Unbounded Hardware Transactional Memory for a Hybrid DRAM/NVM Memory System

Abstract

Persistent memory programming requires failure atomicity. To achieve this in an efficient manner, recent proposals use hardware-based logging for atomic-durable updates and hardware transactional memory (HTM) for isolation. Although the unbounded HTMs are promising for both performance and programmability reasons, none of the previous studies satisfies the practical requirements. They either require unrealistic hardware overheads or do not allow transactions to exceed on-chip cache boundaries. Furthermore, it has never been possible to use both DRAM and NVM in HTM, though it is becoming a popular persistency model. To this end, this study proposes UHTM, unbounded hardware transactional memory for DRAM and NVM hybrid memory systems. UHTM combines the cache coherence protocol and address-signatures to detect conflicts in the entire memory space. This approach improves concurrency by significantly reducing the false-positive rates of previous studies. More importantly, UHTM allows both DRAM and NVM data to interact with each other in transactions without compromising the consistency guarantee. This is rendered possible by UHTM's hybrid version management that provides an undo-based log for DRAM and a redo-based log for NVM. The experimental results show that UHTM outperforms the state-of-the-art durable HTM, which is LLC-bounded, by 56% on average and up to 818%.