Exploring quality-of-service issues in network interface design.

Abstract

This dissertation examines issues related to the design and implementation of communication subsystems for quality-of-service (QoS). End-to-end communication performance is determined by factors such as the underlying network technology, the end-host operating system, and the host-network interface. Increasing network speeds shifts the performance bottleneck to the end hosts, especially to the hardware and software components of the communication subsystem. The tradeoffs involved in providing QoS in communication are explored by implementing real-time communication services using the Ancor VME CIM 250 (CIM), a commercial network adapter that does not provide QoS support. Though QoS guarantees are achieved, architectural deficiencies in the design of the CIM result in throughput which is much less than the capacity of its network link or the host's memory bandwidth. An Emulated Network Device (END) tool is proposed and implemented to address the above deficiencies. END couples a representative executable model of an adapter to a host, allowing the host's communication software to interact with this model in real time. END may be used to design and analyze network adapters before they are built, thus helping avoid costly design errors. A representative model of the CIM was built using END. This model was modified to show how the CIM's throughput could be increased by up to 50%. END is used to examine how QoS support may be divided between host operating systems and network adapters. For various system configurations, these experiments establish that QoS support on adapters significantly enhances delivered QoS for transmission. QoS for reception presents unique challenges since, unlike transmission, the host has no control over incoming data. In an interrupt-driven operating system, high packet arrival rates can result in receive livelock. A novel adapter-based solution to this problem is proposed, and its advantages are demonstrated using END. This research describes the architecture and implementation of END and demonstrates its versatility as a design and analysis tool by using it for: (a) construction of representative models of existing adapters, (b) introducing and evaluating design improvements, (c) evaluating the efficacy of QoS support on the host and/or adapter, and (d) studying reception issues in interface design.