Leveraging Irrevocability to Deal with Signature Saturation in Hardware Transactional Memory

Abstract

In hardware transactional memory, signatures have been proposed to keep track of memory locations accessed in a transaction to help conflict detection. Generally, signatures are implemented as Bloom filters that suffer from aliasing, that is, they can give rise to false conflicts. Such conflicts are more likely as signature fills (saturation), and they can lead a parallel application to perform worse than its serial version. Irrevocability is analyzed to address the signature saturation problem. When a transaction reaches a saturation threshold, the transaction enters an irrevocable state that prevents it from being aborted. Hence, such a transaction keeps running while the others are either stalled or allowed to run concurrently. We propose an analytical model that shows this is a good solution to overcome a high contention scenario. In addition, experimental evaluation shows the benefits in performance and power consumption of the proposed irrevocability mechanisms. Different saturation metrics are considered and a fixed threshold is found that yields maximum performance for the benchmarks evaluated.