Asynchronous Software Thread Integration for Efficient Software Implementations of Embedded Communication Protocol Controllers

摘要

The overhead of context-switching limits efficient scheduling of multiple concurrent threads on a uniprocessor when real-time requirements exist. Existing software thread integration (STI) methods reduce context switches, but only provide synchronous thread progress within integrated functions. For the remaining, non-integrated portions of the secondary threads to run and avoid starvation, the primary thread must have adequate amounts of coarse-grain idle time (longer than two context-switches). We have developed asynchronous software thread integration (ASTI) methods which address starvation through the efficient use of coroutine calls and integration. ASTI allows threads to make independent progress efficiently and reduces the number of context switches needed through integration. Software-implemented protocol controllers are crippled by this problem; the primary thread "bit-bangs" each bit of a message onto or off of the bus, leaving only fragments of idle time shorter than a bit time. This fragmented time may be too short to recover through context switching, so only the primary thread can execute during message transmission or reception, slowing the secondary threads and potentially making them miss their deadlines. ASTI simplifies the implementation of embedded communication protocols on low-cost, moderate speed (1 - 100 MHz, 8- and 16-bit) microcontrollers. We demonstrate ASTI by replacing a standard automotive communication protocol controller (J1850) with software and generic hardware. Secondary thread performance improves significantly when compared with a traditional interrupt-based software approach.