Packets are Not Pages: Flow-Based Addressing Conserves Memory Bandwidth

Abstract

SmartNICs promise hardware offloading for network applications like traffic analysis and virtual host dispatching. However, due to memory bandwidth limitations, SmartNICs are unable to effectively accelerate applications like intrusion detection and deep packet inspection that require high-speed reassembly. We argue that this limitation is due to an architectural mismatch: variable-sized packets arrive out of order, but a SmartNIC's memory is a contiguous address space broken up into fixed-size pages. This mismatch forces multiple unnecessary copies to translate between the two layouts. We propose a new memory addressing scheme, flow-based addressing, which enables software to access the reassembled payloads of a flow's packets as a contiguous region in memory. Flow-based addressing allows software to easily operate on reassembled payloads without copies, dramatically reducing memory traffic. Eliminating copies entirely requires hardware that maps logical flow offsets to non-contiguous physical addresses and assembles misaligned payloads into contiguous cache lines at line rate. The new memory addressing scheme introduces three architectural mechanisms: a flow translation table, a private flow cache, and a specialized DMA engine that assembles unaligned, variable-length packets into contiguous cache lines. We describe a prototype design on an FPGA-based SmartNIC and show that, across synthetic microbenchmarks, university campus network traffic, and data center web-search traffic, flow-based addressing reduces DRAM bandwidth use by 77-83%, lowers execution time by 25-62%, and increases per-core throughput by 1.33-2.66×.