Avoiding unbounded priority inversion in barrier protocols using gang priority management

Large real-time software systems such as real-time Java virtual machines often use barrier protocols, which work for a dynamically varying number of threads without using centralized locking. Such barrier protocols, however, still suffer from priority inversion similar to centralized locking. We introduce gang priority management as a generic solution for avoiding unbounded priority inversion in barrier protocols. Our approach is either kernel-assisted (for efficiency) or library-based (for portability) but involves cooperation from the protocol designer (for generality). We implemented gang priority management in the Linux kernel and rewrote the garbage collection safe-point barrier protocol in IBM's WebSphere Real Time Java Virtual Machine to exploit it. We run experiments on an 8-way SMP machine in a multi-user and multi-process environment, and show that by avoiding unbounded priority inversion, the maximum latency to reach a barrier point is reduced by a factor of 5.3 and the application jitter is reduced by a factor of 1.5.