Reducing False Aborts in STM Systems
Transactional memory (TM) continues to be the most promising approach replacing locks in concurrent programming, but TM systems based on software (STM) still lack the desired performance when compared to fine-grained lock implementations. It is known that the critical operation in TM systems is to ensure the atomicity and isolation of concurrently executing threads. This task is known as the read/write-set validation. In attempt to make this process as fast as possible, STM systems usually use ownership tables to perform conflict detection, but this approach generates false positive occurrences, which result in false aborts. This paper shows the real impact of false aborts and how its relevance increases along with the number of concurrent threads, showing it is an essential factor for TM systems. We propose two different techniques to avoid false aborts, showing its benefits and limitations. The first is a collision list attached to the existing hash table. The second is a full associative memory mapping between the addresses and its version information. We achieved significant performance improvements in some STAMP benchmark programs, resulting in speedups up to 1.5x. We also show that speedups become higher when the number of parallel threads increases.
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