Extending STI for Demanding Hard-Real-Time Systems

Software thread integration (STI) is a compilation technique which enables the efficient use of an application’s fine-grain idle time on generic processors without special hardware support. With STI, a primary function (with real-time requirements on specific instructions) is automatically interleaved with a secondary function to create a single implicitly multithreaded function which minimizes context switching and hence both improves performance and also offers very fine-grain concurrency. In this paper we extend STI techniques to address two challenges. First, we reduce response time for interrupts or other high-priority threads by introducing polling servers into integrated threads. Currently integrated threads disable interrupts, delaying all other work until their completion. Second, we enable integration with long host threads, expanding the domain of STI. With current techniques, if there are frequent interrupts, only host threads which can finish execution before the next interrupt can be integrated. We derive methods to evaluate the response time for threads in systems with and without these new integration methods. We demonstrate these concepts with the integration of various threads in a sample hard-real-time system on a highly-constrained microcontroller. We use an inexpensive 20 MHz AVR 8-bit microcontroller to generate monochrome NTSC video while servicing a high-speed (115.2 kbaud) serial communication link. We have built and tested this system and demonstrate graphics rendering speed-ups of 3.99x to 13.5x