Real-time computing with lock-free shared objects

This article considers the use of lock-free shared objects within hard real-time systems. As the name suggests, lock-free shared objects are distinguished by the fact that they are accessed without locking. As such, they do not give rise to priority inversions, a key advantage over conventional, lock-based object-sharing approaches. Despite this advantage, it is not immediately apparent that lock-free shared objects can be employed if tasks must adhere to strict timing constraints. In particular, lock-free object implementations permit concurrent operations to interfere with each other, and repeated interferences can cause a given operation to take an arbitrarily long time to complete. The main contribution of this article is to show that such interferences can be bounded by judicious scheduling. This work pertains to periodic, hard real-time tasks that share lock-free objects on a uniprocessor. In the first part of the article, scheduling conditions are derived for such tasks, for both static and dynamic priority schemes. Based on these conditions, it is formally shown that lock-free shared objects often incur less overhead than object implementations based on wait-free algorithms or lock-based schemes. In the last part of the article, this conclusion is validated experimentally through work involving a real-time desktop videoconferencing system.