Understanding I/O Direct Cache Access Performance for End Host Networking

Direct Cache Access (DCA) enables a network interface card (NIC) to load and store data directly on the processor cache, as conventional Direct Memory Access (DMA) is no longer suitable as the bridge between NIC and CPU in the era of 100 Gigabit Ethernet. As numerous I/O devices and cores compete for scarce cache resources, making the most of DCA for networking applications with varied objectives and constraints is a challenge, especially given the increasing complexity of modern cache hardware and I/O stacks. In this paper, we reverse engineer details of one commercial implementation of DCA, Intel's Data Direct I/O (DDIO), to explicate the importance of hardware-level investigation into DCA. Based on the learned knowledge of DCA and network I/O stacks, we (1) develop an analytical framework to predict the effectiveness of DCA (i.e., its hit rate) under certain hardware specifications, system configurations, and application properties; (2) measure penalties of the ineffective use of DCA (i.e., its miss penalty) to characterize its benefits; and (3) show that our reverse engineering, measurement, and model contribute to a deeper understanding of DCA, which in turn helps diagnose, optimize, and design end-host networking.