Message communications in a distributed real-time system with a polled bus

High-speed message communications is addressed. Each node in the system consists of multiple processors interconnected by a broadcast bus, but the individual nodes are interconnected by an arbitrary network. The concept of a poll number is proposed to control the access to an intranode bus, minimizing the probability of real-time messages missing their deadlines. When a task generates a time-constrained message, a poll number associated with the message is computed on the basis of the message deadline and the task priority. When the bus is free, the various tasks at a node which desire to use the bus write the poll number onto the bus and read it back, one bit at a time, starting from the most significant bit. If at any time the bit read back is different from the bit written, then that particular task drops out of the competition for the bus. The mechanism provides for not only decentralized control of the intranode bus, but also a high degree of flexibility in scheduling messages by different ways of generating poll numbers. The probability of a message missing its deadline in a token bus is found to be much higher than in a polled bus