Real-Time Virtual Channel Flow Control

Abstract

Real-time communication system support for large scale parallel multicomputers becomes an important issue as the number of real-time applications developed on these systems increases. Flow control is a key component that affects the performance of the communication subsystem. We develop a range of new real-time virtual channel flow control schemes for wormhole networks. The flow control schemes differ in their priority mapping strategies, priority adjustment methods, and arbitration functions. The priority mapping strategy and priority adjustment method of a flow control scheme determine the priority of a message. The priority of a message is used for the virtual channel assignment and the physical channel arbitration. We discuss the trade-off between the performance and the hardware cost of each flow control scheme. A simulator is implemented for studying the performance of the schemes, and simulation experiments are designed to compare the importance of priority mapping, priority adjustment and arbitration toward the system performance. As wormhole networks scale to larger sizes, the average distance between source and destination nodes increases. The flits of messages in wormhole networks, which are buffered in nodes along the path from the source to the destination, consume network resources in these nodes. Therefore, increased scaling may lead to increased resource consumption, congestion, and late messages. In real-time systems, messages lose their value when they miss their deadlines. In order to reduce congestion, we provide a scheme for dropping messages that miss their deadlines.