Sifter, a programmable packet scheduler that offers high accuracy and large capacity while ensuring inversion-free operation. Sifter employs a unique sorting technique called "Sift Sorting" to coarsely sort packets with larger ranks into buckets, while accurately and finely sorting those with smaller ranks using a small Push-In-First-Out (PIFO) queue in parallel. The sorting process takes advantage of the "Speed-up Factor", which is a function of the memory bandwidth to output link bandwidth ratio, to achieve Sift Sorting and ensure accurate scheduling with low resource consumption. Sifter combines the benefits of PIFO's accuracy and FIFO-based schedulers' large capacity, resulting in guaranteed delivery of packets in an accurate scheduling order. Our simulation results demonstrate Sifter's efficiency in achieving inversion-free scheduling, while the FPGA-based hardware prototype validates that Sifter supports a throughput of 100Gbps without packet inversion errors.