Conventional dynamic memory management methods interact poorly with lock-free synchronization. In this article, we introduce novel techniques that allow lock-free data structures to allocate and free memory dynamically using any thread-safe memory management library. Our mechanisms are lock-free in the sense that they do not allow a thread to be prevented from allocating or freeing memory by the failure or delay of other threads. We demonstrate the utility of these techniques by showing how to modify the lock-free FIFO queue implementation of Michael and Scott to free unneeded memory. We give experimental results that show that the overhead introduced by such modifications is moderate, and is negligible under low contention.

We address the problem of integrating lockfree shared data structures with standard dynamic allocation mechanisms (such as malloc and free). We have two main contributions. The first is the design and experimental analysis of two dynamic-sized lockfree FIFO queue implementations, which extend Michael and Scott’s previous implementation by allowing unused memory to be freed. We compare our dynamic-sized implementations to the original on 16-processor and 64-processor multiprocessors. Our experimental results indicate that the performance penalty for making the queue dynamic-sized is modest, and is negligible when contention is not too high. These results were achieved by applying a solution to the Repeat Offender Problem (ROP), which we recently posed and solved. Our second contribution is another application of ROP solutions. Specifically, we show how to use any ROP solution to achieve a general methodology for transforming lockfree data structures that rely on garbage collection into ones that use explicit storage reclamation.

Abduction is inference to the best explanation. In the TACITUS project at SRI we have developed an approach to abductive inference, called “weighted abduction”, that has resulted in a significant simplification of how the problem of interpreting texts is conceptualized. The interpretation of a text is the minimal explanation of why the text would be true. More precisely, to interpret a text, one must prove the logical form of the text from what is already mutually known, allowing for coercions, merging redundancies where possible, and making assumptions where necessary. It is shown how such “local pragmatics” problems as reference resolution, the interpretation of compound nominals, the resolution of syntactic ambiguity and metonymy, and schema recognition can be solved in this manner. Moreover, this approach of “interpretation as abduction” can be combined with the older view of “parsing as deduction” to produce an elegant and thorough integration of syntax, semantics, and pragmatics, one that spans the range of linguistic phenomena from phonology to discourse structure. Finally, we discuss means for making the abduction process efficient, possibilities for extending the approach to other pragmatics phenomena, and the semantics of the weights and costs in the abduction scheme.