Forgive-TM: Supporting Lazy Conflict Detection In Eager Hardware Transactional Memory

Commercial hardware transactional memory (TM) systems commonly use coherence messages to detect data conflicts. When a core inside a transaction receives a coherence request for data, it uses this information to determine whether there was a data conflict. Inherent in this behavior is the fact that data conflicts are detected eagerly, i.e., as soon as possible, and even while both sides of the conflict are speculative. Although it has been shown that lazy conflict detection can lead to better performance, this approach precludes lazy detection. In this paper, we describe a mechanism that allows conventional hardware to support lazy conflict detection, while still keeping the coherence protocol intact. Under Forgive-TM, speculative writes are done immediately to a special buffer, without first obtaining global write permission. The write permission is acquired later, when the transaction is about to commit. In other words, it "acts first, and asks forgiveness later." This effectively allows conflict detection to be done lazily. Using this scheme, ForgiveTM is able to provide 19% overall performance improvement in STAMP.