A comparative study of some network subsystem organizations

The impact of alternative network subsystem design for realizing low end–to–end latencies and high network throughput in a switched LAN are studied in detail through simulation. These alternatives include choices in the disposition of the network interface card (NIC), DMA priorities and OS services. Our simulation model captures the delays of OS services/software layers, message copying DMAs and, in addition, models non–network related traffic on the I/O and memory buses introduced by paging and on–chip cache misses. In a conventional setup, with the NIC placed on the I/O bus, we show that changing traffic priorities on the memory bus to speed up the transfers between the NIC and the DRAM has little impact on overall latency and network throughput as the offered network traffic increases. Improving the speed of the I/O bus produces some performance gains. These performance gains are shown to be quite limited until message demultiplexing capabilities are added to the NIC. The best performance comes from the use of dual–ported DRAMs, with a dedicated connection between the NIC and the added port.