Real-Time Computing with Lock-Free Shared Objects

J.H. Anderson, S. Ramamurthy, and K. Jeffay
Proceedings of the 16th IEEE Real-Time Systems Symposium
Pisa, Italy, December 1995
pp. 28-37.

Abstract: This paper 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 not locked. 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 paper 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 paper, 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 object-sharing approaches typically incur much less overhead than approaches based on lock-based schemes. It is also shown that lock-free objects are surprisingly better suited for real-time computing on uniprocessors than are wait-free objects, even though operations on wait-free objects are guaranteed to complete in bounded time. These conclusions are validated experimentally through work involving a real-time desktop videoconferencing system.

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